Rhodococcus equi is a facultative intracellular bacterium that causes pneumonia in foals.1–4 Expression of VapA by R equi is strongly associated with disease in foals, and detection of this protein or the vapA gene, located on an 85- to 90-kilobase plasmid, is used to classify isolates of this bacterium as virulent.4–6 Only virulent organisms are believed to cause disease; therefore, it is important to determine the virulence status of isolates in epidemiologic studies of R equi. The epidemiologic aspects of pneumonia in foals caused by this bacterium are not clearly characterized. Pneumonia in foals caused by R equi is endemic on some farms and develops intermittently on others but is not found on most farms.2,5 There may be year-to-year variability in foals with pneumonia attributable to R equi at farms where the disease is endemic.7 Generally, the proportion of foals with pneumonia attributable to R equi at endemic farms is approximately 10% to 20%, but proportions > 20% are not unusual.8,9 The reasons for this variation among and within farms remain unknown. Molecular epidemiologic data indicate that recurrence of the problem at a farm does not appear to be attributable to a particular genotypic strain of R equi at that farm, and various genotypes may be isolated from affected foals or their environment at a particular farm.10,11 Although the disease appears to be most common at larger breeding farms that use practices deemed desirable for management of optimum foal health,8,9 these criteria do not reliably differentiate farms with foals that have pneumonia attributable to R equi from farms with unaffected foals.
A possible explanation for the variation in prevalence of this disease among farms is that environmental exposure is greater at farms with affected foals than at farms with unaffected foals. Rhodococcus equi has been isolated from a wide range of environmental samples, including soil, air and airborne dust, chaff and animal feeds, troughs, and stall floors.4,12-18 Results of studies quantifying virulent isolates of R equi in soil samples from breeding farms are exiguous. In Japan, the proportions of virulent isolates in soil samples and fecal samples obtained from foals were significantly greater at an affected farm than at an unaffected farm.19 The proportions of virulent isolates from soil samples obtained from 3 farms with a history of foals with R equi were reportedly higher than historical data from unaffected farms in Japan.20,21 In Australia, however, soil concentrations are not significantly associated with prevalence of pneumonia attributable to R equi among foals at horse breeding farms, whereas airborne concentrations of R equi are significantly associated with disease prevalence.22 To our knowledge, data are lacking for North America regarding the association between disease status of farms with respect to pneumonia attributable to R equi and soil concentrations (or absolute and relative proportions) of virulent R equi. The purpose of the study reported here was to determine whether an association existed between the concentration or proportion of virulent R equi in soil samples and the incidence of foals with pneumonia attributable to R equi among horse breeding farms in central Kentucky. Selected characteristics of these farms also were recorded to account for potentially confounding effects in data analysis.
Virulence-associated protein A
Nalidixic acid, novobiocin, cyclohex-amide, and potassium telurite
American Type Culture Collection
Tris-buffered saline solution
Zero-inflated negative binomial
Adjusted odds ratio
Phosphate-buffered saline solution, pH 7.2, Invitrogen Co, Carlsbad, Calif.
Nitrocellulose membranes, pore size 0.45 Mm, Bio-Rad Laboratories, Hercules, Calif.
Hybridization oven, VWR International, West Chester, Pa.
Nonfat dry milk, Bio-Rad Laboratories, Hercules, Calif.
Tris-buffered saline solution, Bio-Rad Laboratories, Hercules, Calif.
Tween 20 solution, Bio-Rad Laboratories, Hercules, Calif.
Rocking platform, VWR International, West Chester, Pa.
Horseradish peroxidase–conjugated goat IgG fraction against mouse IgG, MP Biomedicals Inc, Aurora, Ohio.
Sodium citrate, Sigma Chemical Co, St Louis, Mo.
Ethylenediaminetetraacetic acid, Sigma Chemical Co, St Louis, Mo.
Dextran sulfate, Sigma Chemical Co, St Louis, Mo.
3,3',5,5'-tetramethylbenzidine, Sigma Chemical Co, St Louis, Mo.
S-PLUS, version 7.0, Insightful Inc, Seattle, Wash.
Stata, version 9.2, Stata Corp, College Station, Tex.
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