The role of cell-mediated immunity has not been fully evaluated. Bunyaviruses are found throughout the world, but each serotype has a limited geographic distribution because it relies on one or, at best, a few arthropod species to maintain its natural cycle. Hantaviruses are maintained in rodent reservoirs and are not arthropod borne. Hantaviruses are maintained in a rodent reservoir. The rodents are asymptomatic and transmit virus to other rodents and humans by way of infected urine and perhaps other body secretions.
Humans are dead-end hosts.
Hantaviruses cause hemorrhagic fever with renal syndrome in Europe and Asia. The most serious disease of bunyavirus origin in the United States is La Crosse encephalitis. First recognized when there was a fatal case in La Crosse, Wisconsin, in 1960, it is now diagnosed not only in the north central United States, but also in much of the eastern portion of the country and in Canada. The closely related California encephalitis, snowshoe hare, and Jamestown Canyon viruses are also occasionally implicated as agents of encephalitis. The vector of La Crosse virus is the woodland mosquito Aedes triseriatus , which breeds in tree holes.