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Mosquito collections by using light traps have been carried out at 10 to 11 stations in Kyoto City area at intervals of about 10 days every year. Mean percent indexes (MPI), being calculated from the data of mosquito collections, were used for comparison of the annual abundance of mosquitoes. It is no doubt that Culex tritaeniorhynchus summorosus has decreased recently and this decrease is correlated with the reduction of human patients of Japanese encephalitis. Wide use of two herbicides, CNP and nitrofen, for rice plant cultivation, may probably be one of the reasons for the decrease of the mosquitoes.

SUMMARY : When vector mosquitoes engorged by feeding on pigs are tested for virus recovery after incubation for 7 to 10 days, the results may show mosquito infection itself. Therefore, seasonal prevalence of infection in each pig population can be estimated from course of the infection rate among mosquitoes. Many mosquitoes of the main vector of Japanese encephalitis virus, Culex tritaeniorhynchus summorosus, collected by light traps everyday or every other day in some pig sheds from 1967 to 1970 were tested for virus recovery after incubation. The tests were positive during about a month period each summer, and the peak infection rate was high being over 10%. The course of the mosquito infection showed a certain pattern with one or two peaks between the initial recovery and the highest peak. From the interval of 12 to 13 days after the first recovery to the peak , cyclic infection b etween the pig and the mosquito may occur at the same interval.

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