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Q fever (Coxiella burnetii)
2009 Case Definition

CSTE Position Statement(s)

  • 09-ID-54

Subtype(s)

  • Q fever, acute
  • Q fever, chronic

Exposure

Exposure is usually via aerosol, is broadly interpreted, and may be unknown (especially for chronic infection), but often includes the presence of goats, sheep, or other livestock, especially during periods of parturition. Direct contact with animals is not required, and variable incubation periods may be dose dependent.

Subtype(s) Case Definition  

Q fever, acute

Clinical Description

Acute fever usually accompanied by rigors, myalgia, malaise, and a severe retrobulbar headache. Fatigue, night-sweats, dyspnea, confusion, nausea, diarrhea, abdominal pain, vomiting, non-productive cough, and chest pain have also been reported. Severe disease can include acute hepatitis, atypical pneumonia with abnormal radiograph, and meningoencephalitis. Pregnant women are at risk for fetal death and abortion. Clinical laboratory findings may include elevated liver enzyme levels, leukocytosis, and thrombocytopenia. Asymptomatic infections may also occur.

Note: Serologic profiles of pregnant women infected with acute Q fever during gestation may progress frequently and rapidly to those characteristic of chronic infection.

Clinical Criteria

Acute fever and one or more of the following: rigors, severe retrobulbar headache, acute hepatitis, pneumonia, or elevated liver enzyme levels.

Laboratory Criteria for Diagnosis

Laboratory confirmed:

  • Serological evidence of a fourfold change in immunoglobulin G (IgG)-specific antibody titer to C. burnetii phase II antigen by indirect immunofluorescence assay (IFA) between paired serum samples, (CDC suggests one taken during the first week of illness and a second 3-6 weeks later, antibody titers to phase I antigen may be elevated or rise as well), OR
  • Detection of C. burnetii DNA in a clinical specimen via amplification of a specific target by polymerase chain reaction (PCR) assay, OR
  • Demonstration of C. burnetii in a clinical specimen by immunohistochemical methods (IHC), OR
  • Isolation of C. burnetii from a clinical specimen by culture.
Laboratory supportive:
  • Has a single supportive IFA IgG titer of ≥1:128 to phase II antigen (phase I titers may be elevated as well).
  • Has serologic evidence of elevated phase II IgG or immunoglobulin M (IgM) antibody reactive with C. burnetii antigen by enzyme-linked immunosorbent assay (ELISA), dot-ELISA, or latex agglutination.
Note: For acute testing, CDC uses in-house IFA IgG testing (cutoff of ≥1:128), preferring simultaneous testing of paired specimens, and does not use IgM results for routine diagnostic testing.

Case Classification

Probable

A clinically compatible case of acute illness (meets clinical evidence criteria for acute Q fever illness) that has laboratory supportive results for past or present acute disease (antibody to Phase II antigen) but is not laboratory confirmed.

Confirmed

A laboratory confirmed case that either meets clinical case criteria or is epidemiologically linked to a lab confirmed case.

Q fever, chronic

Clinical Description

Infection that persists for more than 6 months. Potentially fatal endocarditis may evolve months to years after acute infection, particularly in persons with underlying valvular disease. Infections of aneurysms and vascular prostheses have been reported. Immunocompromised individuals are particularly susceptible. Rare cases of chronic hepatitis without endocarditis, osteomyelitis, osteoarthritis, and pneumonitis have been described.

Clinical Criteria

Newly recognized, culture-negative endocarditis, particularly in a patient with previous valvulopathy or compromised immune system, suspected infection of a vascular aneurysm or vascular prosthesis, or chronic hepatitis, osteomyelitis, osteoarthritis, or pneumonitis in the absence of other known etiology.

Laboratory Criteria for Diagnosis

Laboratory confirmed:

  • Serological evidence of IgG antibody to C. burnetii phase I antigen ≥ 1:800 by IFA (while phase II IgG titer will be elevated as well; phase I titer is higher than the phase II titer), OR
  • Detection of C. burnetii DNA in a clinical specimen via amplification of a specific target by PCR assay, OR
  • Demonstration of C. burnetii antigen in a clinical specimen by IHC, OR
  • Isolation of C. burnetii from a clinical specimen by culture.
Laboratory supportive:
  • Has an antibody titer to C. burnetii phase I IgG antigen ≥1:128 and < 1:800 by IFA.
Note: Samples from suspected chronic patients should be evaluated for IgG titers to both phase I and phase II antigens. Current commercially available ELISA tests (which test only for phase 2) are not quantitative, cannot be used to evaluate changes in antibody titer, and hence are not useful for serological confirmation. IgM tests are not strongly supported for use in serodiagnosis of acute disease, as the response may not be specific for the agent (resulting in false positives) and the IgM response may be persistent. Complement fixation (CF) tests and other older test methods are neither readily available nor commonly used.

Serologic test results must be interpreted with caution, because baseline antibodies acquired as a result of historical exposure to Q fever may exist, especially in rural and farming areas.

Case Classification

Probable

A clinically compatible case of chronic illness (meets clinical evidence criteria for chronic Q fever) that has laboratory supportive results for past or present chronic infection (antibody to Phase I antigen).

Confirmed

A clinically compatible case of chronic illness (meets clinical evidence criteria for chronic Q fever) that is laboratory confirmed for chronic infection.

Comments

The 2009 case definition appearing on this page was re-published in the 2009 CSTE position statement 09-ID-54. Thus, the 2009 and 2010 versions of the case definition are identical.



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