Tularemia

Tularemia is a zoonosis caused by the bacterium Francisella tularensis. It occurs naturally in lagomorphs (rabbits and hares) and rodents, especially microtine rodents such as voles, vole rats and muskrats, and also in beavers. In addition, a wide variety of other mammals have been reported to be infected, and the organism has been isolated from birds, fishes, amphibians, arthropods, and protozoa. Haematophagous arthropods have a substantial role both in the maintenance of F. tularensis in nature and in disease transmission. The disease is characterised by fever, depression and often septicaemia. In humans, there may be ulcers or abscesses at the site of exposure (this is rarely seen in animals) and swelling of the regional lymph nodes. The two clinically most relevant types of F. tularensis are recognised on the basis of culture characteristics, epidemiology, and virulence. Francisella tularensis subsp. tularensis (Type A) is mainly associated with lagomorphs in North America and is primarily transmitted by ticks or biting flies or by direct contact with infected animals. It is highly virulent for humans and domestic rabbits, and most isolates ferment glycerol. Francisella tularensis subsp. holarctica (Type B) occurs mainly in aquatic rodents (beavers, muskrats) and voles in North America, and in lagomorphs (hares) and rodents in Eurasia. It is primarily transmitted by direct contact or by arthropods (primarily ticks and mosquitoes) but may also be transmitted through inhalation or through infected water or food. It is less virulent for humans and domestic rabbits, and does not ferment glycerol. In sensitive animals, clinical signs of severe depression are followed by a fatal septicaemia. The course of the disease lasts approximately 2–10 days, and animals are usually dead when presented for diagnosis. Most domestic species do not usually manifest signs of tularemia infection, but they do develop specific antibodies to the organism following infection. Outbreaks with high mortality caused by the Type A organism have occurred in sheep. Among domestic pets, F. tularensis infection can result in clinical illness in cats but less commonly in dogs. Both have been implicated in transmission of the disease to humans, from cats to humans, most commonly via bites or scratches and from dogs via close facial contact, ticks, and retrieved carcasses, as well as bites. Novel vaccines against tularemia are under development but not yet licensed for human or animal use.