Coxiella burnetii, the causative agent for Q fever, is primarily transmitted to humans via inhalation of particles excreted by livestock such as cattle, sheep, and goats; other forms of transmission (eg, human to human) are rare. Infections may be either acute or chronic. Acute infection is often mild or asymptomatic; however, pregnant women who are infected are at risk of miscarriage, stillbirth, and other adverse outcomes. Chronic Q fever is rare, but often causes endocarditis, which is usually fatal if left untreated. There are distinct antibody responses to two different antigenic phases (phase I and phase II) of C. burnetii infection. The response to phase II antigens is higher in acute infection, and the response to phase I antigens is higher in chronic infection. Laboratory testing for Q fever includes testing for antibody response to both phase I and phase II antigens. For more information, see the Diagnosis and Management of Q Fever recommendations from the CDC.
Quick Answers for Clinicians
Because diagnostic serology tests will return negative results for the first 7-15 days of illness, treatment should begin before laboratory confirmation of Coxiella burnetii infection (Q fever). Treatment should be initiated based on clinical suspicion, which may be based on a combination of symptoms, patient history (including recent animal exposure), and evidence from nonspecific laboratory tests.
Certain standard blood tests, such as CBC and liver function tests, can support a diagnosis of Q fever (and assist in the decision to begin treatment) before confirmatory laboratory tests have been completed. The combination of a low platelet count, a normal leukocyte count, and elevated liver enzymes is suggestive of acute Coxiella burnetii infection.
Fewer than 5% of individuals who contract acute Q fever develop a chronic infection, which may occur a few months to decades after the initial infection. Individuals with acute Q fever should undergo a clinical assessment to determine the risk of developing chronic Q fever. Those with valvular heart disease, arterial aneurysms, and vascular grafts are at high risk, as are individuals who are immunocompromised or who were infected while pregnant.
Indications for Testing
Testing for acute Q fever (ie, C. burnetii infection) should be considered in individuals with flu-like symptoms (such as fever, headache, and chills) and occupational or travel-related exposure to potentially infected livestock.
Testing for chronic Q fever may be considered for individuals with culture-negative endocarditis, particularly in those with a compromised immune system or experiencing other early symptoms such as fatigue and fever.
Serology is the most common diagnostic approach for C. burnetii infection, regardless of whether acute or chronic infection is suspected. For more information, see the CDC Case Definition for Q fever.
The standard test for diagnosing acute C. burnetii infection is indirect fluorescent antibody (IFA) testing on paired acute and convalescent sera. A fourfold increase in phase II immunoglobulin G (IgG) concentration is considered diagnostic for Q fever.
Nucleic Acid Detection
Polymerase chain reaction (PCR) testing on serum or whole blood can be used to detect C. burnetii within the first week of illness before antibodies develop, and PCR in combination with serology is recommended in the early stages of infection for a definitive diagnosis of acute Q fever. However, a negative PCR result does not rule out infection and should not lead to treatment being delayed.
Chronic Q fever can be diagnosed when an individual exhibits both:
- Phase I IgG antibody level of ≥1:1,024 (generally higher than the level of phase II antibodies)
- A persistent focus of infection, such as endocarditis
PCR testing on whole blood is not an appropriate diagnostic test for chronic Q fever due to its lack of sensitivity. PCR or immunohistochemistry (IHC) testing of tissue specimens from the active site of infection may be useful.
ARUP Laboratory Tests
Semi-Quantitative Indirect Fluorescent Antibody
U.S. Department of Health and Human Services, Centers for Disease Control and Prevention. Q fever. Information for healthcare providers. [Last reviewed: Jan 2019; Accessed: Jun 2021]
U.S. Department of Health and Human Services, Centers for Disease Control and Prevention. Diagnosis and management of Q fever — United States, 2013: recommendations from CDC and the Q Fever Working Group. Morbidity and Mortality Weekly Report. [Last reviewed: Mar 2013; Accessed: Jun 2021]
U.S. Department of Health and Human Services, Centers for Disease Control and Prevention. Q fever (Coxiella burnetii): 2009 case definition. [Accessed: Jun 2021]