Prevalence of virulent Rhodococcus equi in isolates from soil collected from two horse farms in South Africa and restriction fragment length polymorphisms of virulence plasmids in the isolates from infected foals, a dog and a monkey

The prevalence of virulent Rhodococcus equi In soil Isolates from two horse farms in South Africa and nine clinical isolates from six foals, a foal fo etus, a dog, and a monkey was investigated. The isolates were tested for th e presence of virulence plasmid DNA and 15- to 17-kDa antigens by immunoblo tting. Rhodococcus equi was isolated from almost all of the soil samples ob tained from the two farms with 5.0 x 10(1) to 3.3 x 10(4) colony forming un its per gram of soil. Virulent R. equi was isolated from three soil samples from one of the farms and appeared In 3.8 % (three of 80 isolates), but no t in any of the 182 isolates from the other farm. Of the three virulent R. equi isolates, one contained an 85-kb type I plasmid and two an 87-kb type I plasmid. Of nine clinical isolates from the foals, foal foetus, dog and m onkey, five from the foals were virulent R. equi which expressed the virule nce-associated antigens and contained a virulence plasmid 85-kb type I, and were all isolated from cases of pneumonia typical of that induced by R. eq uiin young foals living in widely separated areas in South Africa. The isol ates from the other four foals, the dog and the monkey were avirulent R. eq ui.