Molecular Epidemiology of Rhodococcus equi Based on traA, vapA, and vapB Virulence Plasmid Markers

Molecular typing of the actinomycete Rhodococcus equi is insufficiently developed, and little is known about the epidemiology and transmission of this multihost pathogen. We report a simple, reliable polymerase chain reaction typing system for R. equi based on 3 plasmid gene markers: traA from the conserved conjugal transfer machinery and vapA and vapB, found in 2 different plasmid subpopulations. This “TRAVAP” typing scheme classifies R. equi into 4 categories: traA + /vapA + B − , traA + /vapA − B + , traA + /vapAB − , and traA − /vapAB − (plasmidless). A TRAVAP survey of 215 R. equi strains confirmed the strong link between vapA (traA + /vapA + B − plasmids) and horse isolates and revealed other host-related plasmid associations: between traA + /vapA − B + and pigs and between traA + /vapAB − —a new type of R. equi plasmid—and cattle. Plasmidless strains were more frequent among isolates from nonpathological specimens. All plasmid categories were common in human isolates, which possibly reflects the predominantly opportunistic nature of R. equi infection in this host and a zoonotic origin.