NEW AND INCREASINGLY RECOGNIZED PATHOGENS
During the last 5 years, scientists have discovered a number of new infectious organisms (including hepatitis G virus, Chlamydia-like microorganism Z, Legionella-like amebal pathogen, Cache Valley virus, and Babesia species) and have come to appreciate the expanded role of three additional pathogens [Bartonella (Rochalimaea) quintana, Ehrlichia species, and Escherichia coli 0157:H7].
Hepatitis G virus (an RNA virus of the genus Flaviviridae) is a blood-borne agent that frequently causes coinfection along with other hepatitis viruses. The majority of patients become chronic carriers, but there is no evidence that it causes chronic hepatitis, cirrhosis, or hepatocellular carcinoma. In addition, chronic infection with hepatitis G virus does not appear to affect the course, severity, or outcome of liver disease caused by hepatitis B or C virus. An unproven concern is that there may be an association between hepatitis G and aplastic anemia following acute hepatitis.
Microorganism Z is a Chlamydia-like bacterium that has been implicated in patients with community-acquired pneumonia. The illness attributed to microorganism Z is characterized by fever, a nonproductive cough, gastrointestinal symptoms, and a prompt response to erythromycin therapy. Identification of the infection requires isolation of the organism in tissue culture or serologic evidence of antibody.
In other patients with community-acquired pneumonia, infection appears to be caused by a Legionella-like amebal pathogen. The organism, a gram-negative bacterium known as Hall’s coccus, is a nonculturable bacterial pathogen of amebae. The manifestations of pneumonia are not unique. Evidence of the organism’s role in infection is established by serologic detection of a fourfold rise in antibody titer.
Cache Valley virus infection has been reported in a patient in North Carolina. This virus, a member of the family Bunyaviridae, caused encephalitis and multiple organ dysfunction. Presumably, the disease is transmitted through mosquito bites. The diagnosis requires viral isolation. There is no specific treatment, and therapy is supportive.
Babesia-like protozoal organisms that cause infections in humans, particularly patients who have undergone splenectomy, have been newly identified in the western United States (WA1) and in Missouri (MO1). These tick-transmitted disorders are characterized by fever, chills, headache, weakness, myalgia, nausea, and vomiting. The diagnosis is established by demonstration of the intraerythrocytic tetrad (Maltese cross) forms of merozoites and by detection with indirect immunofluorescence of antibody in acute-and convalescent-phase sera. Treatment consists of clindamycin with quinine, perhaps exchange transfusion of red cells, and, if indicated, hemodialysis.
Bartonella (Rochalimaea) quintana, a fastidious, slow-growing, gram-negative bacillus, shares with Bartonella (Rochalimaea) henselae the ability to cause bacillary angio-matosis, bacillary peliosis, relapsing fever, bacteremia, and endocarditis. B. quintana is transmitted by the body louse and appears to be an important cause of blood culture-negative endocarditis. Alcoholism and homelessness appear to be risk factors for the development of endocarditis. Attempts at isolation in blood cultures should include use of the lysis-centrifugation system and routine subculture on chocolate and freshly prepared blood agar in a carbon dioxide-enriched atmosphere. In addition to combination treatment with antibiotics (ampicillin or ceftriaxone with gentamicin), valve replacement is often required to cure the patient with Bartonella endocarditis.
Ehrlichiosis, a tick-borne disease that may be more common than Rocky Mountain spotted fever, is caused by two agents: Ehrlichia chaffeensis, the agent that causes human monocytic ehrlichiosis, and an Ehrlichia species (very nearly identical to Ehrlichia equi) that causes human granulocytic ehrlichiosis. Patients with ehrlichiosis experience fever, malaise, headache, myalgias, and on occasion rigors, nausea, vomiting, arthralgias, cough, confusion, and rash. Symptoms usually develop within 3 days to 1 week of a tick bite (between May and October). Ticks can transmit numerous pathogens (including Borrelia burgdorferi, Babesia microti, Rickettsia rickettsii, Francisella tularensis) in addition to Ehrlichia species, so that patients may experience coinfection or sequential infections; these can result in diagnostic confusion, uncharacteristic findings, and serologic cross-reactions, which contribute to diagnostic difficulty. Laboratory abnormalities can include thrombocytopenia, leukopenia, and elevated liver enzymes. Most patients will not demonstrate morulae (colonies of organisms within the cytoplasm of peripheral leukocytes), and treatment (consisting of 100 mg of doxycycline twice daily for 5 to 7 days) should be initiated when the disease is suspected, as fatalities have occurred from delayed therapy. The diagnosis is usually established by serologic testing (fourfold increase in indirect fluorescent antibody titer between acute and convalescent sera), although early antibiotic therapy can diminish the immune response, and there is a potential for serologic cross-reactions between Ehrlichia and other tick-borne pathogens. The polymerase chain reaction technique has been used for rapid detection of both human ehrlichiosis agents; however, this diagnostic test is not widely available.
Perhaps no pathogen has generated more media attention or occasioned more economic concerns recently than enterohemmorrhagic (0157:H7) E. coli. This organism produces both hemorrhagic colitis and hemolytic uremic syndrome. Initially identified as a contaminant of undercooked ground beef (ingested in fast-food restaurants), the organism has contaminated ham, roast beef, raw milk, yogurt, mayonnaise, apple cider, vegetables, salads, cantaloupe, and more recently alfalfa sprouts. At particular risk are elderly, bedridden persons.
Disease is caused by the toxin(s) elaborated by adherent E. coli, and it ranges from mild diarrhea to severe hemorrhagic colitis. The diarrhea can be accompanied by severe abdominal cramping, vomiting, and abdominal tenderness on examination. The most severe disease occurs in the proximal colon. Most patients do not have fever. Person-to-person transmission occurs, as in medical personnel caring for hospitalized patients.
The mainstay of diagnosis is culture of stool with confirmation by direct immunofluorescence to detect 0157 and H7 antigens. Treatment consists of fluid and electrolyte administration, monitoring for blood loss, and observation for hemolytic uremic syndrome. Antibiotic therapy has not decreased the length or severity of diarrhea, nor has it reduced progression to the hemolytic uremic syndrome. Antimotility agents are also not indicated.
The hemolytic uremic syndrome (acute renal failure, thrombocytopenia, microangiopathic hemolytic anemia) occurs approximately 1 week after the onset of diarrhea and is often associated with central nervous system complications. The syndrome occurs more commonly in persons who are elderly, bedridden, and/or in long-term care facilities. Therapy consists of dialysis and fluid and electrolyte management. There are no convincing data that treatment with a steroid, plasma infusion, IV immune globulin, or plasmapheresis is beneficial. (R.A.G.)
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A gram-negative bacterium (Hall’s coccus) related to Chlamydia appears to cause community-acquired pneumonia.
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Presumably, alfalfa seeds were contaminated with animal feces harboring E. coli 0157:H7.
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A massive recall of meat contaminated with E. coli 0157:H7.
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B. quintana is a cause of endocarditis in homeless patients.
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Legionella-like amebal pathogen as a potential cause of community-acquired pneumonia.
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An illness characterized by fever, cough (nonproductive), gastrointestinal symptoms, and a prompt response to erythromycin.
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Consider Bartonella in the evaluation of culture-negative endocarditis.
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B. quintana as a cause of culture-negative endocarditis.