A 60-year-old farmer presents with 3 months of low-grade fever, weight loss, and a new aortic regurgitant murmur. Three sets of blood cultures drawn before antibiotics show no growth. Serology identifies phase I IgG antibodies against a gram-negative intracellular bacterium transmitted from cattle and sheep. What is the causative agent?

• A Bartonella quintana
• B Chlamydia psittaci
• C Brucella melitensis
• D Coxiella burnetii ✓

Explanation

Coxiella burnetii, the agent of Q fever, is now the most frequently identified cause of culture-negative endocarditis in many series. It is an obligate intracellular gram-negative bacterium acquired from livestock, diagnosed serologically by high-titer phase I IgG antibodies, and it characteristically produces slowly progressive aortic valve disease over months. Bartonella quintana causes culture-negative disease in homeless alcoholic men, Brucella in consumers of unpasteurized dairy, and Chlamydia psittaci is a rare reported cause, but the occupational exposure plus phase I antibody pattern specifically identifies Coxiella.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.