Q fever

Last updated: June 2, 2021

Summary

Q fever is a notifiable zoonotic disease caused by Coxiella burnetii. Infection can occur directly (without a vector) or via vector transmission (e.g., inhalation of aerosols). There are 2 types of Q fever, acute and chronic Q fever. Acute Q fever occurs 2–6 weeks after infection and manifests mainly with flu-like symptoms and possibly atypical pneumonia and/or hepatitis. Chronic Q fever occurs months to years after infection and manifests with low-grade fever and often endocarditis. The best initial test for both, acute and chronic Q fever, is indirect fluorescent antibody test (IFA). Treatment for acute Q fever includes doxycycline and for chronic Q fever doxycycline PLUS hydroxychloroquine.

Epidemiology

Worldwide occurrence
In 2017, 153 cases of acute Q fever and 40 cases of chronic Q fever in the US ^[1]
70% of cases occur in men ^[2]
Peak incidence in April and May ^[1]

Epidemiological data refers to the US, unless otherwise specified.

Etiology

Pathogen

Coxiella burnetii (gram-negative, intracellular)
- Morphological similarities to Rickettsia
- Can survive in harsh environments in a spore-like form

Route of transmission

Direct infection (no vector transmission)
Vector transmission: primary reservoir are cattle, sheep, and goats
- Inhalation of spore-containing aerosols from the amniotic fluid or secretions of infected livestock
- Ingestion of raw milk produced by infected animals

Risk groups

Slaughterhouse workers, farmers, shepherds, veterinarians

Pathophysiology

Development of antigens
- Phase I antigens: seen when C. burnetii is highly infectious
- Phase II antigens: seen when C. burnetii is less infectious
- Antigenic shift is essential to differentiate acute Q fever from chronic Q fever (see “Acute Q fever vs. chronic Q fever” below)
Virulence factors of C. burnetii
- C. burnetii escapes macrophage phagocytosis by:
  - Producing superoxide dismutase to inactive phagolysosomal enzymes
  - Inhibiting cathepsin fusion
C. burnetii infection induces a range of immune responses, ranging from autoantibody production to immunosuppression
One of the two types of Q fever develop depending on the host's immune response:
- Acute Q fever: few C. burnetii organisms and stronger cell response to the pathogen
- Chronic Q fever: multiple C. burnetii organisms and weaker cell response (C. burnetii survives in monocytes and macrophages)

Acute Q fever vs. chronic Q fever

Types of Q fever
		Acute Q fever	Chronic Q fever
Incubation period ^[3]		2–6 weeks	Months to years
Clinical features		Often asymptomatic. Flu-like symptoms, which last for 1–2 weeks High-grade fever, chills, malaise, myalgia Headache, retroorbital pain, photophobia Rhinitis, pharyngitis, laryngitis Nausea, vomiting, diarrhea Atypical pneumonia: generally mild with nonproductive cough and fever Hepatitis, possibly hepatomegaly without jaundice Rare manifestations Meningoencephalitis Acute endocarditis associated with antiphospholipid syndrome Immunosuppression Post-Q fever fatigue syndrome: one of the most frequent sequelae of acute Q fever Fatigue, headache Night sweats Arthralgia, myalgia, fasciculations Blurred vision Painful lymphadenopathy Pregnancy: ↑ risk of spontaneous abortion, intrauterine growth retardation, intrauterine fetal death, and premature delivery	Low-grade fever Night sweats, fatigue Weight loss Swelling arms/legs Endocarditis: chronic Q-fever is the most common cause of culture-negative endocarditis. Culture-negative osteomyelitis
Diagnostics	Serology via IFA (best initial test)	Anti-phase II antibody IgG titers ≥ 200 and IgM titers ≥ 50 IgM antibodies against phase II antigens > antibodies against phase I antigens	Anti-phase I antibody IgG titers > 800 or persistently high levels of anti-phase I antibodies 6 months after completing therapy Antibodies against phase I antigens > antibodies against phase II antigens
	Additional findings	If IFA is negative, perform PCR of serum or tissue samples. Blood cultures are often negative.
	Additional findings	↑ Liver enzymes Leukocytosis, thrombocytopenia Chest x-ray: round opacities in patients with atypical pneumonia	Echocardiography: often nonspecific Modified Duke criteria can be used to diagnose endocarditis.
Treatment		First-line: doxycycline for 2 weeks Second-line: macrolides (e.g., azithromycin) Among pregnant women or children < 8 years with mild disease: trimethoprim-sulfamethoxazole	First-line: doxycycline PLUS hydroxychloroquine for ≥ 18 months Second-line: rifampin PLUS doxycycline/ciprofloxacin Valve repair may be required in the case of endocarditis.

Prevention

Avoid consumption of unpasteurized milk products

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