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Q Fever: Epidemiologic Overview for Clinicians


  • Q fever is caused by the obligate intracellular rickettsial agent C. burnetii.
  • The organism is pleomorphic and has an envelope similar to that of gram-negative bacteria.
  • The organism is resistant to heat, drying, and many common disinfectants.
  • These features enable the bacteria to survive for long periods in the environment.

Geographic Distribution

  • Q fever is found worldwide, except in New Zealand.
  • Human cases of Q fever have been reported from across the United States.
  • On average, approximately 50-60 cases of Q fever are reported in the United States each year, and the average annual reported incidence is 0.28 cases per million persons.
  • Q fever is believed to be under-diagnosed and under-reported in the United States, so the number of reported cases likely does not reflect the true incidence of disease.

Incubation Period

  • The incubation period for Q fever varies depending on the number of organisms that initially infect the patient. Infection with greater numbers of organisms will result in shorter incubation periods.
  • Most persons become ill within 2-3 weeks after exposure.
  • Approximately half of infected persons will be asymptomatic.


  • Infected cattle, sheep, and goats are the primary reservoirs of C. burnetii.
  • Infection has been noted in a wide variety of animals, including other species of livestock, domesticated pets, wild mammals, pigeons, and ticks.


  • C. burnetii does not usually cause clinical disease in infected animals, although abortion in goats and sheep has been linked to infection with this bacteria.
  • Asymptomatic animals can shed C. burnetii for long periods of time in reproductive secretions, milk, urine, and feces.
  • High numbers of C. burnetii are shed in the reproductive fluids and placentas of infected animals, and the risk for disease transmission from these animals is highest during and shortly after giving birth.
  • Infection of humans usually occurs by inhalation of these organisms in small droplets or from inhalation of barnyard dust contaminated with C. burnetii.
  • Infection via ingestion of contaminated dairy products is a less common mode of transmission.
  • Other modes of transmission to humans, including tick bites and human-to-human transmission, are rare.
  • Humans are often highly susceptible to the disease, and very few organisms may be required to cause infection.