Q Fever Serology

Q fever is a zoonosis caused by Coxiella burnetii, an obligate intracellular bacterium shed in the birth products, milk, urine and faeces of cattle, sheep and goats. Humans are infected mainly by inhaling contaminated aerosols and dust, and less often by unpasteurised milk or tick bites. Serology by indirect immunofluorescence (IFA) is the reference test.

It measures IgM and IgG against phase II antigen (acute infection) and phase I antigen (chronic infection). ELISA, complement fixation and PCR on blood, serum or tissue are also used.

Q fever occurs worldwide except New Zealand. Seroprevalence in rural, farming communities is 10 to 30% and higher in some African and Middle Eastern settings. About 60% of infections are asymptomatic. Acute Q fever presents as a self-limiting febrile illness with severe headache, myalgia, atypical pneumonia or hepatitis.

Chronic Q fever (under 5% of infections) occurs in people with valvular heart disease, vascular grafts or aneurysms, immunosuppression and pregnancy, and presents as culture negative endocarditis (the commonest form), vascular infection or chronic hepatitis, with mortality of up to 25 to 60% if untreated. Post-Q fever fatigue syndrome follows in 10 to 20%.

Indications. Fever with atypical pneumonia, hepatitis or severe headache in a person with animal or farm exposure. Culture negative endocarditis, or fever in a patient with known valvular disease or vascular graft and exposure. Pyrexia of unknown origin. Pregnant women with exposure and fever. Follow up of acute infection in high risk patients for development of chronic disease. Outbreak investigation among farmers, abattoir workers and veterinarians.

Specimen. Serum 1 to 2 mL, acute and convalescent samples 2 to 4 weeks apart. EDTA whole blood for PCR (early, before antibiotics and within 2 weeks of onset). Heart valve, vascular graft, placental and other tissue for PCR and immunohistochemistry. The laboratory is warned because the organism is a hazard in culture.

Reference values (assay dependent). IFA phase II IgM and IgG: negative under 1 in 16 to 1 in 64. Phase I IgG: negative under 1 in 64 to 1 in 128. Coxiella DNA: not detected.

Interpretation.

• Acute infection is confirmed by seroconversion or a fourfold or greater rise in phase II IgG between paired sera, or a single phase II IgG of 1 in 256 or more with IgM of 1 in 64 or more.
• Phase II IgG is positive from week 2 to 3 of illness and often negative in the first week. IgM appears with IgG, and persists for months.
• Chronic Q fever: phase I IgG of 1 in 800 or more (1 in 1600 or more is highly suggestive) with phase I IgA, a compatible clinical picture (endocarditis, vascular infection) and positive PCR. Phase I IgG over 1 in 6400 has high positive predictive value for endocarditis.
• Persistently raised or rising phase I IgG after acute infection in a high risk patient signals evolving chronic disease, and monitoring is done at 3, 6, 12 and 24 months.
• PCR of blood or serum is positive in the first 2 to 3 weeks of acute infection, and of blood, valve or tissue in chronic disease (sensitivity 50 to 80%).
• Echocardiography (transoesophageal), PET-CT or CT angiography locate the focus in chronic disease.
• Treatment of chronic Q fever is prolonged: doxycycline with hydroxychloroquine for 18 to 24 months, and serology is monitored every 3 months for a fall of phase I IgG to under 1 in 800.

Critical values and findings needing immediate action.

• Fever, new regurgitant murmur, heart failure or embolic events in a patient with valvular disease and positive phase I IgG: Q fever endocarditis, urgent echocardiography, cardiology and infectious disease review, and surgery for failure or large vegetations.
• Pregnancy with acute or chronic Q fever: risk of miscarriage, preterm birth and foetal death, co-trimoxazole during pregnancy and long term follow up.
• Pulmonary infiltrates with hypoxia in an outbreak setting: supportive care and doxycycline.
• Aortic aneurysm or vascular graft with fever and raised phase I IgG: vascular Q fever, urgent imaging.
• Immunosuppressed patient with persistent fever and hepatitis: chronic infection and treatment.

Interferences and pitfalls.

• Antibody is usually absent in the first 7 to 14 days. Treatment is started on clinical suspicion (doxycycline), and paired sera confirm the diagnosis.
• Early doxycycline can delay or blunt seroconversion.
• Cross-reactions occur with Legionella, Bartonella, Chlamydia, Rickettsia and Epstein-Barr virus (mostly IgM), and polyclonal B-cell activation gives false positive IgM. Phase I IgG is more specific for chronic infection.
• Low level phase I antibodies (1 in 64 to 1 in 400) are common after acute infection and do not indicate chronic disease.
• Antibody levels vary between laboratories and kits, and thresholds for chronic Q fever are validated for IFA.
• Rheumatoid factor and autoimmune disease can interfere with IgM assays.
• Differential diagnosis includes brucellosis, Bartonella, Legionella, mycoplasma, hepatitis viruses, tuberculosis and lymphoma.
• Positive results are notified to public health in many countries because of outbreak risk.

Special groups.

• Children: mostly mild or asymptomatic illness, and chronic disease is rare.
• Pregnancy: infection raises miscarriage, stillbirth and preterm risk, and serology follows every few months.
• Older adults and cardiac patients: higher risk of chronic disease.
• Occupational: farmers, abattoir workers, veterinarians, laboratory staff and people living near goat or sheep farms.
• Regions: outbreaks occur after lambing and in dust from livestock yards, and endemic foci exist in Africa, the Middle East and Asia with limited laboratory capacity.

Follow up tests. Phase I and II IgG, IgM and IgA, Coxiella PCR (blood, tissue), transthoracic and transoesophageal echocardiography, PET-CT or CT angiography, blood cultures (negative), liver function tests, full blood count, ESR and C-reactive protein, chest radiograph, brucella and bartonella serology, and repeat serology at 3, 6, 12 and 24 months.