Properties

 C. burnetii is a small obligate organism that has a membrane similar to Gram-negative bacteria. However, it does not stain with the Gram-stain but does stain with Gimenez. C. burnetii, which causes Q fever, is resistant to drying. This organism may survive pasteurization at 60°C for 30 minutes and can survive for months in dried feces or milk. This may be because of the formation of endospore-like structures by C. burnetii. Coxiellae grow only in cytoplasmic vacuoles.

Antigens and Antigenic Variation

When grown in cell culture, C. burnetii exhibits various phases. These phases are associated with differences in virulence. Phase I is the virulent form that is found in humans with Q fever and in infected vertebrate animals. It is the infectious form of the organism and the lipopolysaccharide expressed during this phase appears to be a key virulence fac tor. Phase II forms are not infectious and occur only by serial passage in cell cultures. Patients with clinical illness mount antibodies to both phase I and phase II antigens.

Epidemiology

 C. burnetii is found in ticks, which transmit the agent to sheep, goats, and cattle, but transmission by ticks to humans is uncommon. Workers in slaughterhouses and in plants that process wool and cattle hides have contracted the disease as a result of handling infected animal tissues. C. burnetii is transmitted by the respiratory pathway rather than through the skin. There may be a chronic infection of the udder of the cow or goat. In such cases, the rickettsiae are excreted in the milk and rarely may be transmitted to humans by ingestion of unpasteurized milk.

Infected sheep may excrete C. burnetii in the feces and urine and heavily contaminate their skin and woolen coat. The placentas of infected cows, sheep, goats, and cats contain the organism, and parturition creates infectious aerosols. The soil may be heavily contaminated from one of the above sources, and the inhalation of infected dust leads to infection of humans and livestock. It has been proposed that endospores formed by C. burnetii contribute to its persistence and dissemination. Coxiella infection is now widespread among sheep and cattle in the United States. Coxiella can cause endocarditis (with a rise in the titer of antibodies to C. burnetii, phase I) in addition to pneumonitis and hepatitis.

Clinical Findings

A. Q Fever

 This disease is recognized around the world and occurs mainly in persons associated with goats, sheep, dairy cattle, or parturient cats. It has attracted attention because of outbreaks in veterinary and medical centers where large numbers of people were exposed to animals shedding Coxiella species.

Infections may be acute or chronic. Acute disease resembles influenza, nonbacterial (atypical) pneumonia, and hepatitis. There is a rise in the titer of specific antibodies to C. burnetii, phase II. Transmission results from inhalation of dust contaminated with the organism from placenta, dried feces, urine, or milk or from aerosols in slaughterhouses.

Chronic Q fever is infection that lasts more than 6 months. Infective endocarditis is the most common form of disease in this phase. Blood cultures for bacteria are negative, and there is a high titer of antibodies to C. burnetii, phase I. Virtually all patients have preexisting valve abnormalities or have some form of immune compromise.

Laboratory Findings

C. burnetii can be cultivated in cell cultures, but this should only be done in experienced biosafety level 3 laboratories. Serology is the diagnostic method of choice, and indirect immunofluorescence is considered the best method. PCR has been useful in diagnosing culture-negative endocarditis caused by C. burnetii.

Treatment

Doxycycline is the drug of choice for the treatment of acute Q fever. The newer macrolides have also been shown to be effective in the treatment of acute pneumonia. Chronic Q fever requires prolonged treatment for 18 months or longer with a combination of doxycycline and hydroxychloroquine. Duration of treatment is long as mentioned earlier and should be determined by decrease in phase I antibody titers. In endo carditis, combination therapy is necessary to prevent relapse; occasionally, valve replacement is required and can prolong survival.

Prevention

 The presently recommended conditions of “high-temperature, short-time” pasteurization at 71.5°C for 15 seconds are adequate to destroy viable Coxiella species.

For C. burnetii, an investigational vaccine made from infected egg yolk sacs is available. This vaccine has been used for laboratory workers who handle live C. burnetii but currently is only commercially available in Australia.

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