12.7 Global strategies for control of communicable diseases*
Oxford Textbook of Public Health
Global strategies for control of communicable diseases*
Robert J. Kim-Farley
Introduction and overview
Definitions of infectious diseases and their control
Global burden of infectious diseases
Chain of infection: agent, transmission, and host
Tools for control of infectious diseases
Control measures applied to the host
General improvement in host resistance
Control measures applied to vectors
Control measures applied to infected humans
Quarantine of potentially infected persons
Restriction of activities
Control measures applied to animals
Restriction or reduction
Chemoprophylaxis and chemotherapy
Control measures applied to the environment
Provision of safe water
Proper disposal of faeces
Design of facilities and equipment
Control measures applied to the agent
Control and prevention programmes
Emerging infectious diseases
Ingenuity, knowledge, and organization alter but cannot cancel humanity’s vulnerability to invasion by parasitic forms of life. Infectious disease which antedated the emergence of humankind will last as long as humanity itself, and will surely remain, as it has been hitherto, one of the fundamental parameters and determinants of human history. (William H. McNeill 1976)
Introduction and overview
Communicable, or infectious, diseases have been and continue to remain a leading cause of morbidity, disability, and mortality worldwide. Their control is a constant challenge that faces health workers and public health officials in both industrialized and developing countries. Only one infectious disease, smallpox, has been eradicated and stands as a landmark in the history of the control of infectious diseases. The international community is now well down the path towards eradication of poliomyelitis and dracontiasis (guinea-worm infection). Other infectious diseases, like malaria and tuberculosis, have foiled eradication attempts or control efforts and are re-emerging as increasing threats in many countries. Some infectious diseases, such as tetanus, will always be a threat if control measures are not maintained. Newer infectious diseases, like AIDS, demonstrate the truth of McNeill’s statement that infectious disease will remain ‘one of the fundamental parameters and determinants of human history’. The history of infectious diseases is an exciting story in itself and readers interested in the subject are referred to McNeill (1976) or to the comprehensive work on the history of human diseases (Kiple 1993).
In the organization of this chapter, a fundamental decision was taken to provide a global and comprehensive view of the control of infectious diseases through examination of the magnitude of disease burden, the chain of infection (agent, transmission, and host) of infectious diseases, the varied approaches to their prevention and control, and the factors conducive to their eradication as well as emergence and re-emergence. Although this chapter provides many examples of infectious diseases that illustrate modes of transmission and approaches to infectious disease control, this chapter does not attempt to be comprehensive in listing all infectious diseases. Detailed recommendations on control measures for any specific disease are outlined periodically in the updated reports of the American Public Health Association, Control of Communicable Diseases Manual (Chin 2000), the comprehensive two-volume work Mandell, Douglas and Bennett’s Principles and Practice of Infectious Diseases (Mandell et al. 1995), and the textbook Infectious Diseases (Gorbach et al. 1998). For readers specifically interested in paediatric infectious diseases there is the comprehensive two-volume Textbook of Pediatric Infectious Diseases (Feigin and Cherry 1998), for infectious diseases in emergency medicine settings there is the textbook Infectious Disease in Emergency Medicine (Brillman and Quenzer 1998), and for tropical infectious diseases there is Tropical Infectious Diseases: Principles, Pathogens, and Practice (Guerrant et al. 1999). A comprehensive treatment of the worldwide distribution and diagnosis of infectious diseases is provided in A World Guide to Infections: Diseases, Distribution, Diagnosis (Wilson 1991). The Centers for Disease Control and Prevention (CDC) publishes up-to-date disease surveillance information for the United States and recommendations for control measures in the Morbidity and Mortality Weekly Reports and provides annual summaries of notifiable infectious diseases in the Summary of Notifiable Diseases, United States (CDC 1999). Many other countries have similar types of publications. The World Health Organization (WHO) publishes worldwide surveillance information and recommendations for control measures in the Weekly Epidemiological Record. A more detailed background on infectious agents as determinants of health and disease is provided in Chapter 2.6.
Definitions of infectious diseases and their control
Infection occurs when an infectious agent enters a body and develops or multiplies. Infectious agents are organisms capable of producing inapparent infection or clinically manifest disease and include bacteria, rickettsia, chlamydiae, fungi, parasites, viruses, and prions. An infectious, or communicable, disease is an infection that results in a clinically manifest disease. Infectious disease may also be due to the toxic product of an infectious agent, such as the toxin produced by Clostridium botulinum causing classical botulism. As this is a textbook of public health, the infectious diseases considered are those that manifest in human hosts and are a result of the interaction of people and their environment. Infectious diseases may be due to infectious agents exclusively found in human hosts such as rubella virus, in the environment such as Legionella pneumophila, or primarily in animals such as Brucella abortus.
Control of infectious diseases refers to the actions and programmes directed towards reducing disease incidence, reducing disease prevalence, or completely eradicating the disease. Control aimed at reducing the incidence of infectious disease can be considered as primary prevention of infectious disease. Primary prevention protects health through the effects of personal as well as community-wide measures, including such actions as maintaining good nutritional status, keeping physically fit, immunizing against infectious diseases, providing safe water, and ensuring the proper disposal of faeces.
Control aimed at reducing the prevalence by shortening the duration of infectious disease can be considered as secondary prevention of infectious disease. Secondary prevention corrects departures from good health through the effects of individual and community actions, including such actions as early detection of disease, prompt antibiotic treatment, and ensuring adequate nutrition. It should be noted that such control efforts in secondary prevention in a group of infected individuals may also result in primary prevention in uninfected persons, a good example being prompt specific drug therapy for tuberculosis patients to produce sputum conversion which renders them no longer a source of infection to others.
Control aimed at reducing or even eliminating long-term impairments of infectious disease can be considered as tertiary prevention of infectious disease. Tertiary prevention reduces or eliminates disabilities, minimizes suffering, and promotes adjustment to conditions that are not remediable through such actions as providing orthopaedic appliances, counselling and vocational training, and prevention of opportunistic infections. The prevention of opportunistic infections in HIV infection, for example, can be considered as tertiary prevention (Osterholm et al. 1995).
Global burden of infectious diseases
Infectious diseases remain a leading cause of morbidity, disability, and mortality worldwide. A WHO analysis of the global burden of disease estimated that 13.3 million deaths out of a total of 53.9 million deaths in 1998 were attributable to infectious diseases (WHO 1999a). Most of these deaths occurred in the economically developing group of countries. Infectious diseases contributed to approximately 25 per cent of all deaths globally, and one in two deaths in developing countries. This should not be construed to mean that infectious diseases are not significant in more developed countries. In the United States, for example, AIDS rose to become the leading cause of death in persons aged 25 to 44 years, and still ranks as an important cause of death in this age group.
The current magnitude of morbidity and mortality due to infectious diseases worldwide is highlighted by the WHO as follows (WHO 1999a).
Acute respiratory infections, including pneumonia and influenza, result in 3.5 million deaths per year. These infections are the highest cause of infant and child mortality in developing countries with almost 2 million deaths in children under 5 years of age.
Diarrhoeal diseases are also a major cause of morbidity and mortality in infants and children in developing countries. Each year there are 2.2 million deaths due to diarrhoeal disease, of which some 1.8 million are among children under 5 years of age.
Malaria is estimated to cause 1.1 million deaths per year worldwide. Aprroximately 800 000 deaths of children under the age of 5 years are attributable to malaria.
Diseases preventable by vaccination, although in decline worldwide, still result in an estimated 1.6 million deaths each year. For example, there are still approximately 900 000 deaths due to measles each year in children under 5 years of age.
Mycobacterium tuberculosis, the causative agent of tuberculosis, now infects about one-third of the world’s population. It is estimated that there are approximately 1.5 million deaths due to tuberculosis each year.
HIV has infected more than 58 million persons since the start of the HIV/AIDS pandemic and it is estimated that approximately 22 million persons with AIDS have died throughout the world.
Sexually transmitted diseases other than AIDS are estimated to have a global annual incidence of some 333 million cases, occurring predominantly in developing countries.
Hepatitis B virus infects some 2 billion people worldwide. There are approximately 350 million chronic carriers and the sequelae of their infections lead to over 1 million deaths per year, almost all avoidable by immunization.
Chain of infection: agent, transmission, and host
The chain of infection is the relationship between an infectious agent, its routes of transmission, and a susceptible host. The prevention and control of infectious diseases depend upon the interaction of these three factors that may result in the human host clinically manifesting disease.
The infectious agent is the first link in the chain of infection and is any micro-organism whose presence or excessive presence is essential for the occurrence of an infectious disease. Examples of infectious agents include the following.
Bacteria: for example, spirochetes and curved bacteria such as Borrelia burgdorferi causing Lyme disease, Gram-negative rods such as Yersinia pestis causing plague, Gram-positive cocci such as Streptococcus pyogenes group A, causing erysipelas, and Gram-positive rods such as Mycobacterium tuberculosis causing tuberculosis.
Rickettsiae: for example, Rickettsia ricketsii causing Rocky Mountain spotted fever, and Rickettsia prowazekii causing epidemic louse-borne typhus fever.
Chlamydiae: for example, Chlamydia psittaci causing psittacosis, and Chlamydia trachomatis causing trachoma and genital infections.
Fungi: for example, Trichophyton schoenleinii and Microsporum canis causing tinea capitis, and Tinea rubrum, Tinea mentagrophytes, and Epidermophyton floccosum causing tinea pedis.
Parasites, for example, helminths such as Trichinella spiralis causing trichinosis, filaria such as Brugia malayi causing filariasis, nematodes such as Enterobius vermicularis causing enterobiasis (pinworm disease), trematodes such as Clonorchis sinensis causing clonorchiasis (oriental liver fluke disease), cestodes such as Taenia solium causing taeniasis (beef tapeworm disease), and protozoa such as Trypanosoma cruzi causing American trypanosomiasis (Chagas’ disease).
Viruses: for example, Paramyxoviridae such as measles virus which causes measles, Togaviridae such as the rubella virus which causes rubella, and arthropod-borne viruses (arboviruses) such as dengue viruses which cause dengue fever.
Prions, which are small proteinaceous infectious particles that cause diseases such as kuru, Creutzfeldt–Jakob disease (and its variant associated with exposure of humans to the bovine spongiform encephalopathy agent), and the Gerstmann–Straussler–Scheinker sydrome.
There is evidence that some infectious agents, often with cofactors, are associated with human tumours. Examples include Shistosoma haematobium with bladder cancer, Shistosoma japonicum with colorectal cancer, Clonorchis sinensis with cholangiocarcinoma, hepatitis B and C viruses with hepatocellular carcinoma, Helicobacter pylori with gastric cancer, and human papillomaviruses with cervical cancer.
Agents can be described by their ability to cause disease (pathogenicity) as well as their ability to cause serious disease (virulence). The pathogenicity of an infectious agent is the extent to which clinically manifest disease is produced in an infected population and is measured by the ratio of the number of persons developing clinical illness to the total number infected. Examples of highly pathogenic infectious agents are the measles virus and the human (a) herpesvirus 3 (varicella-zoster) causing measles and chickenpox, respectively, in which most infected susceptible persons will manifest disease.
The virulence of an infectious agent is the extent to which severe disease is produced in a population with clinically manifest disease. It is the ratio of the number of persons with severe and fatal disease to the total number of persons with disease. An example of a highly virulent infectious agent is HIV, whereby nearly all untreated persons with AIDS will die.
Characteristics of infectious agents that affect pathogenicity include their ability to invade tissues (invasiveness), produce toxins (intoxication), cause damaging hypersensitivity (allergic) reactions, undergo antigenic variation, and develop antibiotic resistance. An example of an infectious agent with high invasiveness is the Shigella organism that can invade the submucosal tissue of the intestine and cause clinically manifest shigellosis (bacillary dysentery). An example of an infectious agent that has a high degree of ability to produce toxins is the Clostridium botulinum organism that can elaborate toxins in inadequately prepared food and cause classical botulism. An example of an infectious agent that is highly allergenic is the Mycobacterium tuberculosis organism which can cause tuberculosis. An example of an infectious agent that has a high degree of antigenic variation is the type A influenza virus which frequently experiences minor antigenic changes—antigenic ‘drift’. Influenza A viruses, on an irregular basis, may also undergo a major antigenic change creating an entirely new subtype—antigenic ‘shift’. Antigenic shifts may result in an influenza pandemic when individuals immune to previous strains of influenza are exposed to the new strain. An example of an infectious agent that can develop antibiotic resistance that challenges control efforts is Neisseria gonorrhoeae that has both chromosomally mediated and resistance transfer plasmid mediated genetic factors for antibiotic resistance.
