Epidemiology of Rhodococcus equi strains on thoroughbred horse farms

Pulsed-field gel electrophoresis of restriction endonuclease-digested genom ic DNA from a large collection of clinical isolates of Rhodococcus equi, an important pathogen of foals, was used to compare strain distribution betwe en farms and over time. Forty-four strains were found among 209 isolates, w ith 5 of these accounting for over half the isolates and the 22 strains iso lated more than once accounting for 90% of the isolates. The average genoty pic diversity on each farm and in each year was found to be less than the g enotypic diversity of the isolates taken as a whole, with 5.2% of the total diversity being due to differences between farms and 5.5% to differences b etween years. A small number of strains on each farm were found to have cau sed at least half the clinical cases of disease, and these varied between f arms and, to a lesser extent, years. Most strains were found on more than o ne farm, and some very similar restriction patterns were found among isolat es from different continents, indicating that strains can be very widesprea d. Multiple strains were isolated in five of the six cases in which more th an one isolate from a single foal was examined, indicating that disease may commonly be caused by simultaneous infection with multiple strains, It was concluded that there are a number of different strains of R, Equi which ca rry the vapA gene, and these strains tend to be widespread, but individual farms tend to have particular strains associated with them.