A 48-year-old dairy farmer presents with low-grade fever and progressive exertional dyspnea over 3 months. Echocardiography shows aortic valve vegetations with culture-negative endocarditis. Serology for Coxiella burnetii shows a Phase I IgG titre of 1:1600 and a Phase II titre of 1:200. The high Phase I antibody titre indicates:

• A Acute Q fever pneumonia
• B Cross-reacting antibodies against Proteus OX-19
• C Past resolved infection requiring no follow-up
• D Chronic Q fever endocarditis ✓

Explanation

In acute Q fever, antibodies are directed mainly against Phase II antigen, while chronic infection, especially endocarditis, is marked by a persistently high Phase I IgG titre, classically exceeding 1:800. A Phase I titre far higher than the Phase II titre in a patient with culture-negative endocarditis therefore confirms chronic Q fever, which requires prolonged combination therapy rather than reassurance.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.