Abstract
Sixteen strains of Salmonella enterica serotype Typhimurium (S. Typhimurium) isolated from foals from 1981 until 1996 in Hokkaido Prefecture, Japan, were examined for their biotype, drug resistance, plasmid profile, virulence plasmid, and virulence in mice and foals. Nine strains isolated from Thoroughbred foals in Hidaka district in 1981 were shown to possess similar properties. Likewise, two strains isolated from crossbred foals in a farm in Sorachi district in 1996, although distinct from the Hidaka isolates, were similar to one another. These 11 strains were all shown to contain a virulence plasmid, and to be virulent in mice and foals. However, five other strains isolated from Thoroughbred foals that were bred in different farms in Hidaka between 1990 and 1993 demonstrated variable properties, and did not possess the virulence plasmid nor show virulence in mice. Thus it was suggested that large outbreaks of S. Typhimurium infection in foals were more likely to be caused by virulent strains possessing the virulence plasmid, although sporadic infections may also be caused by non-virulent S. Typhimurium.
