Seasonal occurrence of Culex tritaeniorhynchus summorosus in Toyama Prefecture, with special references to the effect of aerial application of carbaryl dust on rice-fields and Japanese encephalitis

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  • 富山県におけるコガタアカイエカの消長と農薬空中散布の影響および日本脳炎流行との関連
  • トヤマケン ニ オケル コガタアカイエカ ノ ショウチョウ ト ノウヤク クウチュウ サンプ ノ エイキョウ オヨビ ニホンノウエン リュウコウ ト ノ カンレン

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A series of studies on the seasonal prevalence of Japanese encephalitis virus in relation to the most important vector mosquito, Culex tritaeniorhynchus summorosus, have been conducted since 1965. Five pigpens, two cowhouses and the Toyama Airport, located near the paddy field zone in Toyama Prefecture, have been selected as experimental sites. The results and conclusion are summarized as follows : The prevailing season for the vector mosquito population was, in general, from late July to late August. Numbers of mosquitoes, however, have decreased with the increase of early rice crop in recent years. This pattern seemed to have been affected by helicopter-spraying of the insecticides against rice insects. In 1968, for instance, a spray test with 1.5% carbaryl (0.3kg/acre) at the beginning of August resulted in keeping the vector mosquito population under control, although the mosquitoes still became infected with the virus from the 5th to 26th of August and the virus isolation rate was still high. That is, the vector mosquito population was so small and so was that of infected mosquitoes in 1968. This may explain that the population of swine infected with the virus never reached to 100% and that there was no virologically or serologically diagnosed human Japanese encephalitis cases in Toyama Prefecture in 1968. No attempt has been made in this country to control the adult mosquitoes in paddy fields. It is conceivable, however, that an extensive aerial application of insecticides such as carbaryl over the dry paddy fields for early rice crop, ordinarily seen in Toyama Prefecture, at the time when mosquitoes become infected with the virus, results in much reduction of the vector mosquito population and Japanese encephalitis virus.

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