The bacterium Francisella tularensis affects primarily lagomorphs (rabbits and hares) and rodents in the northern hemisphere, especially western North America, central Europe, and the former USSR, but also in Asia and Africa. However, it has a broad host range, affecting 190 species of mammals and 23 species of birds in addition to a few amphibians and numerous invertebrates. Among the more important hosts in North America are hares, Lepus spp.; New World rabbits, Sylvilagus spp.; water voles, Arviocola sp.; muskrat, Ondatra zibithecus ; American beaver, Castor canadensis ; lemmings, Lemmus spp.; voles, Microtus spp.; hamster, Cricetus cricetus ; and red - backed voles, Myodes spp. In Europe, the disease is common among European brown hare, Lepus europaeus ; varying hare, Lepus timidus ; common vole, Microtus arvalis ; house mouse, Mus musculus ; common shrew, Sorex araneus ; and others. Birds are relatively resistant, though gallinaceous game bird species such as grouse, as well as hawks, owls, and some waterfowl are susceptible.
Francisella tularensis is a highly infectious agent that enters the body in several ways, but primarily via inoculation by blood - feeding arthropods. Other routes of infection include inhalation of aerosols or handling and ingestion of contaminated water or meat. Among the important arthropod vectors are mosquitoes, fleas, tabanid flies, and ticks. A number of ticks from several genera are associated with transmission. Tularemia is primarily an acute disease, infecting the blood and causing infl ammation and necrosis in wildlife. The liver, spleen, bone marrow, and lungs are affected.
In western North America, various host and vector systems are evident. In the eastern USA, cottontail rabbit, Sylvilagus fl oridanus , is infected by ticks and biting fl ies, with tularemia serving as a regulatory mechanism for rabbit populations. In Canada and northern USA, however, muskrat, Ondatra zibethicus, and American beaver, Castor canadensis , are most affected, with the disease apparently water - borne. In both cases, humans are also at risk, both from tick bites and from handling rabbits. The effect on wildlife populations, especially beaver, is signifi cant in North America. In Europe, a different pattern occurs, with the vertebrate host primarily hares and the vector primarily mosquitoes.
Tularemia is rarely a problem for domesticated animals, though cats, dogs, horses, and sheep are occasionally affected. However, it is an issue for humans in areas where tularemia occurs. In some cases, it is advisable to delay the hunting season until cold weather reduces the density of ticks, and the probability of human infection. Also, trappers handling wildlife need to be aware of the risk. Finally, wildlife managers need to be sensitive to the population shifts due to incidence of tularemia, and may need to adjust harvest quotas in furbearers.
