Rhodococcus equi is the most common cause of severe pneumonia in foals, said Keith Chaffin, DVM, MS, associate professor of medicine at Texas A&M University's College of Veterinary Medicine, at the 50th annual American Association of Equine Practitioners (AAEP) Convention in Denver, Colo., Dec. 4-8, 2004. However, despite intensive research, the reasons some foals get this pneumonia and some don't are not known.

A study from two breeding farms in Texas with endemic problems of R. equi has helped shed light on this problem. Chaffin said data from this study showed that peripheral blood lymphocyte (PBL) populations in two- and four-week-old foals are different for foals that later develop pneumonia than for foals that don't develop it. He said the differences were most pronounced during the second week of life, which is the period when foals most commonly become infected with the bacterium.

Chaffin said it is possible that there is a subset of foals that are relatively immunosuppressed compared with other foals, and thus are more susceptible to infection with R. equi. A new finding from this study was that total white blood cell counts and segmented neutrophil concentrations in peripheral blood for foals that later develop R. equi pneumonia were significantly lower than those of unaffected foals at two and four weeks of age.