The infective dose of an infectious agent is the number of organisms needed to cause an infection. The infective dose may vary depending upon the route of transmission and host susceptibility.
Control measures for infectious diseases directed at inactivating the agent are designed according to the type of agent and its reservoirs and sources. An agent like Vibrio cholerae, for example, can be inactivated through adequate chlorination of the water supply. This is a chemical method for provision of safe water to control cholera. An agent like hepatitis B virus can be inactivated through adequate autoclaving of injection and surgical equipment. This is a sterilization method to control hepatitis B. Details of these and other methods of control directed at the agent are provided in the sections in this chapter on control measures applied to the agent and the environment.
Routes of transmission
Control efforts are often designed to break the routes of transmission, the mechanisms by which infectious agents are spread from reservoirs or sources to human hosts. A reservoir of an infectious agent is any person, other living organism, or inanimate material in which the infectious agent normally lives and grows. The source of infection for a host is the person, other living organism or inanimate material from which the infectious agent came. The routes of transmission have been summarized by Chin (2000) as follows.
Direct and essentially immediate transfer of infectious agents to a receptive portal of entry through which human or animal infection may take place. This may be by direct contact as touching, biting, kissing or sexual intercourse, or by the direct projection (droplet spread) of droplet spray onto the conjunctiva or onto the mucous membranes of the eye, nose, or mouth during sneezing, coughing, spitting, singing, or talking (usually limited to a distance of about 1 m or less).
Vehicle-borne Contaminated inanimate materials or objects (fomites) such as (a) toys, handkerchiefs, soiled clothes, bedding, cooking or eating utensils, surgical instruments, or dressings, (b) water, food, milk, and biological products including blood, serum, plasma, tissues, or organs, or (c) any substance serving as an intermediate means by which an infectious agent is transported and introduced into a susceptible host through a suitable portal of entry. The agent may or may not have multiplied or developed in or on the vehicle before being transmitted.
Mechanical Includes simple mechanical carriage by a crawling or flying insect through soiling of its feet or proboscis, or by passage of organisms through its gastrointestinal tract. This does not require multiplication or development of the organism.
Biological Propagation (multiplication), cyclic development, or a combination of these (cyclopropagative) is required before the arthropod can transmit the infective form of the agent to humans. An incubation period (extrinsic) is required following infection before the arthropod becomes infective. The infectious agent may be passed vertically to succeeding generations (transovarian transmission). Trans-stadial transmission indicates its passage from one stage of lifecycle to another, such as nymph to adult. Transmission may be by injection of salivary gland fluid during biting, or by regurgitation or deposition on the skin of faeces or other material capable of penetrating through the bite wound or through an area of trauma from scratching or rubbing. This transmission is by an infected non-vertebrate host and not simple mechanical carriage by a vector as a vehicle. However, an arthropod in either role is termed a vector.
The dissemination of microbial aerosols to a suitable portal of entry, usually the respiratory tract. Microbial aerosols are suspensions of particles in the air consisting partially or wholly of micro-organisms. They may remain suspended in the air for long periods of time, some retaining and others losing infectivity or virulence.
Droplet nuclei Usually the small residues that result from evaporation of fluid from droplets emitted by an infected host. They also may be created purposefully by a variety of atomizing devices, or accidentally as in microbiology laboratories or in abattoirs, rendering plants, or autopsy rooms. They usually remain suspended in the air for long periods of time.
Dust The small particles of widely varying size which may arise from soil (as, for example, fungus spores separated from dry soil by wind or mechanical agitation), clothes, bedding, or contaminated floors.
Control measures for infectious diseases directed at interrupting transmission are designed according to the type of transmission for the agent. Direct transmission of an agent like Neisseria gonorrhoeae, for example, can be reduced by using condoms as a barrier method of control of gonorrhoea. Vector-borne transmission of an agent like Plasmodium falciparum can be reduced by using a residual insecticide against Anopheles mosquitos as a chemical vector control method for malaria. Airborne transmission of an agent like Mycobacterium tuberculosis from sputum-positive pulmonary tuberculosis patients in hospital can be reduced by the use of special ventilation in the patient’s room as an environmental method of control of tuberculosis. It should be recognized that some infectious agents may have more than one route of transmission. Poliovirus, for example, can be spread via direct transmission through the faecal–oral route and pharyngeal spread, or indirect transmission through contaminated food or other materials. Details of these and other methods of control directed at interrupting transmission are provided in the sections on control measures in this chapter.
The host is the final link in the chain of infection. The infectious agent may enter the host through the following portals of entry.
Respiratory tract: infectious agents can be inhaled into the respiratory tract and will be deposited at different levels of the pulmonary tree according to the size of the aerosol, droplet nuclei, or dust particles. Particles between 1 and 5 µm, for example, can reach to the alveoli of the lungs.
Intact skin: some infectious agents, such as Necator americanus that may cause hookworm disease, can penetrate the intact skin.
Gastrointestinal tract: an infectious agent such as Vibrio cholerae may enter via the gastrointestinal tract and result in cholera. Persons who have a compromised gastric function, such as gastric achlorhydria, may be at increased risk of disease.
Mucous membranes: infectious agents, such as measles viruses, may be deposited on mucous membranes by droplet spread or by direct contact with infected persons or contaminated objects.
Urinary tract: some infectious agents, such as Escherichia coli, can enter the urinary tract via an ascending route from the urethra colonized with the organism. Structural abnormalities of the urinary tract and procedures such as urinary catheterization may predispose the host to disease.
Placenta: transplacental transmission is a direct route of transmission to the fetus for infectious agents such as rubella viruses.
Infectious agents also enter the host though mechanisms that get past the body’s natural barriers, including wounds that break the integrity of the skin or mucous membranes; invasive procedures, parenteral injections, parenteral infusions, or organ transplants that may introduce an agent into the body; or insect vectors that may inject agents through the skin.
The most important host factors regarding developing clinically manifest disease and the severity of disease are immune status and age. Infants, young children, and the elderly are at generally higher risk from more severe disease due to immaturity or deterioration of immune systems, respectively.
Many host defence mechanisms help prevent infection or disease. Non-specific host defence mechanisms include the intact skin, nasal cilia, tears, saliva, mucus, and gastric acid. Specific host defence mechanisms include naturally acquired immunity from previous infection, tranplacentally acquired passive immunity in the newborn from the mother, artificially acquired active immunity from immunization, and artificially acquired passive immunity from immunoglobulins and antitoxins.
Host responses to infection that prevent or reduce the severity of infectious disease include (a) polymorphonuclear leukocytosis stimulated by some bacterial infections that increases the number of phagocytic cells, (b) fever that may slow the multiplication of some infectious agents, (c) antibody production that may neutralize some infectious agents or their toxins, and (d) interferon production that may block intracellular replication of viruses.
The manifestation of infection in the host may vary from inapparent (subclinical) infection to severe disease that may even result in death. The interaction between an infectious agent, routes of transmission and host factors determines the spectrum of signs and symptoms. Sometimes the host may become an asymptomatic carrier of the infectious agent and be a source of infection for others.
Control measures for infectious diseases directed at the host are designed according to the immune status of the host and the likelihood of host exposure to certain infectious agents. Measles disease, for example, can be prevented by active immunization with measles vaccine to develop host immunity. Pneumonic plague can be prevented in those in close contact with patients with plague pneumonia by using tetracycline or sulfonamide chemoprophylaxis. Details of these and other methods of control directed at the host are provided in the section on control measures applied to the host in this chapter.
Tools for control of infectious diseases
The primary concern of infectious disease control in public health, whether in developing or industrialized countries, is the reduction, elimination, or even eradication of infectious disease. This is accomplished by directing control measures to the host, the routes of transmission, or the agent. Such control measures include (a) identifying and reducing or eliminating infectious agents at their sources and reservoirs, (b) breaking or interfering with the routes of transmission of infectious agents, and (c) identifying susceptible populations and reducing or eliminating their susceptibility.
The tools for control of infectious diseases are related to the recognition and evaluation of the patterns of diseases and interventions to control them. The most important tool for the recognition and evaluation is surveillance of disease is defined as
the continuing scrutiny of all aspects of occurrence and spread of a disease that are pertinent to effective control. Included are the systematic collection and evaluation of: (a) morbidity and mortality reports; (b) special reports of field investigations of epidemics and of individual cases; (c) isolation and identification of infectious agents by laboratories; (d) data concerning the availability, use, and untoward effects of vaccines and toxoids, immune globulins, insecticides, and other substances used in control; (e) information regarding immunity levels in segments of the population; and (f) other relevant epidemiological data. (Chin 2000)
Therefore surveillance is ‘information for action’. More detailed information on surveillance and field investigations is given in Chapter 6.4 and Chapter 6.16.
There are many tools for control related to interventions:
control measures applied to the host (e.g. active immunization, passive immunization, chemoprophylaxis, behavioural change, reverse isolation, barriers, and improving host resistance)
control measures applied to vectors (e.g. chemical, environmental and biological control)
control measures applied to infected humans (e.g. chemotherapy, isolation, quarantine, restriction of activities, and behavioural change)
control measures applied to animals (e.g. active immunization, restriction or reduction, and chemoprophylaxis and chemotherapy)
control measures applied to the environment (e.g. provision of safe water, proper disposal of faeces, food and milk sanitation, and design of facilities and equipment)
control measures applied to infectious agents (cleaning, cooling, pasteurization, disinfection, and sterilization).
Achieving maximum impact on control of a specific infectious disease may involve more than one of these interventions. For example, the control of hepatitis A infection can be achieved through interventions that may include active immunization, passive immunization, food preparation and handwashing behaviours, provision of safe water, food sanitation, and proper disposal of faeces.
The tools for control can also be considered according to the level at which they are applied: individual, institutional, or community-based. At the individual level, control measures, usually initiated by a clinician, are directed towards the specific infectious disease threats to the particular individual. Examples include chemoprophylaxis to prevent wound infection, pre-exposure prophylactic immunization against rabies for a veterinarian, and use of diphtheria antitoxin in a patient with diphtheria. At the institutional level, control measures are directed to a group of people who are in close contact with each other, such as persons in day-care centres, schools, military barracks, hospital wards, nursing homes, and correctional facilities. Control activities in institutional settings are usually initiated by the officials of the institution. Examples includ administering amantadine hydrochloride or rimantadine for chemoprophylaxis or chemotherapy of influenza A in a high-risk institutional population, quarantining institutionalized young children during a measles outbreak, and hepatitis B immunization of staff and clients of institutions for the developmentally disabled. At the community level, control measures, usually initiated by local, state, or national public health agencies, are directed to the community at large. Examples include childhood immunization programmes, provision of safe water, and recall of contaminated food products. It should be noted that some control measures, such as immunization, may take place at all levels while others, such as the provision of safe water to the community, are more specifically applied at a particular level.
The tools for the control of infectious diseases and their relationship to the chain of infection are the main focus of the remainder of this chapter.
Control measures applied to the host
Control measures applied to the host range from relatively easily administered immunization to behavioural changes that may be extremely difficult for an individual to accept. This section details the types of control measures applied to susceptible hosts and gives examples of their application in the control of selected infectious diseases.
One of the most satisfactory control measures applied to a host is one that renders the host immune from infectious disease by an infectious agent. Active immunization is a cornerstone of public health measures for the control of many infectious diseases and is considered one of the most cost-effective methods of individual, institutional, and community protection for many infectious diseases. The most powerful example of the potential impact of active immunization against an infectious disease is that of smallpox vaccination. Mobilization of political will on a worldwide basis, coupled with full application of the strategies of active surveillance and containment immunization against smallpox, ultimately resulted in the complete global eradication of the disease and cessation of transmission of the infectious agent, variola virus.
