Rhodococcus equi is an organism that lives in soil, requiring warmth and nutrients found in horse manure to grow and spread among equine populations. It is the most common cause of pneumonia in foals one to four months of age, bringing costly losses on affected breeding farms. Recently, a study was conducted at Texas A&M University in an effort to identify characteristics of breeding farms that put them at risk for outbreaks of R. equi in foals.

With the help of 32 area veterinarians, data was collected over a one-year period from 64 breeding farms housing almost 10,000 horses and more than 2,700 foals. Statistical analyses showed that farms most likely to be affected by R. equi pneumonia were the largest farms, those larger than 200 acres with 60 or more acres devoted to horses. These farms often had many horses in residence, which was found to be another risk factor when the population topped 160. Increased density of foals on a farm, greater than 0.25 foals per acre, was found to be a risk factor, as was a large number of resident mare/foal pairs and many transient mare/foal pairs. The authors concluded that large-acreage breeding farms with many horses, many mares, a significant number of transient mare/foal pairs, and a high foal density are at higher risk of R. equi pneumonia in foals.

Chaffin, M.K.; Cohen, N.D.; Martens, R.J. Journal of the American Veterinary Medical Association 222 (4), 467-475, 2003.