Coxiella burnetii
Coxiella burnetii is an obligate intracellular bacterium that causes Q fever, an infection acquired by inhaling aerosols from infected livestock. It classically presents as an atypical pneumonia or hepatitis without a rash.
Coxiella burnetii is a small, gram-negative, obligate intracellular bacterium and the causative agent of Q fever. Although historically grouped with the rickettsiae, it stands apart in ways exams love to probe: it spreads without an arthropod vector, and infection typically produces no rash.
Transmission is airborne. The organism forms a highly resistant, spore-like small-cell variant that survives heat and drying, allowing it to travel in aerosols from cattle, sheep, and goats — especially from the placenta and birth fluids of parturient animals — and in dust from contaminated hides and unpasteurized dairy products. Farmers, ranchers, veterinarians, and slaughterhouse workers are the classic patients, and the infectious dose is extremely low.
Acute Q fever most often presents as a self-limited flu-like illness, atypical (interstitial) pneumonia, or granulomatous hepatitis. Chronic infection is rarer but serious: C. burnetii is a leading cause of culture-negative endocarditis, typically in patients with pre-existing valve disease. Diagnosis relies on serology, since the organism does not grow on routine culture media.
USMLE Step 1 tests Coxiella burnetii as a high-yield microbiology vignette: a farm or slaughterhouse worker with pneumonia or hepatitis and no rash, or a patient with endocarditis whose blood cultures stay negative. Remember the differentiators — no vector, no rash, aerosol spread from livestock — that separate it from the other rickettsial diseases.
Key takeaways
- Coxiella burnetii is an obligate intracellular bacterium that causes Q fever.
- It spreads by inhaled aerosols from livestock — no arthropod vector — and classically causes no rash.
- Acute Q fever presents as flu-like illness, atypical pneumonia, or hepatitis; chronic disease causes culture-negative endocarditis.
- A spore-like resistant form lets the organism survive in the environment with a very low infectious dose.
- Step 1 vignettes feature farmers, ranchers, or veterinarians exposed to animal birth products.