HUMAN INFECTIONS FOLLOWING ANIMAL BITES
Animal bite injuries are common, accounting for about 1% of all emergency department visits and 10,000 hospital admissions. Dogs and cats are responsible for approximately 90% and 10%, respectively, of all animal bites, and these species are the focus of this chapter. However, the variety of animals causing bites is broad, and the spectrum of infections complicating these injuries is continually expanding; for example, “rat-bite fever,” which is caused by Spirillum minor or Streptobacillus moniliformis, can complicate a rodent bite; “seal finger,” an extremely painful condition that responds to tetracycline, can follow a seal bite; and the medical literature contains reports of Acinetobacter osteomyelitis following a hamster bite, Aeromonas soft-tissue infection resulting from a piranha bite, leptospirosis complicating a rat bite, tularemia transmitted by a squirrel bite, and Vibrio infection following a shark bite. Finally, bites can lead to substantial soft-tissue inflammation that can be confused with an infectious disease; the latter clinical scenario commonly follows the bite of the brown recluse spider (Loxosceles).
Although less common than dog bites, cat injuries are associated with a fivefold to 10-fold greater risk of infection. The increased infection rate following feline bites can be attributed, in part, to differences in the nature of the wound (puncture for cats vs. laceration for dogs) and to the higher rate of oropharyngeal carriage of Pasteurella multocida by cats (90% vs. 50% for dogs).
The majority of infections complicating animal bites involve the skin and subcutaneous tissues; however, animal bites can also lead to contiguous infections (osteo-myelitis, septic arthritis), bacteremia with metastatic disease (endocarditis), and unusual conditions, such as cat scratch disease. Further, because bite injuries can be associated with the presence of bone fractures or foreign bodies, a radiographic study should be included in the initial evaluation of selected patients, such as children with dog bites of the hand.
Thorough local wound care is essential in patients presenting with an infected or uninfected animal bite. All wounds should be washed with large volumes of normal saline solution; the purpose of the cleansing is to reduce the number of viable bacteria inoculated during the attack and to remove dirt and other foreign material. Most authorities recommend that puncture wounds be irrigated under high pressure; other experts believe that high-pressure irrigation increases the extent of tissue injury. Similarly, most authorities recommend that all wounds be cleansed with a povidone-iodine solution; others suggest that disinfectant solutions aggravate the soft-tissue insult. Debridement of wounds, including punctures, should be performed under local anesthesia; devitalized skin and subcutaneous tissue must be removed, and involved contiguous structures, such as tendons, joint capsules, and bone, should be explored.
The surgical management of animal bite wounds is rarely straightforward. In general, wounds that are infected at presentation must be left open, and when located on cosmetically unimportant areas, they should be allowed to heal by secondary intention. When present on cosmetically important areas, infected bites can be cleansed, debrided, and packed, and if the infection resolves within 5 to 7 days, they can be revised and closed. Wounds that are open, a few hours old, uninfected at presentation, and amenable to thorough cleansing may be candidates for closure. Some authorities recommend that the primary closure of hand wounds be avoided, regardless of the age of the wound. Cat bites in cosmetically unimportant areas should not be closed; however, because of the rich vascularity and cosmetic importance of the face, many experts recommend that uninfected, fresh facial wounds be sutured primarily. Because of a high risk for infection, the closing of older wounds is not recommended.
Early in the evaluation of the patient with an animal bite, the need for tetanus and rabies immunoprophylaxis should be determined. Concern about the possibility of rabies has increased substantially along the East Coast since the epizootic among raccoons has spread from the mid-Atlantic states to New England. Bats and carnivorous wild animals (raccoons, skunks, foxes) are most likely to be infected with the virus. Any person who experiences an open wound following the bite of a carnivorous mammal should be considered at risk for rabies, unless brain tissue from the animal is negative for the virus on immunofluorescent staining. These wounds must be cleaned thoroughly with soap and water, and previously unvaccinated patients should be given passive and active immunization as soon as possible with human rabies immune globulin (HRIG) and the human diploid cell vaccine (HDCV), respectively. The recommended dose of HRIG is 20 IU/kg of body weight; if feasible, half of the dose should be infiltrated into the tissues around the wound and the balance administered intramuscularly into the gluteal area. The standard dose of HDCV is 1 mL given intramuscularly in the deltoid area at presentation and on days 3, 7, 14, and 28. HRIG and HDCV must not be administered in the same syringe or into the same anatomic site.
The decision concerning the need for antirabies immunoprophylaxis in the patient with a dog or other domestic animal bite will be guided by a number of factors, including the presence or absence of rabies in the region where the injury occurred, circumstances of the incident (provoked vs. unprovoked), and condition of the animal at the time of the attack (unknown, rabid or suspected rabid, healthy and available for observation). Although the animal’s vaccination status should be determined, a history of rabies immunization does not exclude the possibility of the disease; indeed, deaths from rabies have occurred in travelers bitten outside the United States by dogs believed to have been immunized. The bites of squirrels, hamsters, chipmunks, rats, mice, rabbits, and hares almost never require antirabies prophylaxis. Additional details concerning postexposure rabies prophylaxis can be obtained from the Centers for Disease Control and state public health departments.
