STUDY OF JAPANESE ENCEPHALITIS VIRUS IN CHIANGMAI VALLEY, THAILAND IV. VECTOR STUDIES1

Abstract

Gould. D. J., R. Edelman, R. A. Grossman. A. Nisalak and M. F. Sullivan (SEATO Medical Research Laboratory. Bangkok, Thailand). Study of Japanese encephalitis virus in Chiangmai Valley, Thailand. IV. Vector Studies. Am J Epidemiol 100: 49–56, 1974.—As part of an overall study of the ecology of Japanese encephalitis virus (JEV) in the Chiangmai Valley, entomologic studies were made in an effort to identify the JEV vector species and to determine their seasonal abundance, host preferences and patterns of biting activity. During 1970 more than 400, 000 mosquitoes were collected from rural and urban sites in the valley and tested for JEV. Thirteen strains of JEV were isolated from mosquitoes collected between April and October: eight from pools of Culex tritaeniorhynchus, three from Culex gelidus and two from Culex fuscocephala pools. Two strains of Tembusu virus were isolated from C. gelidus and one isolation of this virus was made from a pool containing mosquitoes of the Culex vishnui complex. Twenty-nine other viral agents, as yet unidentified, were also isolated from mosquitoes during this period. Peaks in the population densities of C. fuscocephala, C. gelidus and C. tritaeniorhynchus, as measured by weekly light-trap collections, occurred during the rainy season (April–October) at the time of human and animal JEV infections. Analysis of mosquito blood-meals indicated that, while the three vector species fed on humans and a variety of domestic animals, their most frequent hosts were buffalo and cattle. Feeding activity by the three vector species began after dusk and was maintained throughout the night. These mosquitoes were predominantly exophilic, and very few were collected inside houses. All of these findings suggest that the three incriminated vector species act principally to maintain a cycle of JEV infections in domestic animal populations that only occasionally involves humans in the Chiangmai Valley.