Intense transmission of Japanese encephalitis virus to pigs in a region free of epidemic encephalitis

Epidemic Japanese encephalitis recurs annually in the northern provinces of Thailand, but in the southern provinces cases of human encephalitis are rare. We investigated transmission of Japanese encephalitis virus (JEV) to pigs in southern Thailand. Blood specimens from one hundred young pigs at abattoirs in three southern provinces were tested for JEV hemagglutination inhibiting (HAI) antibodies. Seventy-four percent were positive. Ten seronegative sentinel pigs were placed at five locations in one southern province. Seven of the pigs developed JEV HAI and JEV IgM ELISA antibodies within two weeks of placement. JEV was isolated from all seven seroconverting sentinel pigs from blood specimens collected 3 to 11 days after placement. Fifteen light-trap mosquito collections at the five locations all included known JEV vectors, some in large numbers. We conclude that there is intense transmission of JEV to pigs to southern Thailand despite the rare occurrence of human encephalitis in the same region.