Studies on the population dynamics of the principal vector mosquito of Japanese encephalitis

Abstract

The incidence of Japanese encephalitis (JE) in Japan has decreased in recent years as a secondary consequence of modernization of the agricultural system in this country. The population of the vector mosquito Culex tritaeniorhynchus has decreased in rice fields where new methods for rice cultivation have been applied. Nowadays, rice fields have small amounts of water due to a technology which controls the management of an intermittent water supply, new varieties of rice have been developed to shorten the period of cultivation, and pesticides are extensively used. These facts have transformed the rice fields into inadequate places for mosquito 'breeding'. The following reasons also contribute to the drastic reduction of the vector mosquito with consequent decrease in incidence of JE: very few herds of animals are raised in the farms, the large-scale pig sties are located far from human habitation, and modern construction methods of houses and pig sties hamper the entrance and feeding of mosquitoes. Recently, projects for development of rice fields and pig breeding have been promoted in Southeast Asia with the purpose of increasing food production. Such promotion is giving rise to the spread of JE in Southeast Asia and this is, therefore, imported to Japan. Moreover, global warming, changes in the environment and the emergence of insecticide-resistant mosquitoes increase the risk of epidemics in Japan and other Asian countries.