Q Fever
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Direct answer
Q fever is zoonotic infection with Coxiella burnetii, acquired almost always by inhaling aerosols from the birth products, milk, urine or faeces of infected sheep, goats and cattle — an environmentally hardy intracellular bacterium causing a self-limiting febrile illness, an atypical pneumonia, or granulomatous hepatitis in acute form, and endocarditis or vascular infection in chronic form. Diagnosis is serological with a phase trick: acute disease mounts phase II antibodies (immunofluorescence IgG phase II at 1:200 or above, or a four-fold rise), chronic disease mounts phase I antibodies exceeding phase II — phase I IgG of 1:800 or more suggests chronic infection. Acute Q fever is treated with doxycycline 100 mg twice daily for 14 days; chronic Q fever needs doxycycline plus hydroxychloroquine for a year or longer to prevent relapse, with valvular surgery as needed.
What you must remember
- Reservoir and route: sheep, goats and cattle shed enormous numbers of organisms in placentas and birth fluids; parturient farm animals, abattoir work, veterinary assistance and contaminated straw or dust are the exposure anchors — direct animal contact is not required, and human-to-human spread is rare.
- Phase antibody rule: acute — phase II IgG dominates; chronic — phase I IgG dominates and a titre of 1:800 or more flags chronic disease; seroconversion takes 2-3 weeks.
- Clinical forms: self-limited fever, atypical pneumonia (patchy radiographic shadows), granulomatous hepatitis with raised transaminases and, classically, doughnut granulomas on biopsy; fever with splenomegaly and negative routine cultures is the recognisable stem.
- Chronic faces: culture-negative endocarditis of a pre-damaged or prosthetic valve with vegetations that may be small, plus arterial aneurysm or graft infection, chronic hepatitis, and in pregnancy adverse outcomes — any of these with livestock exposure deserves phase I serology.
- Treatment, acute: doxycycline 100 mg twice daily for 14 days (or longer with pneumonia recovery lag); the fever defervesces within 48-72 hours — failure to do so questions the diagnosis.
- Treatment, chronic: doxycycline 100 mg twice daily plus hydroxychloroquine 200 mg three times daily, typically 18-24 months, hydroxychloroquine raising lysosomal pH so doxycycline can act; monitor for retinal toxicity and photosensitivity.
- Special situations: pregnancy gets co-trimoxazole to lessen fetal risk; valvular heart disease after acute infection carries risk of progression to chronic Q fever, so serological follow-up guides escalation.
- Indian reality: serosurveys from Tamil Nadu, Karnataka and north Indian farm belts report appreciable exposure in livestock handlers, but the diagnosis is rarely made — Q fever belongs in the Indian fever-of-unknown-origin and culture-negative endocarditis differential.
From abattoir to antibody phases
A 44-year-old abattoir worker has ten days of high fever, dry cough and headache; blood cultures are sterile, Widal and dengue negative, transaminases doubled, and the chest radiograph shows patchy lower-zone opacities unresponsive to ceftriaxone. Step one re-reads the exposure: livestock parturition and slaughter-house aerosol place Coxiella on the list ahead of unguided antimicrobial escalation. Step two tests phase II IgG by immunofluorescence — acute and convalescent samples two to three weeks apart, accepting 1:200 or a four-fold rise. Step three starts doxycycline 100 mg twice daily with defervescence within two days, continuing the full two weeks. Step four asks the prognostic question — underlying valve disease: echocardiography now and phase I serology on follow-up, since a seeded pre-damaged valve is the substrate of chronic endocarditis, and persistent phase I titres above 1:800 convert the plan to long doxycycline-hydroxychloroquine therapy.
How the examiner frames it
Coxiella questions cluster around three contrasts. First, the rickettsial family tree: Coxiella is not Rickettsia — no rash, no eschar, no tick history; transmission is aerosol from animals, and it stains poorly with Gram but well with Gimenez. Second, the phase-antibody logic is a ready-made one-best-answer: "IgG phase I 1:1600, phase II 1:400 in a farmer with a prosthetic valve and nine months of low-grade fever" — chronic Q fever endocarditis, treated with doxycycline plus hydroxychloroquine for over a year. Third, the endocarditis culture-negative list: Coxiella and Bartonella are the pair to name before HACEK, and Q fever endocarditis is the diagnosis where serology — not culture — is the reference standard.
Frequently asked questions
Which antibody pattern indicates acute versus chronic Q fever?
Acute disease shows dominant phase II IgG with a four-fold rise; chronic disease shows phase I IgG at 1:800 or higher, exceeding phase II titres.
How is Q fever transmitted to humans?
Inhalation of aerosols or dust contaminated with birth products, urine, faeces or milk of infected sheep, goats and cattle — tick bites and person-to-person spread are rare.
What is the treatment of acute Q fever?
Doxycycline 100 mg twice daily for 14 days, with defervescence typically within 48-72 hours.
Why is hydroxychloroquine combined with doxycycline in chronic Q fever?
It alkalinises the phagolysosome, preventing Coxiella from maintaining its acidic intracellular niche and allowing doxycycline to eradicate infection over 18-24 months.
Where does Q fever sit in the culture-negative endocarditis differential?
Alongside Bartonella at the top of the list, diagnosed by phase I serology rather than culture, particularly in patients with livestock exposure.