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Q fever, or Query fever, is a rickettsial disease caused by Coxiella burnetii and transmitted from animals to humans.

Symptoms and Signs

Only about one-half of all people infected with C. burnetii show signs of clinical illness. Q fever is usually asymptomatic or occurs as a mild disease with spontaneous recovery. Most acute cases of Q fever begin with sudden onset of one or more of the following: high fever, severe headache, general malaise, myalgia, confusion, sore throat, chills, sweats, non-productive cough, nausea, vomiting, diarrhea, abdominal pain, and chest pain. Fever usually lasts for 1 to 2 weeks. Weight loss can occur and persist for some time. Thirty to fifty percent of patients with a symptomatic infection will develop pneumonia. Additionally, a majority of patients have abnormal results on liver function tests and some will develop hepatitis. In general, most patients will recover to good health within several months without any treatment. The disease is fatal in 1 to 2% of people with acute Q fever.

Chronic Q fever, characterized by infection that persists for more than six months, is uncommon but is a much more serious disease. Patients who have had acute Q fever may develop the chronic form as soon as 1 year or as long as 20 years after initial infection. A serious complication of chronic Q fever is endocarditis, generally involving the aortic heart valves, and less commonly the mitral valve. Most patients who develop chronic Q fever have pre-existing valvular heart disease or have a history of vascular graft. Transplant recipients, patients with cancer, and those with chronic kidney disease are also at risk of developing chronic Q fever. As many as 65% of persons with chronic Q fever may die of the disease.

Mode of Transmission

Cattle, sheep, goats, birds and ticks are the primary reservoirs of C. burnetii. Infection has been noted in a wide variety of other animals, including other breeds of livestock and in domesticated pets. Coxiella burnetii does not usually cause clinical disease in these animals, although abortion in goats and sheep has been linked to C. burnetii infection.

Organisms are excreted in milk, urine, and feces of infected animals. Most importantly, during birthing the organisms are shed in high numbers within the amniotic fluids and the placenta. The organisms are resistant to heat, drying, and many common disinfectants. These features enable the bacteria to survive for long periods in the environment. Infection of humans usually occurs by inhalation of these organisms from air that contains airborne barnyard dust contaminated by dried placental material, birth fluids, and excreta of infected herd animals. Very few organisms are required to cause infection in humans.

Ingestion of contaminated milk, followed by regurgitation and inspiration of the contaminated food, is a less common mode of transmission. Other modes of transmission to humans, including tick bites and human to human transmission, are rare.

Coxiella burnetii could be developed for use in bioterrorism. The agent is a highly infectious and is rather resistant to heat and drying. It can become airborne and inhaled by humans. A single C. burnetii organism may cause disease in a susceptible person.

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 patients become ill within 2 to 3 weeks after exposure. Those who recover fully from infection may possess lifelong immunity against re-infection.

Laboratory Diagnosis

Because the signs and symptoms of Q fever are not specific to this disease, it is difficult to make an accurate diagnosis without appropriate laboratory testing. Results from some types of routine laboratory tests in the appropriate clinical and epidemiologic settings may suggest a diagnosis of Q fever. For example, a platelet count may be suggestive because persons with Q fever may show a transient thrombocytopenia. Confirming a diagnosis of Q fever requires serologic testing to detect the presence of antibodies to Coxiella burnetii antigens. The indirect immunofluorescence assay (IFA) is the most widely used method. Coxiella burnetii may also be identified in infected tissues by using immunohistochemical staining and DNA detection methods. Greater accuracy in the diagnosis of Q fever can be achieved by looking at specific levels of classes of antibodies other than IgG, namely IgA and IgM. Combined detection of IgM and IgA in addition to IgG improves the specificity of the assays and provides better accuracy in diagnosis.

The diagnosis of Q fever infection uses serologic tests to detect the presence of antibodies to Coxiella burnetti antigen. Immunologic responses to the two antigenic phases, Phase 1 and Phase 2 are important to distinguish between acute and chronic infection. Phase 2 antibodies are indicative of acute infection and become detectable within the second week of illness. Antibodies to Phase 1 generally take longer to appear and are indicative of continued exposure to the bacteria or chronic infection.

Seroconversion or a fourfold increase or decrease in antibody titre can be considered diagnostic of Q fever. Serological tests should be performed on acute and convalescent sera. The presence of elevated IgG and IgM to Phase 2 antibody is highly indicative of acute infection. High levels to Phase 1 IgG, IgA, and IgM in later specimens in combination with constant or falling levels of Phase II antibodies and other signs of inflammatory disease are suggestive of chronic Q fever or endocarditis. The level of Phase 2 antibodies in acute cases is usually much higher than any detectable Phase 1 antibodies. Significantly higher levels to Phase 1 antibodies are demonstrated in chronic Q fever. Be aware however, that antibodies to Phase 1 and Phase 2 can persist for months or years after initial infection.

For sending specimens to the Central Public Health Laboratory, physicians must call 416-235-6100 during work hours and 416-605-3113 after work hours prior to submission.

Specimens should be handled according to universal precautions and packaged for transport to the Central Public Health Laboratory according to the Transportation of Dangerous Good regulation.


Doxycycline is the treatment of choice for acute Q fever. Antibiotic treatment is most effective when initiated within the first 3 days of illness. A dose of 100 mg of doxycycline taken orally twice daily for 15-21 days is a frequently prescribed therapy. Quinolone antibiotics have demonstrated good in vitro activity against C. burnetii and may be considered by the physician. Therapy should be started again if the disease relapses.

Chronic Q fever endocarditis is much more difficult to treat effectively and often requires the use of multiple drugs. Two different treatment protocols have been evaluated: 1) doxycycline in combination with quinolones for at least 4 years and 2) doxycycline in combination with hydroxychloroquine for 1.5 to 3 years. The second therapy leads to fewer relapses but requires routine eye exams to monitor possible adverse effects of chloroquine on the eye. Surgery to remove damaged valves may be required for some cases of C. burnetii endocarditis.


Q fever is a reportable disease in Ontario under the Health Protection and Promotion Act and must be reported immediately to the local medical officer of health by telephone. The disease should be reported even if it is only suspected and has not yet been confirmed.

For more information
Call the ministry INFOline at 1-800-268-1154
(Toll-free in Ontario only)
In Toronto, call 416-314-5518
TTY 1-800-387-5559
Hours of operation : 8:30am - 5:00pm
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