Author(s): Gilbert Kersh

Infectious Agent

The causative agent of Q fever is the gram-negative intracellular bacterium Coxiella burnetii.

Transmission

C. burnetii is most commonly transmitted through inhalation of aerosols or dust contaminated with dried birth fluids or excreta from infected animals, usually cattle, goats, or sheep. C. burnetii is highly infectious and persists in the environment. Infections via ingestion of contaminated unpasteurized dairy products and human-to-human transmission via sexual contact have been reported, but rarely.

Epidemiology

C. burnetti has a worldwide distribution but is absent from New Zealand. C. burnetti prevalence is greatest in Africa and countries in the Middle East. Reported rates of human infection are higher in France and Australia than in the United States. The largest known Q fever outbreak reported to date involved ≈4,000 human cases during 2007–2010 in the Netherlands. Cases of Q fever in travelers are most often reported in people who visited rural areas or farms with cattle, goats, sheep, or other livestock. During 1990–2013, ≥250 travel-related cases of Q fever were reported in the literature.

Occupational exposure to infected animals, particularly during parturition, poses a high risk for infection among butchers, farmers, meat packers, veterinarians, and seasonal or migrant farm workers. Examples of travel-acquired Q fever include cases in soldiers deployed to rural areas, travelers with livestock contact and consumption of unpasteurized milk, and travelers obtaining treatments that involved the injection of fetal sheep cells. In a 2008 review of 708 returned travelers evaluated for fever, Q fever was diagnosed in 5 people (0.7%).

Clinical Presentation

The incubation period is typically 2–3 weeks but can be shorter after exposure to large numbers of organisms. Estimates suggest that over half of acute infections are mild or asymptomatic. The most common clinical presentation of acute infection is a self-limiting febrile illness, with hepatitis or pneumonia associated with more severe acute infections. Chronic infections occur primarily in patients with preexisting cardiac valvulopathies, vascular abnormalities, or immunosuppression. Without proper treatment, infection during pregnancy poses a risk for adverse pregnancy outcomes. The most common manifestations of chronic disease are endocarditis and endovascular infections. Chronic infections might become apparent months or years after the initial exposure.

Diagnosis

Serologic evidence of a 4-fold rise in phase II IgG by indirect fluorescent antibody test between paired acute and convalescent serum samples collected 3–4 weeks apart is the gold standard for diagnosis. Consider a single high serum phase II IgG titer (>1:64) in conjunction with clinical evidence of infection as indicative of probable acute Q fever. PCR testing of serum or whole blood is useful for confirmation of acute Q fever if samples are taken ≤14 days after symptom onset.

Chronic Q fever diagnosis requires a phase I IgG titer >1:512 and clinical evidence of persistent infection (e.g., endocarditis, infected vascular aneurysm, osteomyelitis). Identifying C. burnetii in whole blood, serum, or tissue samples by PCR, immunohistochemical staining, or isolation can be used to confirm chronic disease. Tests for direct detection of C. burnetii might not be widely available, but the Rickettsial Zoonoses Branch at the Centers for Disease Control and Prevention (CDC) can assist. Information about diagnostic testing is available at CDC's Q Fever webpage or call the CDC Emergency Operations Center (770-488-7100) and ask to be directed to the Rickettsial Zoonoses Branch. Q fever is a nationally notifiable disease.

Treatment

Doxycycline is the most frequently used and most effective treatment for acute Q fever. For pregnant people, children aged <8 years with mild illness, and patients allergic to doxycycline, trimethoprim-sulfamethoxazole is an alternative treatment option. Treatment for acute Q fever is not recommended for asymptomatic people or for those whose symptoms have resolved. Chronic C. burnetii infections require long-term combination therapy, and the combination of doxycycline and hydroxychloroquine for ≥18 months provides the best treatment outcomes. Alternative treatments include trimethoprim-sulfamethoxazole and fluoroquinolones, but these are less effective. Treatment of Q fever also might involve surgery to remove infected tissue.

Prevention

To prevent Q fever, travelers should avoid areas where potentially infected animals are kept and avoid consumption of unpasteurized dairy products. A human vaccine for Q fever has been developed and used in Australia but is not available in the United States.

CDC website: Q fever