A 61-year-old farmer has six weeks of low-grade fever, night sweats, and weight loss. Three sets of blood cultures after 14 days incubation show no growth, yet transoesophageal echocardiography reveals an aortic valve vegetation. Serology shows phase I IgG titre greater than 1:800 against Coxiella burnetii. He reports contact with parturient cattle. What is the most likely causative agent?

• A Aggregatibacter actinomycetemcomitans
• B Bartonella henselae
• C Brucella melitensis
• D Coxiella burnetii ✓

Explanation

Coxiella burnetii, the cause of Q fever, is the most frequently identified cause of culture-negative infective endocarditis worldwide. It is an obligate intracellular organism, so standard cultures remain negative, and diagnosis rests on phase I IgG serology, typically above 1:800. Exposure to livestock, especially products of conception, is the classic risk. Bartonella is the second commonest cause, while HACEK organisms grow in contemporary blood culture systems and are no longer truly culture-negative.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.