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. Why, given the presence of Japanese B encephalitis virus in birds with wide-ranging migratory pat terns, the disease remains localized to certain geographic areas is not entirely clear, but presumably has to do with the specificity of the insect vector in carrying the infectious agent. The mosquito vector, C. tritaeniorhyncus, is found in rice fields and feeds on pigs and birds, as well as human and other hosts. However, the Japanese custom of raising pigs in the fall, after the flooding of rice paddies is over, and of taking the pigs to market early the following year, may also help to account for the general lack of large outbreaks as well as for the usual pattern of sporadic cases. As might be expected with such a pattern, the few large outbreaks tend to occur in more rural areas, street antibody to the virus is relatively common, and clinical cases account for about 2 percent of all of the infections, as judged by antibody surveys.

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