Coxiella

Coxiella burnetii is a small, pleomorphic, obligate intracellular gram-negative coccobacillus in the order Legionellales. It survives within phagolysosomes of macrophages by adapting to the acidic environment, a unique feature among intracellular pathogens. The organism forms a spore-like small cell variant (SCV) that is highly resistant to heat, drying, and disinfectants, allowing it to persist in the environment and become aerosolized from dust, wool, or dried placental tissue.

Transmission occurs primarily via inhalation of contaminated aerosols from infected livestock (cattle, sheep, goats) placental tissue, birth fluids, urine, feces, or milk—no arthropod vector is required for human infection (distinguishing it from other Rickettsial-type organisms). Occupational exposure (farmers, veterinarians, slaughterhouse workers) is a key risk factor.

Clinical presentation: Q fever presents as an acute flu-like illness with high fever, severe headache, myalgias, and atypical pneumonia or hepatitis. Chronic Q fever (occurring in <5% of cases, often in immunocompromised or those with valvular disease) manifests as culture-negative endocarditis, which is a classic board association. Diagnosis is typically serologic (indirect immunofluorescence assay detecting phase I and phase II antibodies), since the organism is difficult to culture on standard media (special cell culture or PCR required due to strict intracellular growth).

Treatment: Doxycycline is first-line for acute Q fever. Chronic Q fever (endocarditis) requires prolonged combination therapy, often doxycycline plus hydroxychloroquine for 18 months to years.

Key boards associations: 'Q' stands for 'query' fever (originally of unknown cause). Remember the mnemonic 'Q fever has no rash, no vector, and one crazy microbe (Coxiella) that hides in vacuoles.' Also tested: it is one of the causes of culture-negative endocarditis (along with HACEK organisms, Bartonella, and Tropheryma whipplei).