Study of homologous and heterologous antibody response in California horses vaccinated with attenuated Venezuelan equine encephalomyelitis vaccine (strain TC-83).

Of 359 horses vaccinated with attenuated Venezuelan equine encephalomyelitis (VEE) vaccine (strain TC-83), 87% developed hemagglutination-inhibition (HI) antibodies to VEE virus within 1 month. Blood from a subsample of 101 of the 359 horses was obtained over a 1-year period. Within 1 month after vaccination, 84% of the 101 horses had developed VEE HI antibodies, 87% had developed VEE-neutralizing (Nt) antibodies, and 78% had developed VEE complement-fixing (CF) antibodies. One year after vaccination, 58% of the horses had VEE HI antibodies and 73% had VEE Nt antibodies. The percentage of horses with VEE CF antibody titers dropped to a low level (46%) within 6 months after vaccination, and most horses were seronegative for VEE CF antibodies 1 year after vaccination. The presence of antibodies to heterologous (western equine or eastern equine encephalomyelitis, or both) alphaviruses suppressed VEE antibody formation in horses vaccinated with TC-83 vaccine. However, the proportion of horses that developed VEE antibodies exceeded levels that generally were believed adequate to suppress a VEE epizootic in a population. There was an 88% correlation between the HI and Nt tests for VEE antibodies.