Résumé
Little is known about the distribution of Rhodococcus equi in the soil environment of native horses in China. One hundred and eight soil samples were collected from native-horse farms in the Hulun Beier grasslands of eastern Mongolia, the Xilin Goler grasslands of southern Mongolia, and Tongliao city in Inner Mongolia of China for investigating the distribution of R. equi in these regions. The isolation rates of R. equi from soil samples from the Hulun Beier and Xilin Goler grasslands ranged from 25.9% to 30.0%. In contrast, isolation rates from soil samples from Tongliao city was as high as 82.3% and the mean number of R. equi in soil samples from Tongliao city was 10 times more than those of samples from the grasslands. The 488 isolates were examined using PCR for the presence of genes that encode virulence-associated 15 000-17 000 antigen protein (VapA) and the 20 000 antigen protein (VapB). All isolates were negative for virulence-associated proteins. Plasmid profiles of these avirulent isolates showed that cryptic plasmids of various sizes were present with an incidence of 13.3% to 21.5%. The results of the present study contrast with those of our recent study, in which we reported that R. equi was absent from Mongolian horses in Ulaanbaatar, Mongolia. It is suggested that the difference between the results of these two studies is due to the mobile pasturing system in Mongolia and nonmobile pasturing system in Inner Mongolia.