Microlobiologic data from studies of infected bites demonstrate that the lesions are usually polymicrobic, and that, on average, three bacteria can be isolated from each wound. The microbes most frequently isolated are Staphylococcus aureus, aerobic streptococci, P. multocida, and a variety of anaerobes, such as Peptococcus species, Bacteroides species, and Fusobacterium species. Although the antimicrobial therapy of infected wounds will be guided by the results of deep-tissue cultures, the initial regimen should have good activity against most of the pathogens listed, especially P. multocida. Most strains of Pasteurella are resistant to the semisynthetic penicillins (e.g., dicloxacillin), first-generation cephalosporins (e.g., cephalexin), clindamycin, and the aminoglycosides. In the outpatient setting, amoxicillin-clavulanate would be a rational choice, as it possesses good in vitro activity against all the pathogens usually associated with infected bite wounds. Penicillin-allegic patients with infected cat bites can be treated with doxycycline or ciprofloxacin, provided other contraindications are not present. Penicillin-allergic adults with infected dog bites can be given clindamycin plus a fluoroquinolone. In the inpatient setting, empiric therapy with ampicillin-sulbactam, ticarcillin-clavulanate, or penicillin plus oxacillin would provide the necessary coverage; the drug treatment can be modified as microbiologic information becomes available. In the penicillin-allergic patient, clindamycin plus doxycycline or a quinolone, which are agents with good in vitro activity against P. multocida, represent acceptable therapies. Of note, the potential toxicities of tetracyclines and quinolones limit their use in children. Erythromycin exhibits only moderate activity against Pasteurella, and life-threatening complications have evolved in patients with pasteurellosis treated with that agent; clarithromycin and azithromycin also have poor activity against the bacterium.
The role of prophylactic antimicrobials in patients with uninfected bite wounds remains controversial. Overall, the risk for infection in a wound that has been vigorously cleaned is low (about 10%); however, the risk appears to be substantially higher in hand lesions. In general, antimicrobials should be considered in patients who present within 12 to 24 hours and in circumstances in which the chances of infection are high or the consequences of infection would be great. Accordingly, prophylactic antimicrobials should be used in puncture wounds, especially cat bites; deep wounds of the hands; wounds involving joints or tendons; wounds in immunosuppressed persons; and perhaps facial wounds. Amoxicillin-clavulanate represents a rational choice; doxycycline alone or doxycycline plus clindamycin would be acceptable in the penicillin-allergic adult patient.
Capnocytophaga canimorsus (dysgonic fermenter-2, or DF-2) is a gram-negative bacillus that is carried in the oropharynx of about 25% of dogs and 15% of cats. In otherwise normal hosts, C. canimorsus rarely causes disease; however, the bacterium can cause a fulminant illness in splenectomized persons, alcoholics, and patients receiving corticosteroids. Usually following a dog bite, the C. canimorsus septicemia syndrome is characterized by fever, cellulitis, bacteremia, endocarditis, meningitis, disseminated intravascular coagulation, shock, renal failure, and peripheral gangrene. Penicillin represents the treatment of choice.
Patients with animal bites treated on an ambulatory basis should be seen within 24 to 48 hours in follow-up. Among patients with initially infected lesions, the response to antimicrobial treatment can be assessed, and the results of wound cultures can be reviewed; the evaluation will determine the need for additional surgical intervention or modifications in drug therapy. Among patients with initially uninfected wounds that are sutured, early follow-up is mandatory, as most infections following animal bites will become manifest within 24 to 48 hours of the injury. All sutures should be removed from infected lesions. As stated before, antimicrobial therapy will be guided by the results of wound cultures. (A.L.E.)
Avner JR, Baker MD. Dog bites in urban children. Pediatrics 1991;88:55.
Young children are at risk for bite injuries from pet dogs, especially German shepherds, pit bulls, rottweilers, and Doberman pinschers.
Brakenbury PH, Muwanga C. A comparative double-blind study of amoxicillin-clavulanate vs. placebo in the prevention of infection after animal bites. Arch Emerg Med 1989;6:251.
In this prospective study, the authors found that amoxicillin-clavulanate was superior to placebo in preventing infection in wounds 9 to 24 hours old; no significant difference was noted in wounds less than 9 hours old.
Butt TS, et al. Pasteurella multocida infectious arthritis with acute gout after a cat bite. J Rheumatol 1997;24:1649.
A geriatric patient who experienced a cat bite to the leg subsequently presented with septic and gouty arthritis.
Capellan J, Fong IW. Tularemia from a cat bite: case report and review of feline-associated tularemia. Clin Infect Dis 1993;16:472.
