Q fever: another query

The μgnome was recently asked about the interpretation of serological tests for diagnosis of Q fever. The case in question had raised titres against both phase I and II antigen.

Making sense of these tests is challenging for several reasons:

  • several serological methods are in common use – enzyme immunoassay (EIA), indirect fluorescent antibody (IFA) and complement fixation test (CFT)
  • antibody to phase II antigen is raised in acute disease, and to phase I antigen in chronic disease (not vice versa, as logic might suggest)
  • antibodies to phase II and phase I antigens may be present at the same time, in either acute or chronic infection

Interpretation of results is therefore best provided by the laboratory that runs the tests, but some general principles can be applied:

  • a more than four-fold rise in antibody titre between acute and convalescent sera supports a diagnosis of recent Coxiella burnetii infection
  • IgM antibodies may persist for a long time after acute infection
  • reliance on serology alone will result in under-diagnosis of Q fever

Q fever (originally query fever) is caused by an obligate intracellular bacterium known as Coxiella burnetii; a small Gram negative cocco-bacillus. Infection is normally by inhalation of aerosols of animal body fluids; usually urine, faeces, milk or birth products from cattle, sheep or goats. In Australia, Q fever is most commonly an infection of abattoir workers, and presents as a short-lived febrile illness after an incubation period of 2-6 weeks.  More severe cases usually present as a pneumonia. In the chronic form of the infection, infective endocarditis is a late onset complication. Other presentations include osteomyelitis, granulomatous hepatitis  and neurological manifestations.

There are no bactericidal antibiotics for C. burnetii infection and there is no place for β-lactam antibiotics in the treatment of Q fever. Treatment hinges on doxycycline for both acute and chronic infection. Combination therapy with either doxycycline/hydroxychloroquine or doxycycline/rifampicin have been used successfully in chronic infections. A minimum duration of 2 years’ treatment has been recommended for chronic C. burnetii infection.

C. burnetii infection can be prevented by vaccination, which is recommended for people in high risk occupations.

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