Transmissions of Japanese encephalitis virus from chick to horse, from horse to horse and from horse to chick by the mosquito Culex tritaeniorhynchus were obtained in these studies. The viremia titers in three horses infected by mosquito bite ranged from less than 100.5 to 101.2 LD50; viremia appeared between the first and fourth days of infection and persisted for from two to six days. One animal, viremic during the first six days of infection, became febrile on the tenth day and developed frank encephalitis. The earliest serological response of these horses was marked by the appearance of hemagglutinin-inhibiting (HI) and complement-fixing (CF) antibodies on days 10 to 14; significant neutralization indices were not detected until the second to the fourth weeks. Both HI and neutralizing antibodies were present in the serum of one horse for more than a year after infection; however, CF antibodies were no longer detectable after that period of time. Challenge of this animal by mosquito bite more than a year after its first infection induced a significant rise in titer of all three types of antibody. Attempts to demonstrate viremia in this horse following challenge were negative.
Present address: Division of Research Services, National Institutes of Health, Bethesda, Maryland.