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

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 ricefields where new methods for rice cultivation have been applied. Nowadays, ricefields have small amounts of water due to technology which controls the management of 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 ricefields 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 teams of animals are raised in the farms, the large-scale pigsties are located far from human habitation, and modern construction methods of houses and pigsties hamper the entrance and feeding of mosquitoes. Recently, projects for development of ricefields and pig breeding are promoted in Southeast Asia with the purpose of increasing food production. Such promotion is giving rise to spread of JE in Southeast Asia and this is, therefore, imported to Japan. Moreover, the global warming, the changes in environment and emergence of insecticide-resistant mosquitoes increase the risk of epidemic in Japan and other Asian countries at any time. Therefore, the development of additional methods for vector control and strengthening of the alert attitude by health professionals and scientists are needed.