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Key Features

Essentials of Diagnosis

  • Exposure to sheep, goats, cattle; some laboratory-acquired infections

  • Acute or chronic febrile illness: headache, cough, prostration, and abdominal pain

  • Pneumonitis, hepatitis, or encephalopathy; less often, vascular infections or chronic fatigue syndrome

  • A common cause of culture-negative endocarditis

General Considerations

  • Q fever, a reportable and significantly underestimated disease in the United States, is caused by the gram-negative intracellular coccobacillus Coxiella burnetii

  • Human infection occurs via inhalation of aerosolized bacteria (in dust or droplets) from feces, urine, milk, or products of conception of infected animals

  • Ingestion and skin penetration are other recognized routes of transmission

  • There is a known occupational risk for animal handlers, slaughterhouse workers, veterinarians, laboratory workers, and other workers exposed to animal products

  • In the United States, over 60% of cases do not report an exposure to potentially infectious animals; drinking raw milk may be an infectious exposure

  • Human-to-human transmission does not seem to occur, but maternal-fetal infection can occur

  • Chronic Q fever is now termed "persistent focalized infections"

Clinical Findings

  • Asymptomatic infection is common

  • For the remaining cases, a febrile illness develops after an incubation period of 2–3 weeks, usually accompanied by headache, relative bradycardia, prostration, and muscle pains

  • Clinical course may be acute, chronic (duration 6 months or longer), or relapsing

  • Pneumonia and granulomatous hepatitis are the predominant manifestations in the acute form, whereas other less common manifestations include skin rashes (maculopapular or purpuric), fever of unknown origin, myocarditis, pericarditis, aortic aneurysms, aseptic meningitis, encephalitis, orchitis, iliopsoas abscess, spondylodiscitis, tenosynovitis, granulomatous osteomyelitis (more often seen in children), and regional (mediastinal) or diffuse lymphadenopathies.

  • Culture-negative endocarditis is most common presentation in patients with persistent focalized infections

    • Risk factors include

      • Immunocompromised state

      • Preexisting valvular conditions

      • Male sex

      • Age above 40 years

      • Valvular prosthesis (mechanical or bioprosthesis)

    • Clinical manifestations of endocarditis are nonspecific with fever, night sweats, and weight loss

    • Sudden cardiac insufficiency, stroke, or other embolic and mycotic aneurysms can develop

  • New infection or reactivation of Q fever can occur in pregnant women and is associated with

    • Spontaneous abortions

    • Intrauterine growth retardation

    • Intrauterine fetal death

    • Premature delivery

    • Oligohydramnios (when infection occurs during first trimester)

Differential Diagnosis

  • Viral, mycoplasmal, and bacterial pneumonias

  • Viral hepatitis

  • Brucellosis

  • Legionnaire disease

  • Kawasaki disease

  • Tuberculosis

  • Psittacosis


Laboratory Findings

  • Elevated liver biochemical tests

  • Leukocytosis

  • A fourfold rise between acute and convalescent sera by indirect immunofluorescence is diagnostic of the infection

  • Real-time PCR for C burnetii DNA is helpful only in early stage of infection

  • Diagnostic tests using Immuno-PCR and combining PCR with ELISA improve the sensitivity and specificity during the first 2 weeks after the onset of symptoms

  • An automated epifluorescence assay has > 95% sensitivity for the detection of phase I antigens in persistent infection

    • Phase variation is the change that occurs in the outer lipopolysaccharide membrane ...

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