Question
‘Q’ fever.
Answer
Q (query) fever is a zoonotic disease caused by Coxiella burnetii, an obligate intracellular Gram-negative bacterium, notable for its extreme environmental resistance due to a spore-like small-cell variant form and its unusual ability to survive and multiply within the acidic phagolysosome of host cells.
Transmission: primarily by inhalation of aerosolized organisms from infected animals (cattle, sheep, goats — a significant occupational hazard for farmers, veterinarians, and abattoir workers) via contaminated dust, placental tissue, birth fluids, or animal products (unlike most rickettsial diseases, an arthropod vector is not required for human infection, though ticks maintain the sylvatic animal cycle); can also be acquired by ingestion of unpasteurized milk.
Clinical features: acute infection typically presents as a self-limiting atypical (interstitial) pneumonia or a flu-like febrile illness, sometimes with hepatitis; a chronic form can develop months to years later, most importantly as culture-negative endocarditis, particularly in patients with pre-existing valvular disease or immunosuppression.
Laboratory diagnosis: serology is the mainstay — indirect immunofluorescence assay (IFA) demonstrating antibodies against Phase II antigen (acute infection) or Phase I antigen (chronic infection, e.g., endocarditis — the “phase reversal” pattern is diagnostically important); PCR for direct detection; culture is hazardous (highly infectious, biosafety level 3 requirement) and rarely attempted in routine laboratories.
Treatment and prevention: doxycycline is the treatment of choice for acute Q fever; chronic Q fever (endocarditis) requires prolonged combination therapy (doxycycline plus hydroxychloroquine) for months to years; prevention relies on occupational hygiene precautions and a vaccine (available in some countries for high-risk occupational groups).