Q fever

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"Q fever" AND "Coxiella burnetii"

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Q Fever

Q fever is a systemic zoonotic infection caused by Coxiella burnetii, a small pleomorphic gram-negative coccobacillus. It was originally classified as a rickettsia due to its intracellular lifestyle and tick association, but is now known to be genetically distinct from the Rickettsiaaceae family.

Microbiology

C. burnetii has several unique features:

Survives in harsh environments (viable in soil, manure, and hay for years).
Escapes intracellular killing in macrophages by inhibiting phagosome maturation, then adapts to the acidic phagolysosome to replicate.
Exists in two antigenic phases: Phase I (virulent, wild-type) and Phase II (avirulent, deleted mutant - more reactive in acute infection serology).
Has an extremely low infectious dose (probably 1-10 viable organisms), making it a potential bioterrorism agent (CDC category B).

Epidemiology

Source: Primary reservoir is infected sheep, goats, and cattle. Birds, cats, dogs, rabbits, and many wild animals can also be hosts.

Transmission:

Inhalation of contaminated aerosols - the dominant route. At parturition, enormous numbers of organisms are shed in placenta, fetal fluids, and membranes, contaminating the environment.
Milk (raw milk ingestion is uncommon but documented).
Rare: person-to-person via childbirth, surgery, autopsy, blood transfusion, organ transplantation, or sexual intercourse.

At-risk groups: Abattoir workers, veterinarians, farmers. However, outbreaks often affect people without any livestock contact (wind-borne spread over miles).

Geography: Worldwide except New Zealand and Antarctica. Endemic in Australia and France. Hyperendemic in French Guiana (40% of community-acquired pneumonias). The largest known outbreak: Netherlands, 2007-2010, with >4,000 reported cases and >40,000 infected, linked to high-density dairy goat farming near urban areas.

Seroprevalence: ~60% of infections are asymptomatic seroconversions. Only ~2% require diagnostic evaluation.

Pathobiology

C. burnetii naturally infects monocytes and endothelial cells, multiplying in the acidic phagolysosome.
Strains are genetically and antigenically heterogeneous; virulence varies.
Control of acute infection is associated with granuloma formation.
In immunocompromised hosts or those with valvular/vascular lesions, the organism can persist and multiply despite high antibody titers, causing chronic infection.

Clinical Manifestations

After infection: ~60% asymptomatic seroconversion, ~38% self-limited disease, ~2% require medical evaluation.

Acute Q Fever

Incubation ~2-3 weeks. Presentations include:

Syndrome	Notes
Self-limited febrile illness	Most common; lasts 1-3 weeks
Atypical pneumonia	35-40% of cases; chest X-ray shows round opacities
Granulomatous hepatitis	~60% have hepatitis; the sole manifestation in ~40%; fever + elevated aminotransferases is a key clue
Prolonged fever of unknown origin	15% of patients
Rare: meningitis, encephalitis, pericarditis, myocarditis, rash (4-18%), erythema nodosum

Key lab features:

WBC usually normal
Thrombocytopenia in ~25%; reactive thrombocytosis during recovery
Autoimmune markers (ANCA, ANA, anti-smooth muscle, antiphospholipid antibodies) often present
Case-fatality rate: ~1-2%

In pregnancy: Premature birth in 35%, abortion or neonatal death in 43% of symptomatic cases. Chronic uterine infection may develop in up to 50% of women infected during pregnancy, risking recurrent abortions later.

Q Fever Fatigue Syndrome

Up to 20% of patients develop prolonged fatigue post-acute infection.
Features: headaches, sweats, arthralgia, myalgias.
Pathogenesis unclear (cytokine dysregulation, genetic/host factors, biopsychological trigger).
Important: A randomized controlled trial showed doxycycline does NOT reduce fatigue vs. placebo - antibiotics should not be prescribed for this syndrome. Cognitive behavioral therapy targeting fatigue-related cognitions showed short-term benefit but this was not maintained at 1 year.

Chronic Q Fever

Develops in 1-5% of all patients (0.2-0.5% of all infected), typically months to years after primary infection (median 13 months; up to >9 years later).

Risk factors: Male sex, older age, valvulopathy, prior valve surgery, aortic aneurysm, vascular prosthesis, renal insufficiency, immunocompromised state.

