An SzP-specific immunoblot is a useful, sensitive measure of diversity among strains of S. zooepidemicus. Single strains with SzP phenotypes similar to those found in tonsils of healthy horses cause pneumonia. Because of the diversity of SzP phenotype and genotype among isolates from animals with pneumonia, SzP phenotype is not an important determinant of invasiveness or epizootic capabilities.

The prevalence of virulent Rhodococcus equi in environmental isolates from 31 horse-breeding farms in Hidaka, Hokkaido was investigated: isolates were tested for the presence of virulence plasmid DNA and 15-to 17-kDa antigens by immunoblotting. R. equi was isolated from almost all of the soil samples obtained from the 31 farms with 102 to 105 colony forming units per gram of soil. However, virulent R. equi at various levels (ranging 1.7 to 23.3 %) was isolated from 24 of the 31 farms and appeared in 6.5% of the isolates (121 of 1,865). The ratio of 85-kb and 90-kb virulence Plasmids in the 121 isolates was about 3:1, which was similar to that previously reported about clinical isolates from infected foals in Hidaka. The similarity between the amounts o f virulence Plasmids obtained from the environment and from infected foals indicates that soil-borne virulent R. equi, which is thought to be a major source of the infection, constitutes a great risk to foals in horse-breeding farms. This study shows that plasmid profiles are useful markers in epidemiological surveys of R. equi infection in foals.

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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.