One of the major foal diseases in the United States is Rhodococcus equi foal pneumonia. Responsible for the deaths of up to 30% of infected foals, it is a serious problem at many large breeding farms. Noah Cohen, VMD, MPH, PhD, Dipl. ACVIM, professor of Large Animal Clinical Sciences at Texas A&M University's College of Veterinary Medicine & Biomedical Sciences, presented a study on the epidemiology ("scientific discipline concerned with quantifying the distribution of disease and determinants of disease and health in populations ") of R. equi at the 2010 American Association of Equine Practitioners Convention, which took place Dec. 5-8 in Baltimore, Md.
In his presentation, he reviewed published research about why some foals contract R. equi pneumonia in the same environment where other foals do not, and why R. equi is more prevalent at some farms but virtually nonexistent at others.
According to Cohen, virulent (disease-causing) R. equi has been isolated from several sources on breeding farms including feces (from both foals and their dams), horse feed, the soil, and the air. Especially at farms with high concentrations of foals, Cohen says it's likely that all foals are exposed to the disease-causing bacterium, but only a small portion of foals actually contract pneumonia.
He noted in the study that the concentration of virulent (disease-causing) R. equi in the dams' feces was not related to the risk of a foal developing pneumonia caused by the bacterium; in other words, mares shedding more R. equi in their feces did not appear to explain the disease. However, that study also observed that nearly all mares shed virulent R. equi in their feces during the period shortly after birth of foals.
There might be genetic factors that influence a foal's susceptibility to R. equi, he said, as veterinarians in the field report that some mares have had multiple foals that have contracted the disease. Moreover, variations in the DNA sequence (known as polymorphisms) for several genes have been associated with somewhat greater risk of disease. He also suggested that some foals could just be particularly immunologically susceptible to the disease.
No matter the cause, he stressed that more research is needed to obtain more accurate information about what predisposes some foals to contract R. equi.
Cohen also examined why some breeding farms seem to have a higher prevalence of R. equi than other farms. Again, combing through the available data provided few definitive answers.
"The density of mares and foals per acre seems to be positively correlated with incidence of R. equi pneumonia," he noted. Foaling at pasture also may reduce risk of this disease, but more work is needed to confirm existing observations.
Cohen also explained that in the studies he examined, R. equi seemed to occur at "well-managed farms that use practices generally deemed to be desirable for preventing infectious diseases of foals.
"This association is not likely causal," Cohen wrote. "But it does indicate that practices effective for preventing other infectious diseases of neonates are of limited benefit against R. equi."
The take-home message, Cohen said, is that researchers don't yet understand why some foals contract the disease while others in the same environment do not. Some farm-level interventions such as reducing density of mares and foals and foaling at pasture need to be systematically evaluated.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.