Lymphoreticular and Hematopoetic Infections
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Q fever is a zoonotic bacterial infection that is worldwide in distribution.
Coxiella burnetii is an obligate intracellular gram-negative bacterium that dwells in the phagolysosome of phagocytes and causes Q fever. It has the ability to survive for long periods of time outside its hosts.
About 50% of patients with infections caused by C burnetii are asymptomatic. After an incubation period of 2–3 weeks, patients who develop symptoms have a rapid onset of high fever (up to 40oC), malaise, myalgia, confusion, sore throat, chills, sweats, nonproductive cough, nausea, vomiting, diarrhea, abdominal pain, and chest pain. If untreated, the fever will last 1–2 weeks. About 30–50% of symptomatic patients will develop pneumonia, if untreated. Liver function tests are usually abnormal; some patients may develop hepatitis. Most patients with acute Q fever recover in about 2 months without complications. The mortality rate for acute Q fever is 1–2%.
Patients who have had acute Q fever may develop a chronic form of this bacterial infection from 1 year to 20 years after the initial infection. A serious complication of chronic Q fever is endocarditis. The aortic valve is the most commonly affected valve, with the mitral valve being the next most commonly affected. Usually patients who develop chronic Q fever have pre-existing valvular heart disease or a history of vascular graft. Transplantation recipients and patients with cancer or chronic kidney disease are also at risk of developing chronic Q fever. The mortality rate is quite high (65%) for patients who have chronic Q fever.
C burnetii is transmitted to humans via aerosols. The organism can exist in an endospore-like form and live for long periods of time in the environment. Once in the lungs, the bacteria are carried in the bloodstream to the liver and bone marrow. The bacteria are then phagocytosed by macrophages and, in spite of phagosome-lysosome fusion, the bacteria survive. Growth in the macrophages can result in T-lymphocyte–mediated granulomas. Elimination of the infection requires release of interferon gamma and a T-lymphocyte response.
There are no unique signs or symptoms of Q fever that would aid in making a definitive diagnosis. Many patients with Q fever do have a transient thrombocytopenia, but that is not unique to the disease. The only method of determining a definitive diagnosis is by serology. Most laboratories use an indirect immunofluorescence assay; other assays are DNA testing and immunohistochemical staining.
C burnetii exists in two antigenic forms, phase I and phase II, which aids in determining the difference between an acute and chronic infection. Antibodies form early to phase II antigens and much later to phase I antigens. Therefore, patients with acute Q fever infections usually have high levels of antibody to phase II antigens and low levels of antibody to phase I antigens (IgG antibodies to phase II and IgM antibodies to phases I and II). Patients with chronic infections usually have high levels of antibody to phase I antigens and low or falling levels of antibody to phase II antigens. Increased levels of IgG and IgA to phase I antigens are predictive of Q fever endocarditis.
Therapy and Prevention
Doxycycline is the drug of choice for treating Q fever, although quinolones have also been shown to be effective. Treatment outcomes are most effective if therapy begins within the first 3 days of symptomatic disease. Relapses are not uncommon; if relapse occurs, the patient must begin treatment again.
Chronic Q fever is much more difficult to treat than acute Q fever. Endocarditis caused by C burnetii usually requires the use of two different antimicrobial agents for extended periods of time. Currently, two treatment regimens have been examined: The first is doxycycline given in combination with quinolones for at least 4 years. The second regimen, which results in fewer relapses, is doxycycline given in combination with hydroxychloroquine for 1½ to 3 years.
Prevention includes instruction on proper disposal of placenta, birth products, fetal membranes, and aborted fetuses at facilities housing sheep and goats. Only pasteurized milk and dairy products should be used. Imported animals should be quarantined.
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