The bacterium is extremely sustainable and virulent: a single organism is able to cause an infection. The common source of infection is the inhalation of contaminated dust, contact with contaminated milk, meat, or wool, and particularly birthing products. Ticks can transfer the pathogenic agent to other animals. Transfer between humans seems extremely rare and has so far been described in very few cases.Some studies have shown more men to be affected than women, which may be attributed to different employment rates in typical professions. "At risk" occupations include:
Veterinary personnel;
Stockyard workers;
Farmers;
Sheep shearers;
Animal transporters;
Laboratory workers handling potentially infected veterinary samples or visiting abattoirs;
People who cull and process kangaroos; and
Hide (tannery) workers. History
Q fever was first described in 1935 by Edward Holbrook Derrick in slaughterhouse workers in Brisbane, Queensland. The "Q" stands for "query" and was applied at a time when the causative agent was unknown; it was chosen over suggestions of abattoir fever and Queensland rickettsial fever, to avoid directing negative connotations at either the cattle industry or the state of Queensland.The pathogen of Q fever was discovered in 1937, when Frank Macfarlane Burnet and Mavis Freeman isolated the bacterium from one of Derricks patients. It was originally identified as a species of Rickettsia. H.R. Cox and Gordon Davis elucidated the transmission when they isolated it from ticks found in the US state of Montana in 1938. It is a zoonotic disease whose most common animal reservoirs are cattle, sheep, and goats.
It is considered a good agent in part because its ID50 (number of bacilli needed to infect 50% of individuals) is considered to be one, making it the lowest known. In animals
Q fever can affect many species of domestic and wild animals, including ruminants (cattle, sheep, goats, bison, deers...), carnivores (dogs, cats, seals...), rodents, reptiles and birds. However, ruminants (cattle, goats and sheep) are the most frequently affected animals, and can serve as a reservoir for the bacteria. Clinical signs
In contrast to humans, even though a respiratory and cardiac infection could be experimentally reproduced in cattle, the clinical signs mainly affect the reproductive system. Q fever in ruminants is therefore mainly responsible for abortions, metritis, retained placenta and infertility. The clinical signs vary between species. In small ruminants (sheep and goats), it is dominated by abortions, premature births, stillbirths and the birth of weak lambs or kids. One of the characteristics of abortions in goats is that they are very frequent and clustered in the first year or two after contamination of the farm. This is known as an abortion storm.In cattle, although abortions also occur, they are less frequent and more sporadic. The clinical picture is rather dominated by non-specific signs such as placental retentions, metritis and, consequently, fertility disorders. Epidemiology
With the exception of New Zealand, which is currently free of Q fever, the disease is present throughout the world. Numerous epidemiological surveys have been carried out.