Pulsed-field gel electrophoresis of restriction endonuclease-digested genomic DNA from a large collection of clinical isolates of Rhodococcus equi, an important pathogen of foals, was used to compare strain distribution between farms and over time. Forty-four strains were found among 209 isolates, with 5 of these accounting for over half the isolates and the 22 strains isolated more than once accounting for 90% of the isolates. The average genotypic diversity on each farm and in each year was found to be less than the genotypic 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 between years. A small number of strains on each farm were found to have caused at least half the clinical cases of disease, and these varied between farms and, to a lesser extent, years. Most strains were found on more than one farm, and some very similar restriction patterns were found among isolates from different continents, indicating that strains can be very widespread. Multiple strains were isolated in five of the six cases in which more than 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 carry the vapA gene, and these strains tend to be widespread, but individual farms tend to have particular strains associated with them.Rhodococcus equi is an important pathogen of foals worldwide, causing pneumonia mainly in the 2-to 4-month-old age group. Foals typically show signs of purulent bronchopneumonia, with secondary enteritis and enteric lymph node abscessation sometimes seen. Incidences of disease and mortality are quite variable between farms, and this variation is believed to be due to a combination of management factors and the prevalence of virulent strains on each farm.Virulence of R. equi in foals is associated with a family of plasmids of 85 to 90 kb, which carry a gene encoding VapA, a 15-to 17-kDa lipoprotein shown to be associated with virulence in foals and mice (16,17). Comparisons of clinical isolates from various species, using ribotyping (6), and of small numbers of isolates from bovine and equine feces (13) and humans and horses (1), using pulsed-field gel electrophoresis (PFGE), have found quite a large degree of diversity among strains. Two surveys of isolates from cases of bovine lymphadenitis (11, 12) found lower levels of diversity, with most isolates appearing to be related. However no studies have examined the genetic diversity of isolates from clinically diseased foals, which almost invariably carry the virulence plasmid.This study examined the genetic relationships between a large number of clinical isolates of R. equi, most of which possessed the vapA gene, in order to compare isolates from different locations and different years, as well as to investigate any association between strain and site of infection. MATERIALS AND METHODSBacterial isolates. Is...
Pulsed field gel electrophoresis of restriction endonuclease digested genomic DNA from a collection of clinical isolates of Rhodococcus equi was used to compare strain diversity on different Thoroughbred horse farms over time. Restricted diversity was found among the isolates tested, as the same strains were detected on multiple farms and in multiple years. Marked variation occurred in strain prevalence with some strains being represented by single isolates, and the most prevalent by 26 isolates. There were dominant strains on some farms and the prevalence of some strains differed between farms. Infection with multiple strains was noted in some cases where multiple isolates from a single foal were examined.
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