Coxiella burnetii - Q-Fever

Q-fever is a worldwide zoonosis caused by Coxiella burnetii and named for a 1985 disease outbreak in Queensland, Australia. For definitive diagnosis in the early stages of illness it is recommended to use serologic tests in combination with PCR of whole blood or serum. Treatment should be initiated as soon as Q fever is suspected and should never be withheld pending the receipt of diagnostic test results.


Indications for Testing

  • Flu-like illness with appropriate risk factors
  • Presence of blood culture-negative endocarditis

Laboratory Testing

  • CDC laboratory confirmation of C. burnetii
  • Nonspecific testing – acute disease
    • CBC – thrombocytopenia common
    • Serum transaminases – moderately elevated in many patients
    • Patients with endocarditis
      • C-reactive protein (CRP)
        • Preferred test to detect inflammatory processes (Choosing Wisely, 2016; American Society for Clinical Pathology)
        • Usually elevated
        • If CRP not available, order erythrocyte sedimentation rate (ESR)
      • CBC – anemia, thrombocytopenia
      • Urinalysis – hematuria
  • Antibody titers by immunofluorescence assay (IFA), enzyme-linked immunosorbent assay (ELISA) – acute disease
    • Perform IgG and IgM, phase I and II, antibody testing
    • Results
      • Phases I and II – IgM increased
      • Phase II – IgG increased
      • Titers may not increase for 2-4 weeks after onset of symptoms
      • May be detected up to 12 weeks after illness onset
  • Chronic disease
    • Phase I – IgG persistently increased
    • Endocarditis – IgG titer ≥1:800 is diagnostic
  • Polymerase chain reaction (PCR) – helpful in patients with suspected disease and low titers
    • Can be performed on tissue fluids (eg, cerebrospinal fluid [CSF], pleural)
    • Most sensitive in first week of illness
    • Sensitivity rapidly diminishes with antibiotic therapy
  • Immunohistochemical staining – not widely available
  • Culture – not recommended
    • Lack sensitivity
    • Available only in research laboratories due to extreme infectivity

Imaging Studies



  • IgG phase I – monitor treatment efficacy; successful therapy should result in decreased IgG
  • Consider repeat serum testing using IgG phase I in patients with known valvular abnormalities, if initial testing was negative
  • Follow-up testing recommended for all patients with acute disease
    • Antibody titers
    • Echocardiogram



  • Incidence – <50/year in U.S.
  • Age – highest prevalence in 30s-60s
  • Sex – M>F
    • Women and children more commonly asymptomatic
  • Transmission
    • Main reservoirs of C. burnetii include cattle, sheep, and goats, as well as the rodents, cats, and birds that feed on them
    • Infection in these animals is enzootic and usually asymptomatic
    • Bacteria infects humans via
      • Inhalation of contaminated dust particles and aerosols
      • Handling/ingestion of infected raw meat or milk


  • Gram-negative coccobacillus
  • Obligate intracellular bacterial pathogen (family Coxiellaceae; order Legionellales) with worldwide distribution
  • Classified as a class B bioterrorism agent

Risk Factors

  • Occupational – farming, veterinary medicine, abattoir work, military work
  • Underlying valve disease – risk factor for endocarditis
  • Ingestion of unpasteurized dairy products


  • C. burnetii exists in two antigenic phases (phases I and II) – antigenic difference is important to diagnosis
    • Antibodies to both phase I and II antigens may persist for months or years after initial infection
  • Acute disease
    • Antibody levels to phase II antigens are usually higher (often by several orders of magnitude) than those for phase I antigens and are usually detected during second week of symptoms
  • Chronic disease
    • Subsequent testing shows high levels of antibodies to phase I antigens and constant or decreasing levels of antibodies to phase II antigens
      • Because antibodies to phase I antigens generally require more time to appear, consistently high levels may indicate continued exposure to the bacteria
    • Signs and symptoms of inflammatory disease

Clinical Presentation

  • Incubation period – ~2-6 weeks
  • Acute disease symptoms
    • Most cases are self-limiting, flu-like illnesses – fever peaks in 2-4 days near 40°C, then gradually declines over period of 1-2 weeks
    • Constitutional – malaise, anorexia, myalgias, weakness, intense headache
    • Pneumonia or bronchitis – tachypnea, rales, rhonchi, cough, wheezing
    • Hepatitis – nausea, vomiting, diarrhea, anorexia, elevated transaminases, rarely jaundice
    • Myocarditis, pericarditis
    • During pregnancy – abortion, prematurity, low birth weight
  • Chronic disease symptoms – rare (<1%)
    • Defined as infection lasting >6 months
    • Presence of endocarditis – pathognomonic for chronic disease
    • Occurs in patients with preexisting heart valve damage (usually aortic and mitral valves), immunosuppression, or chronic renal disease
    • May result in culture-negative endocarditis
    • Symptoms – low-grade fever, cardiac failure, hepatosplenomegaly, clubbing of digits
    • Other organs may be affected, resulting in hepatitis, vascular infection, osteomyelitis, lymphadenitis

ARUP Laboratory Tests

Confirm infectious agent as C. burnetii (Q-fever) in symptomatic patients

Reflex pattern: for IgG or IgM testing, if any phase I or phase II screening result is indeterminate or positive, then titer(s) will be added

Confirm infectious agent as C. burnetii (Q-fever) in symptomatic patients

Recommend testing of acute and convalescent sera

Reflex pattern: if either C. Burnetii antibody IgG phase I and/or phase II result is indeterminate or positive, then titer(s) will be added

Related Tests

Order to differentiate bacterial from viral etiology

Order to rule out associated hepatitis

Preferred test to detect acute phase inflammation (eg, autoimmune diseases, connective tissue disease, rheumatoid arthritis, infection, or sepsis)

Screen for various metabolic and kidney disorders

Nonspecific test used to detect inflammation associated with infections, cancers, and autoimmune diseases

Medical Experts



Marc Roger Couturier, PhD, D(ABMM)
Associate Professor of Pathology (Clinical), University of Utah
Medical Director, Parasitology/Fecal Testing, Infectious Disease Antigen Testing, Bacteriology, and Molecular Amplified Detection, ARUP Laboratories


Patricia R. Slev, PhD
Associate Professor of Pathology (Clinical), Codirector, Clinical Chemistry Fellowship program, University of Utah
Section Chief, Immunology; Medical Director, Immunology Core Laboratory; Medical Director, Serologic Hepatitis and Retrovirus and Immunology Core Laboratory; Co-Medical Director, Microbial Immunology, ARUP Laboratories


Additional Resources
  • 18755387

    Tissot-Dupont H, Raoult D. Q fever. Infect Dis Clin North Am. 2008; 22 (3): 505-14, ix.