Active immunization is usually considered synonymous with the term vaccination, and is the process of administration of an antigen that can induce a specific immune response that protects susceptible hosts from an infectious disease. Some draw a distinction between the two terms. Narrowly defined, vaccination is the process of administration of an antigen and immunization is the development of a specific immune response. Administering an antigen without evoking an immune response is possible, since no vaccine is 100 per cent effective. Conversely, someone can become immunized even if an antigen is administered to someone else (the live attenuated oral polio vaccine viruses, for example, can be transmitted from the recipient to other close contacts).
Active immunization can be accomplished through different types of antigens, including the following.
Inactivated toxins Diphtheria toxoid is an example of a formaldehyde-inactivated preparation of diphtheria toxin that protects against clinically manifest disease, although the immunized person may still become infected with toxin-producing strains of Corynebacterium diphtheria. Tetanus toxoid and Clostridium perfringens toxoid (pig bel vaccine) are other examples of inactivated toxin preparations.
Inactivated complex antigens Whole cell pertussis vaccine is an example of a heat or chemically treated preparation of killed whole pertussis bacteria that protects against clinically manifest disease even if the immunized person may still become infected with Bordetella pertussis. Other examples of inactivated vaccines include inactivated polio vaccine and influenza vaccine.
Purified antigens Acellular pertussis vaccine is an example of a vaccine composed of isolated and purified immunogenic pertussis antigens. Other vaccines with purified components include polyvalent capsular polysaccharide pneumococcal, polysaccharide meningococcal, protein-polysaccharide conjugate Haemophilus influenzae type b, and plasma-derived hepatitis B vaccines.
Recombinant antigens Hepatitis B recombinant vaccine is an example of a vaccine composed of hepatitis B surface antigen subunits made through recombinant DNA technology.
Live attenuated vaccines Measles vaccine is an example of a vaccine containing live infectious agents that are of reduced virulence, but induce protective antibodies against measles viruses. Other live attenuated vaccines include oral polio, mumps, rubella, yellow fever, and bacille Calmette–Guérin (BCG) vaccines.
Protective antibody responses usually take 7 to 21 days to develop. Although most vaccines must be given before exposure to be effective, some vaccines may protect even if administered after exposure to an infectious agent. For example, measles vaccine may provide protection against measles disease if given within 72 hours of exposure.
Duration of protection varies from only months, such as with killed whole-cell cholera vaccine, to years, or even life with some live attenuated vaccines, such as measles vaccine. Some inactivated toxoids and vaccines, such as tetanus toxoid, may require a priming series of doses to be optimally effective and additional booster doses to maintain protective antibodies. Many new technologies are being explored that may increase the number and efficacy of vaccines available against infectious disease, including immune-stimulating complexes, live viral or bacterial vector vaccines, and timed-release vaccines.
It should be recognized that vaccines vary in their efficacy and no vaccine is 100 per cent effective. Vaccine efficacies vary with type of vaccine, storage and handling conditions, skill of administration, age of vaccination, and other host factors. Vaccines for routine use are safe. However, no vaccine is 100 per cent safe. Potential vaccinees, or their parents or guardians, should be screened for contraindications and be informed of potential side-effects.
Immunization schedules for the routine control of infectious diseases preventable by immunization vary between countries and are usually based on expert advice to governments and physicians. For example, in the United States recommended policies for immunization are provided by the Immunization Practices Advisory Committee and are published in the Morbidity and Mortality Weekly Report (ACIP 1994). In addition, the American Academy of Pediatrics periodically publishes comprehensive immunization recommendations in its Report of the Committee on Infectious Diseases (Committee on Infectious Diseases 2000). At the global level, the WHO publishes recommended immunization schedules (WHO 1995b) and recommendations on control of vaccine-preventable diseases are periodically updated by expert advisory groups and published in the WHO Weekly Epidemiological Record.
In outbreak settings, immunization schedules may be modified. For example, the age of immunization for measles may be lowered to 6 months during an outbreak. In such situations, persons receiving vaccine before the routinely recommended age of immunization should be immunized again at the recommended age since immunization at an earlier age may not have been optimally effective.
Immunization programmes include those for routine child, routine adult, travel, selected high-risk populations, and occupational settings. For example, tetanus toxoid is universally recommended, yellow fever vaccine is only recommended in geographical areas of epidemiological risk, typhoid fever vaccine is only recommended for individuals subject to unusual exposure to typhoid, including persons living in the same household as known carriers, and anthrax vaccine is only recommended for veterinarians and persons occupationally exposed to possibly contaminated industrial raw materials.
Besides protection of the individual, vaccination may also provide a degree of community protection. This phenomenon is known as herd immunity. Herd immunity is the relative protection of a population group achieved by reducing or breaking the chains of transmission of an infectious agent because most of the population is resistant to infection through immunization. Herd immunity is a complex phenomenon and varies according to the infectious agent, its routes of transmission, the degree to which immunization protects against infection versus only clinically manifest disease, and the distribution of immunity in the population. The mechanisms of herd immunity are several, including ‘direct protection of vaccinees against disease or transmissible infection and indirect protection of non-recipients by virtue of surreptitious vaccination, passive antibody, or just reduced sources of transmission and, hence, risks of infection in the community’ (Fine 1993).
A particularly difficult problem for vaccine-preventable infectious disease control programmes is complacency by the population that can result from the very success of the programmes. Low rates of vaccine-preventable infectious disease may mistakenly lead parents to consider that vaccination is no longer important for maintaining their children’s health and may result in political leaders reducing funding for immunization programmes. Low disease rates may also focus undue attention on the relatively rare side-effects of vaccination in relation to current rates of disease. Such side-effects should only be compared in relation to rates of disease that would occur without immunization programmes.
A comprehensive discussion of active immunization is given by Plotkin and Orenstein (1999).
Passive immunization is temporary immunity in a host due to the protection afforded by antibody produced in another host. Passive immunity may be acquired either naturally or artificially.
Naturally acquired passive immunity through transfer of maternal antibodies via the placenta is the way that newborn infants are provided with a temporary immunity against many infectious diseases for which the mother is immune. This immunity wanes over time and eventually leaves the infant susceptible to these diseases.
An important use of transplacental immunity as a control measure is in the prevention of tetanus neonatorum (neonatal tetanus) by immunization of women before or during pregnancy with tetanus toxoid. The disease typically occurs when the umbilical cord is cut with an unclean instrument contaminated with tetanus spores or when substances contaminated with tetanus spores are placed on the umbilical stump after delivery. Control by active immunization of the infant cannot be achieved in sufficient time since the average incubation period is only 6 days (with a range from 3 to 28 days). An adequately immunized mother, however, will usually effectively transfer maternal antibodies against tetanus to her newborn and prevent tetanus neonatorum.
Another example of naturally acquired passive immunity is the relative protection against measles disease in a young infant born to a mother who previously had the disease. Typically, such infants are immune for approximately 6 to 9 months or more after birth depending upon how much residual maternal antibodies are present at the time of pregnancy. Other diseases for which there is usually an effective transplacental immunity in infants, for variable amounts of time, include diphtheria, mumps, poliomyelitis, rubella, and varicella (chickenpox). It should be noted that if the mother is not immune, or if residual maternal antibodies have significantly waned, then the infant may be susceptible to disease.
Research is ongoing in other infectious diseases that may be preventable in the neonate or infant though immunization of the mother before or during pregnancy. Examples include Haemophilus influenzae type b, and group B streptococcal and meningococcal diseases (Insel et al. 1994). Many diseases, however, are not prevented by transplacental immunity.
Breast feeding is a form of naturally acquired passive antibody transfer to neonates and infants. Breast milk and colostrum contain secretory immunoglobin A antibodies that may play a protective role in the prevention of infections with such agents as respiratory syncytial virus, rotavirus, and Haemophilus influenzae type b.
Artificially acquired passive immunity through administration of an antibody-containing preparation, antiserum, or immune globulin, has a place in the control of certain infectious diseases. However, unlike active immunization that is appropriate for routine use in the general population, passive immunization is confined to special situations.
Examples of the use of artificially acquired passive immunity to control infectious disease include the following.
Rabies Natural immunity to rabies in humans does not exist and therefore susceptible individuals bitten by an animal known or suspected to be rabid should receive rabies immune globulin to neutralize the rabies virus in the wound. It should be noted that, besides passive immunization with rabies immune globulin, such individuals should also receive active immunization with rabies vaccine.
Hepatitis A In areas where sanitation is poor, hepatitis A infection commonly occurs at an early age and therefore most adults in developing countries are already immune. However, epidemics may occur in industrialized countries. Passive immunization with immune globulin may be given to all household and sexual contacts of patients with hepatitis A, other food handlers in an establishment where hepatitis A has occurred in a food handler, all individuals in an institution where a focal outbreak of hepatitis A has occurred, and persons from industrialized countries travelling to highly endemic areas. It should be noted that vaccines for active immunization for hepatitis A are now available.
Diphtheria Treatment of this disease is an example of the use of an antibody-containing product (diphtheria antitoxin) produced in an animal (only diphtheria antitoxin from horses is available) administered as part of the treatment regimen for secondary prevention of disease. In suspected cases of diphtheria, the antitoxin must be given as soon as possible because it is only effective in neutralizing diphtheria toxins not yet bound to cells.
Other important infectious diseases, including hepatitis B, measles, tetanus, and varicella Depending upon the circumstances of exposure, susceptibility of the host and status of the host’s general immune system there are circumstances under which hepatitis B immune globulin, tetanus immune globulin, varicella-zoster immune globulin, or immune globulin may be warranted.
Chemoprophylaxis is the prevention of infection or its progression to clinically manifest disease through the administration of chemical substances, including antibiotics. Chemoprophylaxis can also consist of the treatment of a disease to prevent complications of that disease (Solomon and Fraser 1998). Chemoprophylaxis may be specifically directed against a particular infectious agent or it may be non-specifically directed against many infectious agents. The use of antibiotics before surgical procedures is an example of non-specific chemoprophylaxis to prevent wound infections in the postoperative period. Examples of specific chemoprophylaxis are given below.
The use of chemoprophylaxis to prevent development of infection is illustrated by using chloroquine to prevent malarial parasitaemia caused by Plasmodium vivax, Plasmodium ovale, Plasmodium malariae, and chloroquine-sensitive strains of Plasmodium falciparum. For some chloroquine-resistant strains of Plasmodium falciparum, alternative regimes include the addition of pyrimethamine and sulfadoxine (Fansidar®), pyrimethamine and dapsone (Maloprim®), or mefloquine. To reduce the risk of a relapse from intrahepatic forms of Plasmodium vivax and Plasmodium ovale after chloroquine is stopped, primaquine may be given. Determination of a specific malaria chemoprophylactic regimen is complex. It must take into account the geographical area, the possibility of pregnancy, the weight of an individual (dose size for children is determined by body weight), and the risks of adverse reactions to the chemoprophylactic regimen.
Other examples of prevention of development of infection include the following:
the use of silver nitrate, erythromycin or tetracycline instilled into the eyes of a newborn to prevent gonococcal ophthalmia by transmission of Neisseria gonorrhoeae from an infected mother during birth
the use of tetracycline, sulfonamides (including sulfadiazine and trimethoprim-sulfamethoxazole), chloramphenicol, or streptomycin in close contacts of confirmed or suspected cases of plague pneumonia to prevent plague pneumonia by transmission of Yersinia pestis
the use of benzathine penicillin in those in sexual contact with confirmed cases of early syphilis to prevent syphilis by transmission of Treponema pallidum.
An example of the use of chemoprophylaxis to prevent the progression of an infection to active manifest disease is the use of isoniazid to prevent the progression of latent infection with Mycobacterium tuberculosis to clinical tuberculosis. Persons less than 35 years of age who are tuberculin-test positive should receive isoniazid to prevent clinical tuberculosis. The decision to use isoniazid, especially in individuals more than 35 years of age, must be determined based on such information as length of infection, closeness of association with a current case, status of the immune system, presence of acute liver disease, possibility of pregnancy, and risk of adverse reactions.
