Although about 30% of foals that develop Rhodococcus equi foal pneumonia do not survive, the majority of foals can be treated successfully and proceed with a normal life. But why is it that only foals are affected by this respiratory disease? According to M. Julia B. Felippe, DVM, MS, PhD, Dipl. ACVIM, of Cornell University’s College of Veterinary Medicine, one of the contributing factors is that some aspects of the foal’s immune system take time to develop and, thus, contribute to susceptibility to disease.

Felippe summarized research findings on the topic at the 2010 American Association of Equine Practitioners Convention, held Dec. 4-8, 2010, in Baltimore, Md.

“The unique susceptibility of young foals to R. equi disease is still puzzling, despite many studies investigating their innate and acquired immune systems,” Felippe said. Part of the puzzling aspect is why only certain foals contract the disease, while other foals remain healthy.

Despite the fact that many foals with naive immune systems (they haven’t been exposed to the pathogen in question before) are exposed to R. equi, only some of them develop disease in the face of that exposure. According to Felippe, this points towards some foals having individual risk factors that make them more susceptible to the R. equi bacterium