SummaryHorses e~perimentally infected with a cloned virus of equine infectious anemia (EIA) exhibited a chronical disease with periodical relapses of fever and simultaneously increased viremia. Virus isolates taken at successive febrile attacks proved to be changed in their antigenic specificity. These virus variants could be differentiated from one another by neutralizing, but not complement-fi~ing or precipitating antibodies, suggesting a modification of surface antigens only. No alteration of infectivity among these variants could be demonstrated. The supposition is made that the persistence of virus in the blood of infected horses might be maintained by a consecutive development of antigenically distinct virus populations not susceptible to neutralizing antibodies previously produced, and that the appearance and increase of such variants in the blood might be responsible for febrile relapses.

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A primary enzootic of equine Getah virus infection involving 722 of 1,903 racehorses occurred at a training center in Japan between September and November of 1978. Sixty-two viral agents were isolated from the plasma of 209 sick horses which exhibited pyrexia with rectal temperatures ranging from 38.5--40 degrees C, urticarial rash on various portions of the body, and edema of the hind legs. The viruses were antigenically related to the AMM 2021, Haruna, and Sagiyama strains of Getah virus. Infection and disease were produced experimentally in horses when inoculated by the intramuscular or intranasal routes.

Electropherotypes (ET), serotypes, and subgroups of equine rotaviruses isolated from foals in Japan were determined. The ETs of 136 isolates from 1981 through to 1991 were divided into six groups: ET-A-ET-F. The ET-A, -B, -C, -D, -E, and -F were present in 3, 1, 121, 9, 1, and 1 strains, respectively. Representative viruses of ET-A, -B, -C, and -D were identified as serotype G3. Viruses of ET-E and -F were identified as serotypes G 10 and G 5, respectively. The four representative viruses of serotype G 3 did not belong to either subgroup I or II. The two viruses of serotypes G 5 and G 10 belonged to subgroup I. Serotype G 3 strains possessing ET-C were prevalent among the foals throughout the study period.