Other examples of prevention of progression of an infection to active manifest disease include the following:
use of co-trimoxazole or pentamidine to prevent subclinical latent infection with Pneumocystis carinii from progression to clinically manifest pneumocystis pneumonia in immunosuppressed persons such as HIV-infected individuals
use of mebendazole, albendazole, or pyrantel pamoate to prevent infection with Necator americanus, Ancylostoma duodenale, and Ancylostoma ceylanicum progressing to the clinically manifest anaemia of hookworm disease
use of pyrimethamine–sulfadiazine–folinic acid to prevent asymptomatic infants congenitally infected with Toxoplasma gondii from clinically manifest chorioretinitis and other sequelae of congenital toxoplasmosis.
In some situations, establishing screening programmes to detect and treat asymptomatic or unrecognized infections in defined populations is useful. An example is the screening for Chlamydia trachomatis in sexual partners of persons infected with Chlamydia trachomatis, women with mucopurulent cervicitis, sexually active women less than 20 years of age, and women 20 years of age or older who meet certain criteria. A more detailed background on screening as a public health function is given in Chapter 12.6.
An example of the use of chemoprophylaxis to treat an infectious disease to prevent complications of the disease is the use of penicillin (or erythromycin in penicillin-sensitive patients) to treat streptococcal sore throats caused by Streptococcus pyogenes group A to prevent acute rheumatic fever.
Other examples of prevention of complications of an infectious disease include the following:
tetracycline for adults, or penicillin for children, for treatment of Lyme disease caused by Borrelia burgdorferi in the erythema chronicum migrans stage to prevent or reduce the severity of late cardiac, arthritic, or neurological complications
benzathine penicillin for treatment of syphilis in its primary, secondary, or early latency period to prevent late manifestations of the disease such as cardiovascular syphilis
ketoconazole for treatment of blastomycosis caused by Blastomyces dermatitidis in its early stages to prevent progression of chronic pulmonary or disseminated blastomycosis that may lead to death.
Potential problems with the use of chemoprophylaxis may include compromise of the host’s own non-specific defence mechanisms, other replacement infectious agents causing disease by growing in the place of the infectious agent affected by the specific chemoprophylactic regimen, and emergence of resistant strains of the infectious agent. The development of antibiotic resistance can be reduced by using antibiotics only when needed, selecting the proper antibiotic (or, in some situations, the appropriate multidrug therapy) for the infectious agent, and ensuring compliance with the appropriate regimen for the duration of treatment.
Perhaps the most challenging tool for the control of infectious diseases, and sometimes one of the most powerful and cost-effective, is behaviour change in the host that reduces or eliminates risk of exposure to an agent. Everyone has developed habits of living (lifestyles) that are not easily changed. Some of these behaviours are protective against infectious diseases. Others render the individual at higher risk of infection.
Examples of higher risk of exposure to infectious agents through behaviour, and behaviour changes that can have an impact on the chain of transmission, include the following.
Many infectious agents are transmitted through the direct transmission route of sexual intercourse, including Chlamydia trachomatis causing chlamydial genital infections, Neisseria gonorrhoeae causing gonorrhoea, Treponema pallidum causing venereal syphilis, Calymmatobacterium granulomatis causing granuloma inguinale, Haemophilus ducreyi causing chancroid, herpes simplex virus causing herpes simplex, Trichomonas vaginalis causing trichomoniasis, human papillomaviruses causing condyloma acuminata, HIV causing AIDS.
Abstinence behaviour, i.e. refraining from sexual activity with other persons, eliminates the risk of transmission of these agents through sexual contact. The delaying of age of first sexual intercourse avoids the risk of transmission of these agents at an early age. Restricting sexual activity to having sex only between two uninfected persons who do not have sexual activity with any other persons virtually eliminates the risk of transmission of these agents through sexual intercourse. The exceptions are due to other routes of transmission of some of these agents (e.g. HIV acquired through intravenous drug use in one partner being transmitted through sexual intercourse to the other partner). Limiting the number of sexual partners, and limiting those sexual partners to persons who also have few sexual partners, reduces the risk of exposure. However, at the individual level, if one of these sexual partners has an infectious agent transmissible by sexual intercourse, the risk of transmission may still be high. Finally, condom use during sexual intercourse in high-risk situations will markedly reduce, but not eliminate, transmission. A more detailed background on sexually transmitted diseases is provided in Chapter 9.13.
Intravenous drug use behaviour
Injection of drugs using non-sterile needles and syringes previously used by other intravenous drug users may transmit infectious agents in blood through the vehicle-borne route of indirect transmission, including HIV causing AIDS, hepatitis B virus causing viral hepatitis B, and Plasmodium vivax, Plasmodium malariae, and Plasmodium ovale causing malaria.
Abstinence behaviour, i.e. refraining from intravenous drug use, eliminates the risk of transmission of such agents through contaminated needles and syringes. Using a sterile needle and sterile syringe for intravenous drug use will break the chain of transmission of these infectious agents through this route. Some community public health programmes, in addition to promoting drug abstinence and drug rehabilitation, conduct needle and syringe exchanges and education in methods of decontamination to help promote the use of sterile injection equipment among intravenous drug users.
Eating certain foods may result in exposure to infectious agents through the vehicle-borne route of indirect transmission. These behaviours include consuming raw molluscs by which an infectious agent like the hepatitis A virus can cause viral hepatitis A, eating raw eggs by which an infectious agent like a Salmonella serotype can cause salmonellosis, and consuming raw beef by which an infectious agent like Taenia saginata can cause beef tapeworm infection.
Although food and diet are strongly ingrained behaviours, modification of dietary patterns is possible. Cooking foods like beef, pork, and eggs can markedly reduce risk of transmission of infectious agents. In addition, reducing risks by elimination of infectious agents from the food may be possible (see the section on control methods applied to the environment). Handwashing before eating also reduces risk of transmission of many infectious agents, such as Shigella dysenteriae, Shigella flexneri, Shigella boydii, and Shigella sonnei, which are spread through direct or indirect routes of faecal–oral transmission.
In certain occupations many behaviours may result in exposure to infectious agents and should be targets for control programmes in occupational health and safety. Specific examples include the following:
dental workers improperly performing procedures with bare hands which may result in exposure to hepatitis B viruses from infected patients
health workers improperly handling used needles which may result in needlestick injuries leading to exposure to HIV from infected patients
hospital laboratory workers processing specimens containing infectious agents without proper glove or eyewear protection
veterinarians who do not properly handle animals which may result in exposure to Brucella abortus, Brucella melitensis, Brucella suis, or Brucella canis.
Occupational hazards related to non-infectious materials may predispose an individual to increased risk of infectious diseases. For example, working conditions and behaviours in industrial plants and mines that lead to silicosis due to long-term inhalation of free crystalline silica dust will greatly increase the risk of developing tuberculosis.
Working behaviours appropriate for the particular occupational setting may include wearing protective clothing, eyewear, and gloves, handwashing and changing clothes after work, meticulous adherence to needle disposal and equipment sterilization procedures, and using hooded laboratory benches when handling certain specimens.
Other behaviours may affect the transmission of infectious agents:
scheduling outdoor activities at periods of low vector activity, applying insect repellents, and sleeping under bednets reduces the indirect transmission of vector-borne agents of infectious diseases like malaria
searching oneself for attached ticks every 3 to 4 h when playing or working in tick-infested areas reduces the indirect transmission of vector-borne agents of infectious diseases like Rocky Mountain spotted fever
avoiding sharing utensils, cups, toothbrushes, or towels reduces the indirect transmission of vehicle-borne agents of infectious diseases like mononucleosis
wearing shoes reduces the direct transmission of infectious agents like those causing hookworm disease
frequently bathing and regular washing of clothes in hot soapy water controls body lice
breast feeding reduces diarrhoeal diseases in the infant, although it may transmit HIV from HIV-infected mothers
large family sizes and crowding may facilitate airborne transmission of infectious agents in droplet nuclei for infectious diseases like tuberculosis.
Some of these other behaviours, like crowding, are conditioned by circumstances such as poverty that are not directly amenable to programmes promoting behavioural change.
A more detailed background on behaviour and behavioural modification is provided in Chapter 2.3 and Chapter 7.3 respectively.
Certain rare circumstances exist where a means of avoiding transmission of an infectious disease to a highly susceptible host is to provide reverse, or protective, isolation. Such isolation attempts to protect infection-prone patients from potentially harmful infectious agents. Reverse isolation procedures range from provision of a private room with the use of masks, gloves, and gowns by all persons entering the room, to elaborate facilities with laminar airflow rooms and sterilization of all food. Protective isolation is usually conducted for a limited time until the normal immune system recovers, a regimen of passive immunization is begun, or a bone marrow transplant has been successful. Examples of persons who may need periods of reverse isolation include those who have such diseases as X-linked agammaglobulinaemia, DiGeorge’s syndrome, and severe combined immunodeficiency; or those who have received therapies, such as some forms of cancer chemotherapy, that have severely compromised the person’s immune system in combating many infectious diseases.
One tool of control that can be applied to the host is the use of barriers between the host and the infectious agent. The effectiveness of such barriers, however, may be dependent on the behaviour of the host to use them consistently. Examples of barriers include the following:
screens, bednets (including bednets impregnated with benzyl benzoate), long-sleeved shirts and trousers (with the cuffs tucked into boots as a mechanical barrier), and repellents (such as diethyl-m-toluamide) to prevent transmission of malaria through the bite of infected female Anopheles mosquitoes
condoms to prevent transmission of HIV and other sexually transmitted infectious agents through sexual intercourse
masks to prevent transmission of tuberculosis through airborne droplet nuclei from patients with sputum-positive pulmonary tuberculosis.
General improvement in host resistance
Improving host resistance through general improvement of the immune system is a non-specific approach, but may be important in certain settings. Kwashiorkor, marasmus, and other forms of malnutrition debilitate the host’s immune system and may make an individual more susceptible to some infectious diseases. Moreover, persons who are malnourished and succumb to an infectious disease are at higher risk of the disease being of greater severity and leading to other complications.
Malnutrition also encompasses micronutrient deficiencies. Vitamin A deficiency, for example, has been linked to higher rates of mortality associated with measles disease. Correcting vitamin A deficiency, through programmes of supplementation, fortification, and dietary modification in high-risk populations, can reduce mortality rates due to measles.
A complex interaction exists between infectious diseases, such as diarrhoeal diseases, and malnutrition. A downward spiral of infection may lead to malnutrition that, in turn, leads to more infections, and so on. If unchecked, especially in developing countries, this downward spiral can ultimately result in death.
The special situation of international travel combines many control measures applied to the host already mentioned. The increase in the numbers of travellers, the speed of travel, and the ability to reach areas previously infrequently visited have reduced the effectiveness of surveillance for infectious diseases at ports of arrival and increased infectious disease risks. Advice for prevention against infectious diseases must be both general and specific. General advice includes such issues as avoidance of eating and drinking potentially contaminated food or drink (including ice) and swimming or bathing in polluted water. Specific advice must be provided based on information about the area to be visited and may include such measures as active immunization against yellow fever, active or passive immunization against hepatitis A, chemoprophylaxis against malaria, repellents against potentially infected mosquitoes, and not walking barefoot in areas of risk for infection with hookworms Strongyloides stercoralis and Strongyloides fuelleborni. A more detailed background on international travel and health is provided in the annually updated WHO publication International Travel and Health: Vaccination Requirements and Health Advice (WHO 1999b).
Control measures applied to vectors
Vector-borne transmission is the only or main route of transmission for many infectious diseases. For example, there exist more than 100 arthropod-borne viruses that may produce clinically manifest diseases in humans. Control of vector-borne diseases include measures to change behaviour and create barriers to the susceptible host discussed above, to reduce or break the chain of transmission of the infectious agent from an infected host to the vector (which includes some of the same behaviour and barrier measures used to prevent infection in a susceptible host discussed above as well as some of the control measures applied to infected hosts discussed below), and to directly control the vector population itself. Chemical, environmental, and biological controls are the primary means of directly controlling the vector population.
