Microbiology

Coxiella burnetii-the etiologic agent of Q fever-is a small, pleomorphic coccobacillus that has a gram-negative cell wall and is located intracellularly.

Epidemiology and Pathogenesis

A worldwide disease, Q fever is a zoonosis. Cattle, sheep, and goats are responsible for most cases of human infection; many other animals can serve as vectors of transmission or as reservoirs of disease.

• C. burnetii localizes to the uterus and mammary glands of infected female mammals. It is reactivated in pregnancy and is found at high concentrations in the placenta. At parturition, the organism is dispersed as an aerosol, and infection usually follows inhalation.
• Abattoir workers, veterinarians, farmers, and other persons who have contact with infected animals, particularly with newborn animals or infected products of conception, are at risk.
• In the United States, there are 28-54 cases per year; in Australia, there are 30 cases per 1 million population per year.

Clinical Manifestations

The specific presentation of acute Q fever differs geographically (e.g., pneumonia in Nova Scotia and granulomatous hepatitis in Marseille), potentially reflecting differences in routes of infection or infecting strains; chronic Q fever almost always implies endocarditis.

• Acute Q fever: After an incubation period of 3-30 days, pts may present with flulike syndromes, prolonged fever, pneumonia, hepatitis, pericarditis, myocarditis, meningoencephalitis, and infection during pregnancy.
  • Signs and symptoms are often nonspecific (e.g., fever, fatigue, headache, chills, sweats, nausea, vomiting, diarrhea, cough, and occasionally rash).
  • Multiple rounded opacities on CXR in pts in endemic areas are highly suggestive of Q fever pneumonia.
  • The WBC count is usually normal, but thrombocytopenia occurs. During recovery, reactive thrombocytosis can develop.
  • Prolonged fatigue, along with a constellation of nonspecific symptoms (e.g., headaches, myalgias, arthralgias), can follow Q fever (post-Q fever fatigue syndrome).
• Chronic Q fever: Pts with C. burnetii endocarditis typically have prior valvular heart disease, immunosuppression, or chronic renal failure.
  • Fever is absent or low grade; pts may be ill for >1 year before diagnosis.
  • Valvular vegetations are seen in 21-50% of cases with transesophageal echocardiography but in only 12% with transthoracic echocardiography. The vegetations differ from those in bacterial endocarditis of other etiologies and manifest as endothelium-covered nodules on the valve.
  • The disease should be suspected in all pts with culture-negative endocarditis.
  • Although C. burnetii can be isolated by a shell-vial technique, most laboratories are not permitted to attempt isolation because of the organism's highly contagious nature. PCR testing of tissue or biopsy specimens can be used, but serology is the most common diagnostic tool; indirect immunofluorescence is the method of choice.