The amount and type of Rhodococcus equi in a farm’s soil is not an indicator of an increased likelihood of having foal pneumonia cases caused by this bug, reported researchers from Texas A&M University. Additionally, farms with a greater density of mares and foals on the property are more likely to have cases of foal pneumonia attributable to Rhodococcus equi than farms with fewer horses.

Rhodococcus equi (R. equi) is a bacterium that causes pneumonia in foals. In North America the pathogenic (disease-causing) form of R. equi is endemic on some farms and approximately 10-20% of foals on endemic farms develop clinical signs of pneumonia.

“To date, it is unclear why some foals on some farms develop pneumonia caused by the pathogenic strain of R. equi and not others,” explained Noah Cohen, VMD, MPH, PhD, Dipl. ACVIM, of Texas A&M’s Department of Large Animal Clinical Science, and lead author on this study. “We performed this study to determine if an association existed between soil concentrations of the pathogenic strain of R. equi and the number of foals with R. equi-associated pneumonia.”

Investigators obtained soil samples from 37 farms in Central Kentucky and measured the concentrations of both total R. equi and the disease-causing form of R. equi in January, March, and July of 2006.

Sixteen of the 37 farms had foals affected with R. equi-associated pneumonia in 2006, while the remaining 21 farms were free of the disease.

Soil concentrations of total or pathogenic R. equi did not differ between the farms during the study period and the disease-causing strain of R. equi was identified in all of the 37 analyzed farms.

These results are important because they suggest that soil concentration of the bacterium and exposure of foals to the virulent (i.e., disease-causing) strain of R. equi do not predict which foals will develop pneumonia. That is, exposure to the pathogenic strain of R. equi is necessary, but not sufficient, to cause disease.

“Our study also found that farms with a greater density of mares and foals were more likely to have foals with pneumonia attributable to R. equi,” summarized Cohen.

According to Cohen, farm-to-farm differences in pneumonia are likely related to a complex and as-yet unidentified interaction between the bacterium, foal immune system, environment, and farm management practices. Research in this field is ongoing.

The study, “Association of soil concentrations of Rhodococcus equi and incidence of pneumonia attributable to Rhodococcus equi in foals on farms in central Kentucky,” was published in the March 2008 edition of the American Journal of Veterinary Research.

For more on earlier R. equi research, see R. Equi Pneumonia Linked to Large Farms, Lots of Horses.