Chemicals used in the control of vectors include minerals, natural plant products (botanicals), chlorinated hydrocarbons, organophosphates, carbamates, and fumigants. Chemical control measures include the following public health interventions.
Spraying chemical insecticides such as organochlorine insecticides (for example, dichlorodiphenyltrichlorothane or DDT, and dieldrin), organophosphorus insecticides (for example, malathion and fenitrothion), and carbonate insecticides (for example, propoxur and carbaryl) to prevent malaria through control of mosquitoes.
Spraying chemical biodegradable insecticides such as temephos (Abate®) to prevent onchocerciasis through control of Simulium fly vectors.
Using traps impregnated with decamethrin to prevent African trypanosomiasis (sleeping sickness) through reduction of the population of infective species of Glossina (tsetse fly) vectors.
Treating snail breeding places with chemical molluscicides to prevent schistosomiasis due to the free-swimming cercariae (larval forms) of Schistosoma mansoni, Schistosoma haematobium, and Schistosoma japonicum that develop in snails.
Treating step-wells and ponds with chemical insecticides such as temephos (Abate®) to prevent dracontiasis due to infected cyclops (a crustacean copepod).
Suppressing rat populations by poisoning, preceded or accompanied by measures to control fleas, as an additional measure to supplement environmental sanitation to control rodent populations to prevent human plague.
The use of spraying for control of mosquitoes has been complicated due to concerns of environmental contamination by chemicals such as DDT and dieldrin which have led to their being banned in many countries. In addition, the emergence of mosquito vectors resistant to the insecticides has diminished their effectiveness in many areas. New methods of application, such as ultra low-volume spraying of malathion, have reduced the amounts of insecticide used.
Environmental control of vectors includes the following public health interventions.
Eliminating breeding sites of mosquito larvae by filling and draining areas where there is stagnant water and removing objects around houses that may collect water.
Destroying the habitats of the tsetse fly vector.
Properly implementing landfill procedures, placing lids on rubbish bins, covering food for human consumption, screening privies, cleaning up spilled food, and appropriately storing food.
Placing cockroach and fly traps.
Constructing rat-proof houses.
Eliminating rodent habitats.
It is also important to note that certain development projects may have an impact on the environment that facilitates the growth of vector or intermediate host populations and results in increased infectious diseases. Construction of artificial waterways may serve as breeding sites for Simulium fly vectors that can transmit Onchocerca volvulus resulting in onchocerciasis. Irrigation schemes can foster the growth of snail intermediate hosts required for the transmission of Schistosoma species resulting in schistosomiasis. Carefully conducted environmental impact studies that include consideration of the impact of a construction project on the vector and intermediate host populations, and ways to modify the project to reduce such populations, are important environmental control measures.
Biological control of vectors includes the following public health interventions.
Introduction of predators and parasites: the introductions of Gambusia affinis, a small fish that feeds on mosquito larvae, and of Coelomomyces, a fungus, are examples of control measures that are effective against Aëdes mosquitoes.
Insect growth regulators: the use of such regulators may result in death or sterility of vectors by interfering with normal insect development. An example is the use of methoprene (Altosid®) to control flood-water mosquitoes.
Control measures applied to infected humans
Control measures may be applied to infected persons in the community, institution, or hospital setting.
The hospital setting is a unique situation which requires special efforts to prevent and control nosocomial infections. Infection control programmes for hospitals should ideally include the following elements:
an infection control committee responsible for overall co-ordination of infection control activities
one or more infection control practitioners responsible for nosocomial disease surveillance, analysis of data, and consultation with and training of hospital staff
a hospital epidemiologist to supervise the infection control practitioners, data collection and analysis, and carrying out of any necessary emergency infection control measures
an engineer to direct engineering and preventive maintenance operations, especially ventilation equipment
a sanitarian who helps to develop procedures for sanitation of water, ice, food, and proper disposal of liquid and solid wastes
effective guidelines for patient care practices
surveillance of patient care practices, patient infections, and environmental contamination by infectious agents
co-ordination with other departments (microbiology laboratory, central services, housekeeping, food service, and laundry)
thorough investigation of problems.
Examples of specific control measures that may be applied to infected humans at the individual, institutional, and community levels are detailed below.
Treatment of persons with infectious diseases or subclinical infections may be a control tool for some infectious diseases. Such treatment may or may not have an impact on disease progression in the patient. It should be noted that rapid case detection and prompt application of appropriate chemotherapeutic agents are needed to limit infectivity.
Some important examples of control by chemotherapy include the following.
Treatment of patients with sputum-positive pulmonary tuberculosis with appropriate multidrug therapy will usually result in sputum conversion rendering them non-infectious to others within a few weeks. Recommended treatment regimens include isoniazid combined with one or more of the following antibiotics: rifampin, streptomycin, ethambutol, and pyrazinamide. The WHO has recommended that adherence to a complete course of multidrug therapy be directly observed by another responsible person as part of the directly observed treatment, short-course (DOTS) global strategy for the control of tuberculosis.
Patients with leprosy treated with appropriate multidrug therapy are considered no longer infectious within 3 months of regular and continued treatment. Recommended treatment regimens for multibacillary leprosy include the following antibiotics: rifampin, dapsone, and clofazimine.
Treatment of patients with streptococcal sore throats with penicillin (or erythromycin for penicillin-sensitive patients) will usually no longer be infectious after 24 to 48 h.
Patients with pertussis treated with antibiotics such as erythromycin or trimethoprim-sulfamethoxazole, although they may not affect the patient’s symptoms, will usually result in the patient no longer being infectious after 5 to 7 days.
Of special note is the situation of treatment of persons who are carriers. A carrier is
a person or animal that harbours a specific infectious agent without discernible clinical disease and serves as a potential source of infection. The carrier state may exist in an individual with an infection that is inapparent throughout its course (commonly known as healthy or asymptomatic carrier), or during the incubation period, convalescence, and postconvalescence of an individual with a clinically recognizable disease (commonly known as incubatory carrier or convalescent carrier). Under either circumstance the carrier state may be of short or long duration (temporary or transient carrier, or chronic carrier) (Chin 2000).
A chronic carrier of diphtheria, for example, may shed the infectious agent Corynebacterium diphtheriae for 6 months or more, but appropriate antibiotic therapy will usually promptly stop the carrier state. Another example is that of untreated patients with typhoid fever due to Salmonella typhi. Between 2 and 5 per cent of such patients will become permanent carriers. Treatment with appropriate antibiotics may be effective in ending the carrier state.
Antibiotic treatment may not always eliminate a carrier state for some infectious agents. For example, the treatment of persons with salmonellosis with an antibiotic may not terminate the period of communicability and can even result in emergence of antibiotic-resistant strains. However, antibiotic therapy may be still warranted under certain circumstances.
In some situations, establishing screening programmes in defined target populations for identification of asymptomatic or unrecognized infections that could be transmitted to others may be appropriate. Such screening should include the necessary follow-up with appropriate chemotherapy and counselling. An example would be screening close contacts of diphtheria patients with nose and throat cultures for the presence of Corynebacterium diphtheriae. Identified carriers with positive cultures should be treated with appropriate antibiotic therapy. See Chapter 12.6 on screening as a public health function.
Isolation is the ‘separation, for the period of communicability, of infected persons or animals from others in such places and under such conditions as to prevent or limit the direct or indirect transmission of the infectious agent from those infected to those who are susceptible to infection or who may spread the agent to others’ (Chin 2000).
Category-specific isolation precautions in hospital settings have been summarized by Chin (2000) as quoted below. All categories have two common requirements: ‘(a) hands must be washed after contact with the patient or potentially contaminated articles and before taking care of another patient; and (b) articles contaminated with infectious material should be appropriately discarded or bagged and labelled before being sent for decontamination and reprocessing’. The specific categories are as follows.
Strict isolation ‘This category is designed to prevent transmission of highly contagious or virulent infections that may be spread by both air and contact. The specifications, in addition to those above, include a private room and the use of masks, gowns and gloves for all persons entering the room. Special ventilation requirements with the room at negative pressure to surrounding areas are desirable’ (Chin 2000). Examples of infectious diseases for which patients are recommended to be placed under strict isolation precautions include the acute febrile period of Argentine haemorrhagic fever and Bolivian haemorrhagic fever caused by Junin virus and Machupo virus respectively, pharyngeal diphtheria caused by Corynebacterium diphtheriae, and pneumonic plague caused by Yersinia pestis.
Contact isolation ‘For less highly transmissible or serious infections, for diseases or conditions which are spread primarily by close or direct contact. In addition to the basic requirements, a private room is indicated, but patients infected with the same pathogen may share a room. Masks are indicated for those who come close to the patient, gowns are indicated if soiling is likely, and gloves are indicated for touching infectious material’ (Chin 2000). Examples of infectious diseases for which patients are recommended to be placed under contact isolation precautions include cutaneous diphtheria caused by Corynebacterium diphtheriae, rubella, and disseminated herpes simplex caused by herpes simplex virus.
Respiratory isolation ‘To prevent transmission of infectious diseases over short distances through the air, a private room is indicated, but patients infected with the same organism may share a room. In addition to the basic requirements, masks are indicated for those who come in close contact with the patient; gowns and gloves are not indicated’ (Chin 2000). Examples of infectious diseases for which patients are recommended to be placed under respiratory isolation precautions include pertussis caused by Bordetella pertussis, mumps caused by mumps virus, and patients in hospital with measles caused by measles virus through the fourth day of rash. Although isolation of patients with measles not in hospital is not practical in the general population, schoolchildren should remain out of school through at least the fourth day of rash.
Tuberculosis isolation ‘For patients with pulmonary tuberculosis who have a positive sputum smear or a chest X-ray that strongly suggests active tuberculosis. Specifications include use of a private room with special ventilation and closed door. In addition to the basic requirements, respirator-type masks are used by those entering the room. Gowns are used to prevent gross contamination of clothing. Gloves are not indicated’ (Chin 2000).
Enteric precautions ‘For infections transmitted by direct or indirect contact with faeces. In addition to the basic requirements, specifications include use of a private room if patient hygiene is poor. Masks are not indicated; gowns should be used if soiling is likely and gloves are to be used for touching contaminated materials’ (Chin 2000). Examples of infectious diseases for which patients are recommended to be placed under enteric precautions include acute diarrhoea caused by strains of Escherichia coli that are enterotoxigenic, enteroinvasive, or enteropathogenic, and patients in hospital with acute poliomyelitis caused by poliovirus. It should be noted that such enteric precautions for acute poliomyelitis patients in the home setting are of limited value since the highest risk of transmission would have already occurred during the prodromal phase of illness.
Drainage/secretion precautions ‘To prevent infections transmitted by direct or indirect contact with purulent material or drainage from an infected body site. A private room and masking are not indicated; in addition to the basic requirements, gowns should be used if soiling is likely and gloves should be used for touching contaminated materials’ (Chin 2000). Examples of infectious diseases for which patients are recommended to be placed under drainage/secretion precautions include tularaemia with open lesions caused by Francisella tularensis, chlamydial genital infections caused by Chlamydia trachomatis, and brucellosis with draining lesions caused by Brucella abortus, Brucella melitensis, Brucella suis, or Brucella canis.
Chin (2000) also states that:
[The] CDC has recommended that universal precautions be used consistently for all patients (in-hospital settings as well as outpatient settings) regardless of their bloodborne infection status. This practice is based on the possibility that blood and certain body fluids (any body secretion that is obviously bloody, semen, vaginal secretions, tissue, cerebrospinal fluid, and synovial, pleural, peritoneal, pericardial, and amniotic fluids) of all patients who are considered infectious for HIV, HBV, and other bloodborne pathogens. Universal precautions are intended to prevent parenteral, mucous membrane, and non-intact skin exposures of healthcare workers to bloodborne pathogens. Protective barriers include gloves, gowns, masks and protective eyewear or face shields. A private room is indicated if patient hygiene is poor.