Main forms:

Endocarditis (most common chronic form) - culture-negative; vegetations absent on echo in most cases; manifests as endothelium-covered valve nodules, aortic root abscesses, or rapid valve insufficiency.
Infected aneurysm or vascular prosthesis - can lead to aortoenteric, aortocaval, or arteriocutaneous fistula (1 in 6 vascular cases).
Osteomyelitis / vertebral osteomyelitis / prosthetic joint infection.
Lymphadenitis, chronic hepatitis, chronic pericarditis.

Clinical features: Low-grade or absent fever, progressive heart failure, weight loss, night sweats, fatigue, malaise. CRP often low or normal. Digital clubbing, splenomegaly, and hepatomegaly in endocarditis. Complications include emboli (cerebral), renal insufficiency, and progressive heart failure. Mortality of Q fever endocarditis: ~10%.

Diagnosis

Serology (Reference Method)

Indirect immunofluorescence assay (IFA) is the gold standard.

Condition	Diagnostic Criterion
Acute Q fever	Seroconversion or 4-fold rise in phase II IgG; or single sample IgG ≥200 + IgM ≥50 (phase II)
Chronic Q fever	Phase I IgG ≥800 (or ≥1600) AND IgA ≥100; a high phase I titer (e.g., >512) is suggestive but not diagnostic alone

Phase II antibodies dominate in acute disease; phase I antibodies rise in chronic disease.

Other Methods

PCR: Useful for culture-negative endocarditis and tissue samples. Blood PCR is negative in the majority of chronic Q fever patients.
Culture: Possible in BSL-3 facilities only.
FDG-PET/CT: Detects vascular and prosthetic valve infection, osteomyelitis, and lymphadenitis. Even after serology and PCR, FDG-PET/CT changed diagnosis or treatment in 20% of suspected chronic Q fever patients. Should be considered when chronic Q fever is suspected. Adding FDG valve uptake as a Duke criterion increased definite endocarditis diagnoses 1.9-fold.
Echocardiography: Vegetations detected in only 12% by TTE; 21-50% by TEE in Q fever endocarditis.

Treatment

Acute Q Fever

Doxycycline 100 mg twice daily (or 200 mg/day) for 14 days - first-line.
Quinolones are also effective.
Pregnancy: Doxycycline is contraindicated. Use TMP-SMX (cotrimoxazole 320/1600 mg daily) for the duration of pregnancy.
Patients with hepatitis and excessive immune response may benefit from a short glucocorticoid course (e.g., prednisolone 10 mg/day for 3 days).

Prophylaxis in High-Risk Patients

Patients with valvulopathy, prosthetic heart valves, aneurysm, or vascular prosthesis who have acute Q fever should receive doxycycline + hydroxychloroquine for 6-12 months to prevent progression to chronic disease.

Chronic Q Fever / Endocarditis

Doxycycline (200 mg/day) + Hydroxychloroquine (600 mg/day, adjusted to plasma level 1 mg/mL) for 18-36 months, guided by serial phase I IgG titers.
Major adverse effect: photosensitivity - patients must avoid sun exposure.
Alternative: Doxycycline + ofloxacin for ≥3 years.
Monitor treatment response via phase I antibody titers.
Valve replacement may be required and can prolong survival; combination therapy is essential to prevent relapse.
Consult an infectious disease specialist for chronic Q fever treatment decisions.

Prevention

Veterinary control of infected animal herds.
Q-VAX vaccine (phase I, formalin-inactivated): Available in Australia for occupationally at-risk individuals. Pre-vaccination skin test and serology required to avoid severe hypersensitivity reactions in previously sensitized individuals. No widely available vaccine elsewhere.

Key Distinguishing Points

Feature	Acute Q Fever	Chronic Q Fever
Serologic phase	Phase II antibodies dominant	Phase I antibodies dominant (IgG ≥800)
Onset	Weeks after exposure	Months to years post-infection
Main presentations	Pneumonia, hepatitis, fever	Endocarditis, vascular infection
Blood cultures	Negative	Negative
Treatment duration	14 days	18-36 months

Sources: Harrison's Principles of Internal Medicine 22E (2025), Goldman-Cecil Medicine, Jawetz Melnick & Adelberg's Medical Microbiology 28E

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