California Encephalitis

The California serogroup within the genus Bunyavirus comprises more than a dozen viruses isolated from mosquitoes or vertebrates in various parts o< the world. The best studied and most important human pathogen in the group is La Crosse virus, which causes encephalitis in children and forestry workers in wooded areas of the United States. The virus is endemic, with about 100 cases of encephalitis reported annually, but serological surveys indicate that there are about 300,000 human infections annually, mostly in summer, throughout the Midwest; fortunately fewer than 1 case occurs for every 1000 infections in children. The ecology of the virus is described in the legend to Fig. 33-3. It has been said that "no two isolates are the same," probably because of the high frequency of genetic drift and perhaps occasional genetic reassortment.

Summer cycle

Clinical disease

California Encephalitis

^ Venereal

Overwintering cycle

Fig. 33-3 Cycle of California (La Crosse) encephalitis virus During the warmer months there is a cycle of the La Crosse virus between Aedes friscnatus, a woodland mosquito, and chipmunks and tree squirrels The virus is also maintained indefinitely in mosquitoes by transovarial transmission and is amplified by venereal transmission between male and uninfected female mosquitoes, which can in turn transmit either to a vertebrate by biting or to the next generation of mosquito by transovarial transmission Humans, who are dead-end hosts, are the only host known to develop clinical disease (Modified from R. T. Johnson, "Viral Infections of the Nervous System." Raven, New York, 1982.)

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