Etiology and Epidemiology

Two variants (biovars) of the causative organism are recognized. F. tularensis biovar tularensis (type A) has been isolated in nature only in North America and is the most virulent in human beings. The second is designated F. tularensis biovar palaearctica (type B) and is found in all areas where tularemia is endemic in the Northern Hemisphere.

Tularemia is unique in the number of ways in which humans can become infected, and the clinical picture of the disease depends upon the infection route. The most common route is via the skin, either by insect bite or by direct passage through intact skin by contact with infected carcasses or a scratch from an infected animal. Of the numerous insects that transmit the disease, the tick is the most important. The wood tick (Dermacentor andersoni) and three species of rabbit tick are especially important in the United States. Biting insects such as the deer fly (Chrysops discalis) and the stable fly (Stomoxys calcitrans) also carry the disease to humans. Of the several species of mosquitoes shown to harbor the disease organism, only two species act as vectors to humans: Aedes cinereus and Aedes excrucians in Sweden and the former Soviet Union. Infection of the intestinal canal follows ingestion of contaminated water and undercooked meat. Humans can also contract infection via the respiratory route by inhaling the organism from such sources as contaminated hay and wool.

Susceptibility to tularemia is independent of age, sex, race, and health status. That men are more often infected is related to their intrusion into the transmission cycle through hunting and handling of infected, fur-bearing animals. Human-to-human transmission is extremely rare, and the disease is largely confined to rural areas. The disease may occur in any season but is least prevalent in winter when insect vectors are least abundant and small animals are not much hunted.

The fatality rate in North America, prior to the widespread use of streptomycin in the late 1940s, ranged from 5 to 9 percent. Today that figure has been reduced to less than 1 percent. In Europe, the mortality rate has always been much lower, in the realm of 1 percent owing, probably, to the lower virulence of F. tularensis strain. An attack confers relatively solid lifelong immunity. A live attenuated vaccine is now available that reduces the severity of the ulceroglandular infection and reduces the incidence of typhoid-type tularemia. However, the vaccine is still being investigated and used primarily for laboratory workers who are always at high risk in working with the tularemia organism.

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