Q fever is a zoonotic disease caused by Coxiella burnetii. These bacteria are distributed globally and can produce atypical pneumonia, fever, hepatitis or endocarditis. C. burnetiiinfects a wide range of animals, as well as many tick species. Infection of humans is most commonly acquired by breathing infectious aerosols or contaminated dust.
Clinical features: Most acute cases of Q fever include the three following symptoms: flu-like syndrome, pneumonia and hepatitis. Pneumonia can affect up to thirty to fifty percent of patients with a symptomatic infection, although most cases are relatively mild. Chronic Q fever, characterised by ineffectively treated infections that persist for more than 6 months, show as a main sign the inflammation of the heart’s inner lining, named endocarditis. Other less common characteristics of Q fever are cirrhosis and lung scarring (interstitial pulmonary fibrosis).
Diagnosis: The diagnosis of Q fever is based on serological methods since isolation from clinical samples is difficult.
IgM antibodies appear since the second week until the fourth month of the acute phase of illness. After 4-8 weeks, the maximum level of IgG antibody is detected. In those cases of acute infection where IgG antibodies are detected at higher titres, it is highly probable to find IgM as well. In the chronic phases (endocarditis), IgM is not detectable.
Most diagnostic laboratories use either the indirect fluorescent antibody assay or enzyme immunoassay. Both tests are sensitive and specific. The indirect fluorescent antibody assay is generally used when equipment or space is limited or when small numbers of samples are tested. An advantage of the indirect fluorescent antibody assay is the ability to use phase I and phase II antigens. The enzyme immunoassay is highly sensitive, easy to perform, has great potential adaptability for automation, and can be applied in epidemiological surveys.
The two antigenic forms of C. burnetii that are important for serologic diagnosis of Q fever are the phase I (i.e. virulent microorganism with smooth LPS [S-LPS]) and phase II (i.e. avirulent microorganism with rough LPS [R-LPS]) whole-cell antigens. Determining antibodies against phase I and phase II C. burnetii can help distinguish acute and chronic Q fever. In sera from acute Q fever patients, the magnitude of antiphase II titers exceeds those of antiphase I titers. However, in chronic Q fever patients, the antiphase I titers exceed those of anti-phase II titers, and patients with chronic Q fever endocarditis can have high levels of serum IgA.
Treatment: antibiotic treatment is the most effective one for Q fever. Doxycycline is the treatment of choice for acute Q fever. Chronic Q fever often requires the use of multiple drugs, usually doxycycline in combination with hydroxychloroquine. Q fever endocarditis patients generally receive therapy for several months.