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Q Fever (Coxiella burnetii)
1999 Case Definition

Clinical Description

Acute infection: A febrile illness usually accompanied by rigors, myalgia, malaise, and retrobulbar headache. Severe disease can include acute hepatitis, pneumonia, and meningoencephalitis. Clinical laboratory findings may include elevated liver enzyme levels and abnormal chest film findings. Asymptomatic infections may also occur.

Chronic infection: Potentially fatal endocarditis may evolve months to years after acute infection, particularly in persons with underlying valvular disease. A chronic fatigue-like syndrome has been reported in some Q fever patients.

Laboratory Criteria for Diagnosis

  • Fourfold or greater change in antibody titer to Coxiella burnetii phase II or phase I antigen in paired serum specimens ideally taken 3-6 weeks apart, OR
  • Isolation of C. burnetii from a clinical specimen by culture, OR
  • Demonstration of C. burnetii in a clinical specimen by detection of antigen or nucleic acid.

Case Classification


A clinically compatible or epidemiologically linked case with a single supportive Immunoglobulin G (IgG) or Immunoglobulin M (IgM) titer. Cutoff titers are determined by individual laboratories. CDC tests for IgG antibodies with an indirect immunofluorescence assay (IFA), and uses a titer of 1:128 as the cutoff for significant antibody.


A clinically compatible or epidemiologically linked case that is laboratory confirmed.