Hemagglutination-inhibition tests with different strains of equine infectious anemia virus

The serologic relationships between 6 strains of equine infectious anemia (EIA) viruses were investigated by hemagglutination-inhibition (HI) tests. Cross HI tests, using sera from horses in the early stage of infection, revealed that all strains were inhibited only by homologous strain antisera and that HI antibody was always detectable before virus-neutralizing antibody. In the later stages of infection, both homologous and heterologous HI antibodies were detected in a sera of most of the horses, and the order of appearance of heterologous HI antibodies was random in 2 horses inoculated with the same virus strain. The viruses sequentially isolated from 1 horse had serologically the same hemagglutinating antigenicity to their parent virus. These results indicate that hemagglutinating antigens of EIA virus were type specific, but different from the antigens detectable by neutralization. The possible role of EIA virus hemagglutinin and HI antibody in causing periodic anemia is discussed.