Q fever was identified for the first time in Australia in 1935 and few years later in the United States. Its cause was unknown for several years and it was denominated Q fever from “query”.
The disease has a worldwide distribution and affects the lungs and less frequently other organs, such as the liver. Infection of livestock (cattle, sheep, goats) is very common, and once the microorganisms are shed by the animals, disease can be transmitted to humans. Disease is usually contracted by humans through the airborne route after manipulation of contaminated animal tissue or more rarely by ingestion of contaminated milk.
The causative agent
Coxiella burnetii is a rickettsia, an obligate intracellular bacterium that cannot be cultured in the lab without a host cell. It is a gram-negative coccoid or rod-shaped microorganism and is carried by insects. C. burnetii differs from other rickettsiae because it does not get transmitted by an insect bite and it is resistant to heat and desiccation. It produces a spore-like structure that makes it particularly resistant to killing. This feature explains the ability of this bacterial agent to survive in the air and effectively be transmitted to animals and humans via inhalation. Farmers, livestock, dairy workers, and laboratory personnel are the groups more at risk of becoming exposed to the agent.
The characteristics described above make C. burnetii a successful respiratory pathogen. This bacterium is highly infectious, as very few organisms are required to cause disease. It is included in the list of agents that could potentially be used in bioterrorism. In the laboratory, C. burnetii is handled under biosafety level 3 (BSL3) similarly to pathogens such as Francisella tularensis and Bacillus anthracis.
The disease
Following inhalation, the microbe settles in the lower respiratory tract and multiplies in the terminal part of the lungs. The incubation period is approximately three weeks and patients will present with high fever, severe headache, chills and occasionally nausea, vomiting and diarrhea. Most patients will recover without further complications, but sometimes the disease will progress to atypical pneumonia. More rarely the disease will invade the liver, causing a hepatitis-like illness, or the central nervous system.
Although most cases of Q fever are acute, a chronic form may develop weeks or even years after initial disease onset. The most susceptible hosts of this disease type are pregnant women, immunocompromised patients and individual with pre-existing heart conditions. These patients may develop severe endocarditis, which is highly fatal if untreated.
Diagnosis, antibiotic therapy and prevention
Q fever can be easily misdiagnosed because signs and symptoms in patients are aspecific and may resemble those of a viral illness. Physicians will often use medical history and information on patient's travel to rural areas with livestock as clues to determine the cause of the disease. Q fever is mostly diagnosed serologically, although tests may appear negative for the first week from initial onset. Two different antigens are used to distinguish between disease types. Antibodies to phase 2 are seen in the acute form of the disease, while antibodies to both phase 1 and 2 are observed in chronic disease.
Infection is generally treated with the antibiotics doxycycline or other tetracyclines. A vaccine composed of inactivated C. burnetii cells was developed in Australia for prevention against occupational exposure, but is currently not available in the United States. Other preventive measures include proper handling of the microorganism in a laboratory setting, careful manipulation of livestock and animal quarantine when necessary. Milk and its derivatives should always be pasteurized before consumption.
Sources
- CDC website, Q fever
- Medical Microbiology, Second Edition. Mims, Playfair, Roitt, Wakelin, Wlliams.
- Biology of Microorganisms, Eighth Edition. Brock.
Disclaimer: The information contained in this article is for educational purposes only and should not be used for diagnosis or to guide treatment without the opinion of a health professional. Any reader who is concerned about his or her health should contact a doctor for advice.
Damiana Chiavolini - I have written over ten articles published in peer-reviewed scientific journals, but my appetite for writing goes beyond microbiology, ...
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