Risk Factors Associated with Pneumonia
Rhodococcus equi is a common cause of bacterial pneumonia in foals 1 to 3 months old. Despite its prevalence, researchers still don’t know why some foals on a farm contract it while others don’t, or why it will affect a farm’s foal crop one year but not the next.

So researchers from Texas A&M University’s (TAMU) College of Veterinary Medicine and Biological Sciences, in College Station, performed a retrospective study of an affected farm to find out. Michelle Coleman, DVM, PhD, Dipl. ACVIM, assistant professor of Large Animal Internal Medicine at TAMU, presented their results at the 2016 American Association of Equine Practitioners Convention, held Dec. 3-7 in Orlando, Florida.

Most foals are exposed to R. equi in their first few days or weeks of life, but only some develop and show signs of disease, said Coleman. If researchers could identify risk factors for foals developing R. equi, they could help reduce the burden of disease on the animal, farm, personnel, and veterinarian.

In general, the higher a farm’s stocking density, the greater the risk for clinical R. equi cases. Management practices likely play a role in R. equi transmission. The bacteria spread particularly quickly on large farms, most notably through the air (via secretions from coughing and sneezing or in airborne dust).

R. equi is present in feces, soil, and the air. Because of its high incidence on breeding farms, it is not unusual for the farm veterinarian to administer hyperimmune plasma (HIP) to foals to boost immune function. But the scientific data regarding HIP’s efficacy and the amount and frequency of administration is mixed.

In their study, Coleman and colleagues set out to identify foal-level risk factors at a farm in Texas with a recurring R. equi problem 2009-2011. During this time, 27% of foals born on the farm developed R. equi pneumonia.

Based on their results, the researchers found no association between disease and birth month, duration of gestation, age of dam, or whether the dam was biological or surrogate (such as a nurse mare). Plasma antibody concentration (the amount of immunity conferred to the foal after consuming his dam’s colostrum, or first milk) at 24 hours of age was not a factor. Surprisingly, said Coleman, foals that had a concurrent respiratory illness were less likely to develop R. equi.

About 97% of foals received either one or two liters of HIP within the first 24 hours after birth. Foals that received two liters of HIP were less likely to develop R. equi, compared to foals who received 1 liter or none at all, said Coleman. However, the method of HIP administration varied by year, confusing the results, so further study is warranted to determine the ideal method of administering HIP.

Take-Home Message

“Based on the results of this study, we cannot make recommendations regarding the use of HIP to foals,” Coleman said. “In general, evidence for the use of HIP is conflicting. Highly effective preventative strategies of R. equi pneumonia are still lacking and in the absence of more effective strategies, transfusion of HIP continues to be the mainstay for control and prevention on many farms.”

The question of why foals with concurrent respiratory diseases were less likely to develop R. equi remains to be answered but, perhaps, has to do with other illnesses improving immune function, she said. In future studies, she added, researchers should also analyze the effects of colostrum and disease heritability on R. equi disease development.