Q fever is a zoonotic disease caused by the intracellular bacterium Coxiella burnetii. Although it has a wide and diverse host range, in animals this organism is primarily known as a cause of reproductive losses in domesticated ruminants.
C. burnetii is a highly pleomorphic coccobacillus with a gram-negative cell wall. It measures 0.3 x 1 μm ; unlike true rickettsiae it enters the cell by a passive mechanism. Within the cell it survives within the phagolysosome - the low pH of this environment is necessary for the metabolic functioning of C. burnetii.
People usually get Q fever by breathing (aerosol) contaminated barnyard dust or by direct contact with infected animals while assisting with the delivery of newborn animals. Occasionally people can get Q fever by drinking (oral) contaminated milk or from tick bites (vector).
Humans infected with C. burnetii often seroconvert without clinical signs or develop a mild, self-limited, flu-like illness. However, this organism can cause more serious syndromes, including pneumonia and reproductive losses. A few people, generally those with pre-existing abnormalities of heart valves or blood vessels, develop life-threatening sequelae.
Sheep, goats and cattle are most likely to get Q fever. Other animals that can get the disease include dogs, cats, rabbits, horses, pigs, camels, buffalo, rodents, and some birds.
Two characteristics of the organism are important in the epidemiology of the disease. These are its ability to withstand harsh environmental conditions, probably as a result of spore formation, and its extraordinary virulence for man. A single organism can cause disease in man.
Zoonotic disease of Q fever