The authors describe a middle-aged man in whom ulceroglandular and pneumonic tularemia developed following a cat bite, and they review 14 previously reported cases of cat-associated tularemia.
Centers for Disease Control. Capnocytophaga canimorsus sepsis misdiagnosed as plague—New Mexico, 1992. MMWR Morb Mortal Wkly Rep 1993;42:72.
The article reports a patient with fatal C. canimorsus sepsis that was initially diagnosed as plague, and it summarizes some of the epidemiologic data derived from 200 human cases of disease caused by this pathogen.
Dire DJ. Cat bite wounds: risk factors for infection. Ann Emerg Med 1991;20:973.
In this prospective survey of 186 patients with 216 cat bites or scratches, the authors found that among the risk factors for infection were age over 50 years, a full-thickness lesion, and a long interval between injury and presentation. Among the subjects with bite injuries only and with uninfected wounds at presentation, infections were more likely to develop in patients with puncture or lower extremity wounds who did not receive prophylactic antimicrobials.
Feder HM Jr, Shanley JD, Barbera JA. Review of 59 patients hospitalized with animal bites. Pediatr Infect Dis J 1987;6:24.
In this review of pediatric and adult patients, the authors present the microbiology of infected wounds, noting that the Gram’s stain of drainage from these lesions is not accurate in predicting results of cultures.
Fishbein DB. Rabies. Infect Dis Clin North Am 1991;5:53.
A thorough review of animal and human rabies in the United States, with discussions on preexposure and postexposure interventions.
Francis DP, Holmes MA, Brandon G. Pasteurella multocida. Infections after domestic animal bites and scratches. JAMA 1975;233:42.
P. multocida produces a rapidly evolving cellulitis, with most patients experiencing symptoms within 24 hours following the bite injury.
Goldstein EJ. Bite wounds and infection. Clin Infect Dis 1992;14:633.
A thorough discussion of the microbiology and management of animal (and human) bite wounds.
Hantson P, et al. Fatal Capnocytophaga canimorsus septicemia in a previously healthy woman. Ann Emerg Med 1991;20:93.
Although most serious infections caused by C. canimorsus (DF-2) occur in immunosuppressed patients, this report illustrates the capacity of the bacterium to produce a fatal illness in ostensibly fit persons.
Hicklin H, Verghese A, Alvarez S. Dysgonic fermenter-2 septicemia. Rev Infect Dis 1987;9:884–890.
The clinical manifestations, course, and outcome of the dramatic infection caused by DF-2 (C. canimorsus) are reviewed.
Janda DH, et al. Nonhuman primate bites. J Orthop Res 1990;8:146.
Persons bitten by Macaca monkeys are at risk for a number of infections, including disease caused by Herpesvirus simiae (B virus), which can produce a fatal encephalomyelitis.
Levin JM, Talan DA. Erythromycin failure with subsequent Pasteurella multocida meningitis and septic arthritis in a cat-bite victim. Ann Emerg Med 1990;19:1458.
Erythromycin and the newer macrolides have inadequate activity against P. multocida.
McDonough JJ, Stern PJ, Alexander JW. Management of animal and human bites and resulting infections. Curr Clin Topics Infect Dis 1987;8:11.
This comprehensive review of the problem includes a discussion of the potential legal ramifications of treating animal bite wounds, such as the need to testify in a personal injury suit. To address that possibility, the authors emphasize the importance of meticulous documentation.
Rollof J, Nordin-Fredriksson G, Holst E. Pasteurella multocida occurs in high frequency in the saliva of pet dogs. Scand J Infect Dis 1989;21:583.
In this survey of 21 pet dogs, 17 (81%) were found to harbor P. multocida.
Shinall EA. Cat-scratch disease: a review of the literature. Pediatr Dermatol 1990;7:11.
A review of the clinical manifestations of the illness, with a discussion of the role of antimicrobials in normal and immunosuppressed patients.
Talan DA, et al. Bacteriologic analysis of infected dog and cat bites. N Engl J Med 1999;340:85.
In a study of the microbiology of the infected wounds of 57 patients with cat bites and 50 patients with dog bites, the authors found an average of 5 bacterial isolates per culture. Pasteurella species were the microbes most frequently recovered from these patients and were isolated from 75% of patients with cat bites and 50% of those with dog bites.
Valtonen M, et al. Capnocytophaga canimorsus septicemia: fifth report of a cat-associated infection and five other case. Eur J Clin Microbiol Infect Dis 1995;14:520.
Although usually associated with dog bites, the C. canimorsus sepsis syndrome can occur in susceptible patients following bites or the licking of skin ulcers.
Weber DJ, et al. Pasteurella multocida infections: report of 34 cases and review of the literature. Medicine (Baltimore) 1984;63:133.
The clinical spectrum of infections associated with this microbe is reviewed.
Wright JC. Reported cat bites in Dallas: characteristics of the cats, the victims and the attack events. Public Health Rep 1990;105:420.
In this survey, the author found that most cat bites are caused by stray female cats and that most victims are women.