Quarantine of potentially infected persons
Quarantine is the ‘restriction of the activities of well persons or animals who have been exposed to a case of communicable disease during its period of communicability (that is, contacts) to prevent disease transmission during the incubation period if infection should occur’ (Chin 2000). Two categories of quarantine are as follows (Chin 2000).
Absolute or complete quarantine The limitation of freedom of movement of those exposed to a communicable disease for a period of time not longer than the longest usual incubation period of that disease, in such manner as to prevent effective contact with those not so exposed.
Modified quarantine A selective, partial limitation of freedom of movement of contacts, commonly on the basis of known or presumed differences in susceptibility and related to the danger of disease transmission. It may be designed to accommodate particular situations. Examples are exclusion of children from school, exemption of immune persons from provisions applicable to susceptible persons, or restriction of military populations to the post or to quarters. It includes: personal surveillance, the practice of close medical or other supervision of contacts to permit prompt recognition of infection or illness but without restricting their movements; and segregation, the separation of some part of a group of persons or domestic animals from the others for special consideration, control or observation; removal of susceptible children to homes of immune persons; or establishment of a sanitary boundary to protect uninfected from infected portions of a population.
Examples of diseases where quarantine may be considered include the following.
Pneumonic plague: persons who have been in the same household or in face-to-face contact with patients with pneumonic plague and who do not accept chemoprophylaxis should be placed under absolute quarantine with strict isolation, including careful surveillance, for 7 days.
Measles: although absolute quarantine is impractical, a modified quarantine is recommended in settings where young children are living in dormitories, wards, or institutions. When measles occurs in an institutional setting, strict segregation of infants is recommended.
Lassa fever: close personal surveillance of all close contacts is recommended. Such persons include those who live or are in close contact with lassa fever patients as well as laboratory personnel testing specimens from such patients.
Restriction of activities
Controlling infectious disease transmission by restriction of the activities of a person in the community who is potentially infectious to others may be appropriate in certain circumstances. Examples of this include the following.
Individuals with a diarrhoeal disease should be excluded from handling food and caring for patients in hospital, children, and elderly persons.
Known carriers of Salmonella typhi should be excluded from foodhandling and care of patients.
Persons with staphylococcal disease should avoid contact with debilitated persons and infants.
Persons with rubella should be excluded from school or work for seven days after the onset of rash and from contact with pregnant women.
Behaviour change in an infected person to protect others may be difficult to accomplish. However, this should be considered in preventing the transmission of infectious agents in the following situations.
Examples of infectious agents transmitted through sexual intercourse are discussed in the section above on control measures applied to the host and in more detail in Chapter 9.13. Individuals who suspect that they may have a sexually transmitted disease should be encouraged to have health-seeking behaviours. Persons with a sexually transmissible infectious agent should be treated and co-operate with health officials to trace their sexual contacts. Patients with diseases such as lymphogranuloma venereum and syphilis, for example, should refrain from sexual contact until all lesions are healed. HIV-infected individuals should be counselled to treat genital ulcer disease promptly since such disease may increase transmissibility of HIV. Also, HIV-infected persons should avoid sexual intercourse with HIV-negative individuals or, if having sexual intercourse with HIV-negative individuals, use methods to reduce the risk of transmission, including condoms and a spermicide. For a more detailed overview of HIV and AIDS see Chapter 9.14.
Intravenous drug use behaviour
Besides counselling to abstain from intravenous drug use and establishing drug rehabilitation programmes to help individuals who wish to abstain, promoting behaviour change in the use of injection equipment is important. Discouraging the sharing of injection equipment and education on methods for the decontamination of needles and syringes for intravenous drug use reduces risks of transmission of infectious agents through contaminated injection equipment.
Food preparation behaviour
Individuals who should be restricted from handling food (e.g. carriers of Salmonella typhi) should be counselled regarding their condition and potential to infect others if they handle food. Foodhandlers who have an infectious disease that is potentially transmissible through the vehicle-borne means of food should be discouraged from handling food for others. The importance of handwashing, especially after defecation and before handling food, should be stressed.
Other behaviours that may reduce risk of transmission of infectious agents to other persons include the following:
patients with infectious diseases directly transmitted by droplet spread or airborne transmitted by droplet nuclei (e.g. patients with sputum-positive tuberculosis) should cover their mouth and nose when coughing or sneezing
persons suffering from dracontiasis should avoid entering a source of drinking water if they have an active ulcer or blister
patients with the vector-borne disease of African trypanosomiasis (sleeping sickness) with trypanosomes in their blood should prevent tsetse flies from biting
individuals who are infected with HIV or who have sexual and other behaviours that have placed them at increased risk for HIV infection should not donate blood, plasma, tissues, cells, semen for artificial insemination, or organs for transplantation.
Control measures applied to animals
‘An infection or infectious disease transmissible under natural conditions from vertebrate animals to humans’ is a zoonosis (Chin 2000). A detailed approach to zoonoses is given in the comprehensive CRC Handbook of Zoonoses (Beran and Steele 1994). Many approaches are used in the control of zoonoses, including the following.
An example of an infectious disease in animals in which some control can be achieved through immunization in selected animal populations is rabies. The reservoir of the rabies virus is varied and includes dogs, foxes, wolves, skunks, raccoons, and bats. Preventive measures include efforts to vaccinate all dogs.
Other examples of immunization of animals under certain conditions include immunization of young goats and sheep using a live attenuated strain of Brucella melitensis and calves using a strain of B. abortus in areas of high endemicity for brucellosis, and immunization of animals at risk for acquiring infection with Bacillus anthracis that could be transmitted to humans causing anthrax.
Restriction or reduction
The example of rabies can also be used to illustrate the use of restriction or reduction of an animal population to help control an infectious disease. Chin (2000) recommends a programme to:
educate pet owners and the public that restrictions for dogs and cats are important (for example, that pets be leashed in congested areas when not confined on owner’s premises; that strange-acting or sick animals of any species, domestic or wild, may be dangerous and should not be picked up or handled; that it is necessary to report such animals and animals that have bitten a person or another animal to the police and/or the local health department; that confinement and observation of such animals is a preventive measure against rabies); and that wild animals should not be kept as pets… Euthanize immediately non-immunized dogs or cats bitten by known rabid animals; if detention is elected, hold the animal in an approved secure pound or kennel for at least 6 months under veterinary supervision, and immunize against rabies 30 days before release. If previously immunized, reimmunize and detain (leashing and confinement) for at least 45 days… Cooperative programmes with wildlife conservation authorities to reduce fox, skunk, raccoon, and other terrestrial wildlife hosts of sylvatic rabies may be used in circumscribed enzootic areas near campsites and areas of human habitation. If such focal depopulation is undertaken, it must be maintained to prevent repopulation from the periphery.
In epizootic situations:
in urban areas of the United States and other developed countries, strict enforcement of regulations requiring collection, detention and euthanasia of ownerless and stray dogs, and of non-immunized dogs found off owners’ premises, and control of the dog population by castration, spaying or drugs have been effective in breaking transmission cycles. (Chin 2000)
Other examples of restricting or reducing animal populations include the following:
rat-proofing dwellings and reduction of the rat population to prevent rat bites that may transmit the infectious agents Streptobacillus moniliformis and Spirillum minus causing the rat-bite fevers of streptobacillosis and spirillosis respectively
rat suppression by poisoning (after achieving flea control) in rodent populations with a high potential for epizootic plague
elimination of animals infected with Brucella abortus, Brucella melitensis, Brucella suis, and Brucella canis by segregation or slaughter to prevent brucellosis
slaughtering dairy cattle that test positive for infection with Mycobacterium bovis, the infectious agent of bovine tuberculosis.
Chemoprophylaxis and chemotherapy
Psittacosis is an example of a zoonosis controlled by chemoprophylaxis or chemotherapy in selected animal populations. The infectious agent, Chlamydia psittaci, can be directly transmitted to humans from infected birds when the dried droppings, secretions, or dust from the feathers of such infected birds are inhaled. Imported psittacine species of birds should be placed under quarantine and receive an appropriate antibiotic chemotherapeutic regimen such as chlortetracycline administered in their feed for 30 days.
Another example is chemoprophylaxis in selected dogs at high risk of infection with Echinococcus granulosus. This infectious agent can be transmitted to humans through hand to mouth transmission of the tapeworm eggs from dog faeces causing echinococcosis due to E. granulosus, or cystic hydatid disease. Such high-risk dogs should periodically receive antihelminth treatment with a chemotherapeutic agent such as praziquantel (Biltricide®).
Control measures applied to the environment
Control measures applied to the environment are designed to interrupt the routes of transmission by which an infectious agent may be spread through the environment. Just as the routes of transmission are varied, so too are the control methods that can be applied. Control measures that affect transmission that can be applied to the host, agents, vectors, infected humans, and other animals are reviewed elsewhere in this chapter. Environmental factors may also have a direct impact on the host, agent, or vector. For example, low humidity may predispose to certain infections due to a greater permeability of mucus membranes in the host; cold, dry climates inhibit development of the infective larvae agent of hookworm disease; and higher altitudes and colder climates limit the mosquito vector.
The recognition of the relationship between disease and filth led to a sanitary revolution in industrialized countries that markedly reduced infectious diseases even before the arrival of the antibiotic era. Improved methods for storing and preserving food, better housing, and smaller families with a resultant decrease in the risk of infections in infancy all contributed to reductions in infant and child mortality rates.
This section focuses on general environmental control measures not mentioned elsewhere. Some of these methods, such as provision of safe water, have the potential to prevent several different infectious diseases and significantly reduce rates of disease in the community.
Provision of safe water
It has been estimated that about 1.3 billion people in the developing world lack access to clean and plentiful water (World Bank 1993). Contaminated drinking water, sometimes the result of poorly designed or maintained systems of sewerage, may lead to the water-borne indirect transmission of such infectious agents as Giardia lamblia causing giardiasis, pathogenic serotypes of Salmonella causing salmonellosis, and Cryptosporidium species causing cryptosporidosis.
Purification of water can occur though natural methods or human intervention. Examples of natural methods that contribute to water purification include the processes of evaporation and condensation, filtration through the earth, plant growth, aeration, and reduction and oxidation of organic material by bacteria. Purification of water for public consumption is conventionally done through such processes as coagulation of colloids by aluminium salts or with other techniques, filtration through such materials as coal, sand, or diatomaceous earth, and disinfection with such chemicals as chlorine derivatives. In special situations, boiling and distillation can be used for purification (Solomon and Fraser 1998).
Proper disposal of faeces
It has been estimated that nearly 2 billion people in the developing world do not have an adequate system for proper disposal of faeces (World Bank 1993). Infectious agents in faeces that may result in infectious diseases include poliovirus causing poliomyelitis, Shigella dysenteriae, Shigella flexneri, Shigella boydii, and Shigella sonnei causing shigellosis, and Entamoeba histolytica causing amoebiasis.
Infectious agents in faeces may be transmitted by the direct transmission route (including the faecal–oral mode), the vehicle-borne route (including water as noted in the previous section), and the vector-borne route (including the simple mechanism of flies carrying infected faeces on their feet). Public health environmental control measures to interrupt these routes of transmission by ensuring the proper disposal of faeces include appropriate on-site disposal through such means as properly constructed sanitary privies in rural areas with no sewerage systems, on-site disposal of domestic wastewater (such as use of septic tanks or cesspools), and sewerage systems with treatment of wastewater. Wastewater treatment may include preliminary treatment, sedimentation, chemical coagulation and flocculation, biological treatment (such as activated sludge units and trickling filters), stabilization ponds, sludge management, and disinfection (usually with chlorine) of effluents discharged into drinking, bathing, or shellfish-growing waters. The importance of personal health-promoting behaviours of using toilets, keeping toilets clean, and handwashing after defecation are a part of control efforts aimed at the proper disposal of faeces.
Food-borne infectious diseases remain a problem in both industrialized and developing countries. In the United States alone, an estimated 76 million persons are affected each year resulting in some 5000 deaths annually. Significant food-borne outbreaks and sporadic cases continue to occur due to factors such as the following.
