Virus isolations from mosquitoes collected during the 1982 Japanese encephalitis epidemic in northern Thailand

From 16 June to 15 August, 1982 CDC light traps were used to collect mosquitoes in the province of Kamphaengphet, N. Thailand. 353,042 mosquitoes comprising 59 species were collected and identified, and 345,173 were placed in pools for attempted virus isolation by inoculation of C6/36 Aedes albopictus mosquito cell cultures. Viruses were isolated from 63 mosquito pools. These comprised 56 flaviviruses, identified as 35 isolates of Japanese encephalitis (JE) virus strains, 18 strains of Tembusu (TEM) virus and three untyped flaviviruses (FLA); three alphaviruses, identified as the first isolates of Getah (GET) virus to have been made in Thailand; and four viruses which are still unidentified. Most virus isolates were from Culex tritaeniorhynchus mosquitoes collected in carbon dioxide baited light traps. JE virus was isolated only over a ten-day period and the last isolate was obtained one week before the peak of admission of human encephalitis cases at Kamphaengphet Provincial Hospital. Rapid screening of isolates grown on Ae. pseudoscutellaris (LSTM-AP-61) mosquito cells by indirect immunofluorescence using flavivirus group-specific and JE-specific monoclonal antibodies showed a high degree of correlation with plaque reduction neutralization tests. An antigen capture enzyme immunoassay (EIA) test successfully identified about 50% of the JE virus positive pools, but the method saved considerable processing time.