VIII.74 - Japanese B Encephalitis

Summary

Japanese B encephalitis is a relatively uncommon disease, even in areas where the infection is endemic. The disease is one of several caused by arthropod-borne viruses (arboviruses); carried by mosquitoes of the genus Culex, this one is a member of the family Togaviridae and genus Flavivirus and thus is an RNA virus. The species of Culex that is the most common insect vector for Japanese B encephalitis is Culex tritaeniorhyncus. The disease was first recognized and described in 1871, and the virus was first isolated in 1935. The infection may appear in epidemic or in sporadic outbreaks, and is carried particularly in swine, but also has been isolated from a variety of birds and from equine animals. The virus is distributed principally in East and Southeast Asia.

Epidemic outbreaks of Japanese B encephalitis, like those of arboviruses in general, tend to occur in regions that are usually dry and arid and, therefore, relatively free of viral activity; such areas may accumulate a large number of individuals who, because of lack of previous exposure, are relatively susceptible. Then with rain and the appearance of conditions favorable to the proliferation of the insect vector, epidemic outbreaks may occur, particularly where there are relatively high population densities of the human host and of the amplifying hosts such as equine or porcine animal species. In addition, there is evidence that for some arboviruses a change occurs in the relative virulence of the infecting strain, which may also account for an epidemic outbreak.