Microbiology and epidemiology of Q fever
- Didier Raoult, MD, PhD
Didier Raoult, MD, PhD
- Faculté de Médecine
- Aix Marseille Université
Q fever is a zoonotic infection caused by the pathogen Coxiella burnetii, which can cause acute or chronic disease with protean manifestations . The designation Q fever (from Query) was made in 1935 following an outbreak of febrile illness in an abattoir in Queensland, Australia.
The microbiology and epidemiology of Q fever will be reviewed here. The clinical features, diagnosis, and treatment of Q fever are discussed separately. (See "Clinical manifestations and diagnosis of Q fever" and "Q fever endocarditis".)
C. burnetii is a short, pleomorphic rod that is a strict intracellular bacterium. While previously designated as a Rickettsia, C. burnetii has been re-classified as a Proteobacteria, which is closer to Legionella and Francisella . C. burnetii can be cultivated in embryonated eggs, laboratory animals, and in vitro cell culture systems [3,4]. It can also be cultured in axenic (host cell-free) media [4,5].
In mammals, the usual host cell of C. burnetii is the macrophage, which is unable to kill the bacterium. C. burnetii lives and multiplies in a single, large, acidic vacuole, which results from the fusion of cell lysosomes . In addition, a sporulation-like process protects the organism from the external environment, where it can survive for long periods of time.
An important characteristic of C. burnetii is its antigenic variation, called "phase variation". When C. burnetii express phase I antigen it is highly infectious and a single bacterium is sufficient to infect a human. This is the form that is isolated from animals or humans. After subculturing C. burnetii in cells or embryonated eggs, modification of its lipopolysaccharide (LPS) capsule results in an antigenic shift to the phase II form, which is not infectious. This antigenic shift can be measured and forms the basis for differentiating acute from chronic Q fever .
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