Contamination of meat, poultry, and eggs with infectious agents, including pathogenic serotypes of Salmonella, Yersinia pseudotuberculosis, Yersinia entercolitica, and Listeria monocytogenes.
Problems in food storage, handling, and preparation in commercial eating places and in homes.
Larger and more centralized production and processing facilities, coupled with increasingly extensive distribution networks, which may result in transmission of infectious agents to many persons if a commercial product becomes contaminated.
Industrialized countries have significantly reduced the transmission of some infectious agents through major public health programmes in food sanitation, including inspection of eating and drinking establishments, meat and poultry inspection, shellfish sanitation, and promotion of adequate cooking, canning techniques, and refrigeration (Solomon and Fraser 1998).
Examples of vehicle-borne indirect transmission of infectious agents through food that can be controlled though a comprehensive public health food sanitation programme include the following.
Pathogenic serotypes of Salmonella transmitted by ingesting food made from infected animals or contaminated by the infectious agent in faeces that may cause salmonellosis. Control is achieved through ‘(a) handwashing before, during, and after food preparation; (b) refrigerating prepared foods in small containers; (c) thoroughly cooking all foodstuffs derived from animal sources, particularly poultry, pork, egg products, and meat dishes; (d) avoiding recontamination within the kitchen after cooking is completed; and (e) maintaining a sanitary kitchen and protecting prepared foods against rodent and insect contanimation … Inspect for sanitation and adequately supervise abattoirs, food-processing plants, feed-blending mills, egg-grading stations, and butcher shops’ (Chin 2000).
Staphylococcus aureus causing staphylococcal food intoxication by ingesting food containing the staphylococcal enterotoxin. Control is achieved through means to ‘(a) educate food handlers in strict food hygiene, sanitation and cleanliness of kitchens, proper temperature control, handwashing, cleaning of fingernails; and to the danger of working with exposed skin, nose, and eye infections and uncovered wounds; (b) reduce food handling time (initial preparation to service) to an absolute minimum, with no more than 4 hours at ambient temperature. Keep perishable foods hot (> 60°C) or cold (below 10°C; best is less than 4°C) in shallow containers and covered, if they are to be stored for more than 2 hours; and (c) temporarily exclude people with boils, abscesses, and other purulent lesions of hands, face, or nose from food handling’ (Chin 2000).
Trichinella spiralis transmitted by ingesting raw or improperly cooked meat or meat products, mainly pork, containing infectious encysted larvae that may cause trichinosis. Control is achieved through means to ‘(a) educate the public on the need to cook all fresh pork and pork products and meat from wild animals at a temperature and for a time sufficient to allow all parts to reach at least 71°C, or until meat changes from pink to gray, which allows a sufficient margin of safety. This should be done unless it has been established that these meat products have been processed either by heating, curing, freezing, or irradiation adequate to kill trichinae; (b) grind pork in a separate grinder or clean the grinder thoroughly before and after processing other meats; (c) adopt regulations to encourage commercial irradiation processing of pork products. Testing carcasses for infection with a digestion technique is useful. Immunodiagnosis of pigs with an approved ELISA test is also useful; (c) adopt and enforce regulations that allow only certified trichinae-free pork to be used in raw pork products that have a cooked appearance or in products that traditionally are not heated sufficiently to kill trichinea during final preparation; and (d) adopt laws and regulations to require and enforce the cooking of garbage and offal before feeding to swine’ (Chin 2000).
Milk may be a vehicle for indirect transmission of such infectious agents as: Mycobacterium bovis causing tuberculosis, Corynebacterium diphtheriae causing diphtheria, Listeria monocytogenes causing listeriosis, and Campylobacter jejuni and Campylobacter coli causing Campylobacter enteritis.
Public health control measures to break the chain of transmission include the following:
mechanization and sanitization of milking processes
refrigeration of milk
pasteurization of milk by high-temperature short-time, batch, ultra-pasteurization, or ultra-high-temperature methods
monitoring milk quality by testing for bacteria using a standard bacterial plate count, by testing for density of coliform organisms, and by use of the phosphatase test to assay for pasteurization
periodic testing of cows for tuberculosis and brucellosis.
The use of raw milk for human consumption and postpasteurization contamination may result in outbreaks of milk-borne diseases.
Design of facilities and equipment
The design and proper maintenance of buildings, rooms, and equipment can help break the chain of transmission of infectious agents. Laminar airflow hoods in laboratory workbenches, ventilation systems in hospitals, and disposable intravenous equipment are examples of systems designed to reduce risk of transmission. Routine maintenance needed to retain the original design standards for control of transmission of infectious agents include replacement of air filters, cleaning of cooling towers, monitoring of positive pressure rooms and airlocks, and replacement of indwelling peripheral venous catheters.
Examples of infectious agents whose transmission can be reduced through proper design and maintenance include the following.
Legionella species, the infectious agents responsible for legionellosis, are usually transmitted through airborne transmission via aerosol production. Transmission of the agent from cooling towers can be reduced by periodically cleaning off any scale or sediment, routinely using biocides to kill slime-forming organisms, and draining such towers when not in use.
Staphylococcus aureus, the infectious agent responsible for staphylococcal disease in medical and surgical wards, can be controlled by enforcing strict aseptic technique, including procedures to change intravenous infusion sites at least every 48 hours and replace indwelling peripheral venous catheters every 72 hours.
Bacillus anthracis, the infectious agent responsible for anthrax, can be transmitted, among other ways, through inhalation of anthrax spores. Proper design of industrial plants that handle raw animal fibres include providing facilities for adequate ventilation and control of dust, washing and changing clothes after work, and eating away from the places of work.
In addition to the environmental methods of control of transmission of infectious agents already mentioned, the following methods, some specific and some general, should also be noted.
Improvement of housing conditions to reduce crowding (as measured by the number of persons per room and not total population density) is a general measure that can reduce the transmission of infectious agents, especially direct transmission from direct contact or direct projection (droplet spray).
Improvement in working conditions can affect the risk of infectious disease. For example, control of particulate matter by proper ventilation in occupations such as textile mill workers, metal grinders, pottery factory workers, etc. can reduce inflammation of the lungs and thus decrease the risk of developing tuberculosis. Excessive physical exertion and the stress of exhausting work can also increase the risk of tuberculosis.
Improved irrigation and agricultural practices and removing vegetation or draining and filling of snail-breeding sites can reduce or eliminate the freshwater snail hosts of such infectious agents as Schistosoma mansoni, Schistosoma haematobium, and Schistosoma japonicum that cause schistosomiasis in humans.
Adequate screening of blood, serum, plasma, tissues, or organs can break the chain of vehicle-borne transmission from such biological products. Examples include screening for hepatitis B surface antigen and HIV antibodies in donated blood to prevent transmission of hepatitis B and HIV respectively.
Installation of screened living and sleeping quarters and the use of bednets, including bednets impregnated with a synthetic pyrethroid such as permethrin, can reduce exposure to mosquitoes infected with the infectious agents of malaria.
Control measures applied to the agent
Control of some infectious diseases can be achieved through means that remove the infectious agents from the environment or inactivate the agents. Physical measures (such as heat, cold, ultraviolet light, and ionizing radiation) and chemical measures (such as liquid disinfectants and antiseptics, gases, and chlorination) can be used. Examples of control measures applied to infectious agents include the following.
Cleaning is the removal of infectious agents from surfaces through such physical actions as vacuum cleaning or washing and scrubbing using soap or detergent and hot water. Cleaning also helps remove organic materials that might support the growth or survival of infectious agents.
Cooling may inhibit bacterial multiplication, and some infectious agents, such as Trichinella cysts and Taenia solium larvae (cysticerci), can be killed by freezing temperatures.
Pasteurization is the heating to a temperature of 75°C for 30 min to kill pathogenic vegetative bacteria. It does not inactivate bacterial spores. Pasteurization is a commonly used process to help ensure safety of milk and to prolong its storage quality.
Disinfection is the reduction or killing of vegetative harmful bacterial infectious agents outside the body or on objects. Disinfection may not inactivate all bacterial spores and viruses. Disinfectants are used to eliminate pathogenic bacteria from the skin surface and from contaminated inanimate surfaces and include alcohols, halogens such as iodine and chlorine, surface-active compounds such as the quaternary ammonium compound benzalkonium chloride, phenolics, alkylating agents such as glutaraldehyde and formaldehyde. Antiseptics are a class of disinfectant that can be applied on body surfaces; they have a lower toxicity than environmental disinfectants and are usually less effective in killing micro-organisms.
Sterilization is the complete removal or killing of all infectious agents in or on an object. Sterilization of equipment for surgery and wound dressings, during the parenteral administration of drugs, vaccines, or nutrients, during catheterization, and during dental work are all important means of controlling infectious diseases by killing infectious agents. Sterilization can be accomplished using fire, steam (such as in an autoclave), heated air, certain gases (such as ethylene oxide), ultraviolet light, ionizing radiation, liquid chemicals, and filtration. The method of sterilization chosen depends on the type of equipment to be sterilized.
The use of sterilized disposable equipment, such as disposable needles, syringes, and catheters, has the potential to reduce the risk of transmission of infectious agents. However, it must be ensured that such equipment is disposed of properly and is not reused. For example, disposable syringes cannot be properly resterilized because the plastic from which they are made cannot withstand the heat necessary for sterilization. Technologies such as the single-use disposable needle and syringe developed for immunization programmes help to ensure that such disposable equipment is not reused.
Control and prevention programmes
The preceding sections have considered the issues and given examples of control measures for infectious diseases at individual, institutional, and community levels and of the tools for control directed at the host, routes of transmission, and the agent. Control and prevention programmes using these tools must be developed according to a number of factors including the risk of disease, the magnitude of disease burden (as measured by mortality, degree of disability, morbidity, and economic costs), the feasibility of control strategies, the cost of control measures, the effectiveness of such measures (on current levels of disease and impact on future cases or outbreaks), the adverse effects or complications of the control measures, and the availability of resources. Public heath planning for the control of infectious diseases must consider these issues to design optimal rationally based control and prevention programmes.
The tools of disease surveillance for recognition and evaluation of the patterns of disease can provide the information on the risk and magnitude of disease burden to individuals, persons in institutions, subgroups of populations, and the community at large. Establishment and maintenance of the infrastructure for surveillance, including a system for the reporting of notifiable infectious diseases and unusual events, must be a high priority.
Feasibility of possible control and prevention strategies must be assessed through operational research, pilot projects, or from field experience. The fact that a particular measure can help control a disease does not mean it can be applied on a sufficient scale to have the desired impact. The cost of control activities (in both human and material resources) can be assessed through costing studies that can also provide the data needed to conduct more rigorous cost–benefit and cost-effectiveness analyses. A costly measure, even if it provides a high degree of control for an infectious disease, may not be affordable to the society or reasonable to apply in the light of other less expensive alternative strategies. Effectiveness of control measures may be assessed through epidemiological studies to find out their impact on reduction in the incidence or prevalence of disease.
The availability of resources for prevention and control programmes forces public health planners to set priorities by taking into account all these factors and then designing programmes that have maximum impact within available resources. Planners have a responsibility to mobilize additional necessary resources by raising public awareness and generating political will. Effective communication of disease burden and the results achievable through well-managed and effective control programmes can be a powerful tool for advocacy. Ideally, communities should actively participate in the planning, execution, and evaluation of public health programmes.
Prevention effectiveness is ‘the measure of the impact on health (including effectiveness, safety, and cost) of prevention policies, programmes, and practices. The assessment of prevention effectiveness is the ongoing process of applying evaluation tools to prevention practices’ (CDC 1995). Recognizing that systems for assessing the effectiveness of prevention strategies (including prevention strategies for infectious diseases) are weak or non-existent in both developing and industrialized countries alike, the CDC has suggested the following objectives for prevention effectiveness activities: ‘evaluate the impact of prevention, use results of evaluation research to establish programme priorities, and establish or apply standardized methods to compare the benefits and effectiveness of alternative prevention strategies’ (CDC 1995).
