Japanese encephalitis in Sarawak: Virus isolation and serology in a land Dyak village

55 viruses were isolated from approximately 174,300 mosquitoes collected in 2 areas of Sarawak between November 1968 and December 1969: 36 isolates, of which 4 have been provisionally identified as strains of Japanese encephalitis virus, came from C. tritaeniorhynchus and 7 isolates, of which 1 is a strain of Japanese encephalitis, from C. gelidus. The infection rate with Japanese encephalitis virus in the human population of Kampong Tijirak was estimated to be approximately 6% per annum. In pigs the infection rate was very much higher and was not uniform throughout the year. Pigs appear to become infected more frequently with increasing age and there was a considerable increase in the pig infection rate during the last quarter of the year coincident with the seasonal peak in C. tritaeniorhynchus population. The lower infection rate in man than in pigs is probably attributable to the considerably lower frequency with which man is bitten by C. tritaeniorhynchus. A considerable proportion of wild birds and of village dogs had Japanese encephalitis virus antibody and a few bats and ducks also had antibody. Fowls, geese and rodents do not appear to play a role in the maintenance of the virus.