Prevalence of Virulence Plasmids in Environmental Isolates of Rhodococcus aqui from Horse-Breeding Farms in Hokkaido

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 of 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.