The current situation of international migration of many people worldwide presents an additional complexity to the design of programmes for the control of infectious diseases. Pertinent issues include refugee camps, legal status of migrants in recipient countries, and temporary return migration. Public health officials must consider the most effective mix of combined control measures applied to the host, agent, and routes of transmission when designing suitable control and prevention programmes (Gellert 1993).
International commerce and transportation are specific areas of concern for public health infectious disease control programmes, especially as the speed of travel has increased. The tools of control include measures such as spraying insecticides effective against mosquito vectors of malaria in aircraft before departure, in transit, or on arrival, and rat-proofing or periodic fumigation to control rats on ships, docks, and warehouses to prevent plague. Specific international control measures relating to aircraft, ships, and land transportation for infectious diseases have been specified in the International Health Regulations (WHO 1983).
The challenge facing infectious disease control programmes is to design an optimal set of interventions at local, institutional, and community levels supported and accepted by the political leadership and the persons to whom these measures are applied.
A unique endpoint in the control of infectious diseases is that of eradication. Eradication is the cessation of all transmission of infection by extermination of the infectious agent. To date, only one infectious disease, smallpox, has been eradicated. The WHO World Health Assembly in May 1980 confirmed its global eradication some 3 years after the last naturally acquired case of smallpox in October 1977 (Fenner et al. 1988). The magnitude of this accomplishment is appreciated when one realizes that in the early 1950s it was estimated 50 million cases of smallpox still occurred each year in the world, some 150 years after Edward Jenner performed the first vaccination and wrote: ‘it now becomes too manifest to admit of controversy, that the annihilation of the Small Pox, the most dreadful scourge of the human species, must be the final result of this practice’ (Fenner et al. 1988).
The goal of global eradication has been set by the World Health Assembly for two other infectious diseases, poliomyelitis caused by wild poliovirus and dracontiasis, the latter caused by the infectious agent Dracunculus medinensis. A high level of sustained political will, aggressively applied disease surveillance, and effective control measures are the required elements to achieve eradication of the infectious agents for these diseases.
Impressive progress has been made towards the global eradication of poliomyelitis since the 1988 World Health Assembly set the goal for its eradication by the year 2000. The entire region of the Americas has succeeded in interrupting transmission of indigenous wild poliovirus since August 1991. The Western Pacific region has succeeded in interrupting transmission since 1997. In other regions of the world, countries endemic for poliomyelitis are carrying out the necessary strategies to eradicate the poliovirus. Poliomyelitis control measures that will lead to eradication include the following.
Achieving and maintaining high levels of routine coverage of infants with at least three doses of oral polio vaccine.
Mass application of oral polio vaccine in countries where poliomyelitis is endemic through national immunization days, usually by providing oral polio vaccine to every child less than 5 years of age twice each year, separated by 4 to 6 weeks, and conducted during the low season of poliovirus transmission.
‘Mopping-up’ operations after the use of national immunization days has reduced transmission of disease to defined focal geographical areas, usually by providing oral polio vaccine house-to-house to all children less than 5 years of age on two occasions separated by 4 to 6 weeks.
Aggressive action-oriented surveillance for acute flaccid paralysis. Such surveillance includes case investigation, a laboratory network for isolation and characterization of polioviruses in suspect cases of poliomyelitis and people in close contact with them, and limited outbreak response immunization providing one house-to-house round of oral polio vaccine to children less than 5 years of age living in the same village or neighbourhood as the patient.
Significant strides in the eradication of dracontiasis have also been made. Over the last decade the total number of dracontiasis cases have declined by more than 95 per cent. The disease is now limited to only certain parts of some African countries in a band between the Sahara desert and the equator. India was certified as dracontiasis free in February 2000. Dracontiasis control measures that are leading to ultimate eradication include the following.
Establishing a national programme office, conducting baseline surveys, and preparing and refining a national plan of action.
Educating the population in endemic areas that the source of guinea-worm comes from their drinking water.
Ensuring that persons with blisters or emerging worms do not enter sources of drinking water through behaviour changes and by converting step-wells into draw-wells.
Promoting the boiling or filtering of water through a fine mesh cloth to remove copepods. Treating drinking water with chlorine or iodine will also kill the copepods and infective larvae.
Providing non-infected water through construction of wells or rainwater catchments.
In selected endemic villages, controlling copepod populations with temephos (Abate®) insecticide placed in reservoirs, tanks, ponds, and step-wells.
Implementing an intensified surveillance and aggressive case-containment strategy as programmes get close to achieving eradication.
The eradication of a disease requires a unique set of conditions, including the following: the infectious agent must have a defined accessible reservoir; affordable and effective control measures that can interrupt the chain of infection directed at the host, agent, or route of transmission; a surveillance mechanism adequate to monitor and ultimately certify the disappearance of the infectious agent.
It is likely that measles may be targeted for global eradication in the future. Some countries and geographical regions have already targeted measles for elimination—a term sometimes used to describe the eradication of a disease from a large geographical area. Other diseases that may potentially be targeted for eradication in the future include mumps, rubella, hepatitis B, leprosy, and diphtheria.
Emerging infectious diseases
New, emerging, and re-emerging infectious diseases have become a focus for the attention of public health prevention and control programmes in both industrialized and developing countries. Such infectious diseases have thwarted any expectation that infectious diseases will soon be eliminated as public health problems and resulted in a widening spectrum of diseases, many of which were once thought to be almost conquered. Krause has reflected on this as follows:
Microbes and vectors swim in the evolutionary stream, and they swim faster than we do. Bacteria reproduce every 30 minutes. For them, a millennium is compressed into a fortnight. They are fleet of foot, and the pace of our research must keep up with them, or they will overtake us. Microbes were here on earth 2 billion years before humans arrived, learning every trick for survival, and it is likely that they will be here 2 billion years after we depart. (Krause 1998)
Many factors contribute to the emergence of new diseases or the re-emergence of those previously known (Lederberg et al. 1992; CDC 1994; Murphy 1994), including the following.
Human demographic change by which persons begin to live in previously uninhabited remote areas of the world and are exposed to new environmental sources of infectious agents, insects, and animals. A more detailed overview of population dynamics as a determinant of health and disease is provided in Chapter 7.2.
Breakdowns of sanitary and other public health measures in overcrowded cities and in situations of civil unrest and war.
Economic development and changes in the use of land, including deforestation, reforestation, and urbanization.
Other human behaviours, such as increased use of child-care facilities, sexual and drug use behaviours, and patterns of outdoor recreation.
International travel and commerce that quickly transport people and goods vast distances.
Changes in food processing and handling, including foods prepared from many different individual animals and transported great distances.
Evolution of pathogenic infectious agents by which they may infect new hosts, produce toxins, or adapt by responding to changes in the host immunity.
Development of resistance of infectious agents such as Mycobacterium tuberculosis and Neisseria gonorrhoeae to chemoprophylactic or chemotherapeutic medicines.
Resistance of the vectors of vector-borne infectious diseases to pesticides.
Immunosuppression of persons due to medical treatments or new diseases that result in infectious diseases caused by agents not usually pathogenic in healthy hosts.
Deterioration in surveillance systems for infectious diseases, including laboratory support, to detect new or emerging disease problems at an early stage.
Examples of emerging infectious disease threats include the following.
Toxic shock syndrome, due to the infectious toxin-producing strains of Staphylococcus aureus, illustrates how a new technology yielding a new product, superabsorbent tampons, can create the circumstances favouring the emergence of a new infectious disease threat.
Lyme disease, due to the infectious spirochete Borrelia burgdorferi, illustrates how changes in the ecology, including reforestation, increasing deer populations, and suburban migration of the population, can result in the emergence of a new microbial threat that has now become one of the most prevalent vector-borne diseases in the United States.
Shigellosis, giardiasis, and hepatitis A are examples of emerging diseases that have become threats to staff and children in child-care centres as the use of such centres has increased due to changes in the work patterns of societies.
Opportunistic infections, such as pneumocystis pneumonia caused by Pneumocystis carinii, chronic cryptosporidiosis caused by Cryptosporidium species, and disseminated cytomegalovirus infections, illustrate emerging disease threats to the increasing number of persons who are immunosuppressed because of cancer chemotherapy, organ transplantation, or HIV infection.
Foodborne infections such as diarrhoea caused by the enterohaemorrhagic strain 0157:H7 of Escherichia coli and waterborne infections such as gastrointestinal disease due to Cryptosporidium species are examples of emerging disease threats that have arisen due to such factors as changes in diet, food processing, globalization of the food supply, and contamination of municipal water supplies.
Hantavirus pulmonary syndrome, first detected in the United States in 1993 and caused by a previously unrecognized hantavirus, illustrates how exposure to certain kinds of infected rodents can result in an emerging infectious disease.
Nipah virus disease, first detected in Malaysia in 1999 and caused by a previously unrecognized paramyxovirus, demonstrates how close contact with pigs can result in an emerging infectious disease.
Emergence of the new toxigenic Vibrio cholerae 0139 strain of cholera in Asia is an example of a new strain of an infectious agent for which there is no protection from prior infection with other strains or with current vaccines.
Antimicrobial drug resistance as a major factor in the emergence and re-emergence of infectious diseases deserves special attention. Although significant reductions in infectious disease mortality have occurred since the introduction of antimicrobials for general use in the 1940s, antimicrobial drug resistance has emerged because of their widespread use in humans.
Drugs that once seemed invincible are losing their effectiveness for a wide range of community-acquired infections, including tuberculosis, gonorrhoea, pneumococcal infections (a leading cause of otitis media, pneumonia, and meningitis), and for hospital-acquired enterococcal and staphylococcal infections. Resistance to antiviral (for example, amantadine-resistant influenza virus and acyclovir-resistant herpes simplex), antifungal (for example, azole-resistant Candida species), and antiprotozoal (for example, metronidazole-resistant Trichomonas vaginalis) drugs is also emerging. Drug-resistant malaria has spread to nearly all areas of the world where malaria occurs. Concern has also arisen over strains of HIV resistant to antiviral drugs. Increased microbial resistance has resulted in prolonged hospital admissions and higher death rates from infections; has required much more expensive, and often more toxic, drugs or drug combinations (even for common infections); and has resulted in higher health care costs. (CDC 1994).
Antimicrobial drug resistance has also emerged because of the use of antimicrobials in domesticated animals. For example, the use of fluoroquinolones in poultry has created a reservoir of quinolone-resistant Campylobacter jejuni that has now been isolated in humans.
An aggressive public health response to these new, emerging, and re-emerging infectious disease threats must be made to characterize them better and to mount an effective response for their control. For example, the outbreak of West Nile fever in New York City and surrounding areas in 1999 demonstrates how a viral encephalitis, initially classified as St Louis encephalitis and later confirmed to be due to West Nile-like virus, can reach far beyond its normal setting.
The WHO has outlined the following high-priority areas (WHO 1995a):
strengthen global surveillance of infectious diseases
establish national and international infrastructures to recognize, report, and respond to new disease threats
further develop applied research on diagnosis, epidemiology, and control of emerging infectious diseases
strengthen the international capacity for infectious disease prevention and control.
Another unfortunate source of a new or emerging disease threat is the spectre of biological warfare or bioterrorism, especially in an age where terrorist acts are frequent events (Christopher et al. 1997). Several countries are developing rapid-response capability to deal with such contingencies.
Only through worldwide concerted action will the effort to control infectious disease be effective. We have now entered an era where, as Nobel Laureate Dr Joshua Lederberg has stated, ‘The microbe that felled one child in a distant continent yesterday can reach yours today and seed a global pandemic tomorrow’ (quoted in CDC 1994). As Hans Zinsser stated over 60 years ago:
Infectious disease is one of the few genuine adventures left in the world. The dragons are all dead and the lance grows rusty in the chimney corner … About the only sporting proposition that remains unimpaired by the relentless domestication of a once free-living human species is the war against those ferocious little fellow creatures, which lurk in the dark corners and stalk us in the bodies of rats, mice and all kinds of domestic animals; which fly and crawl with the insects, and waylay us in our food and drink and even in our love. (Hans Zinsser 1934, quoted in Murphy 1994)
*The author alone is responsible for the views expressed in this publication.
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