Published March, 2015

Fernanda Cesar, DVM, MS, PhD candidate at the University of Kentucky (UK) Gluck Equine Research Center, discussed using hyperimmune plasma (HIP) to prevent Rhodococcus equi in foals at the 6th Annual Kentucky Breeders’ Short Course, held Jan. 24 in Lexington. Other authors on this paper included Macarena Sanz, DVM, MS, Dipl. ACVIM, and David Horohov, PhD, also of the Gluck Center.

R. equi, a bacterium that is found worldwide, is the leading cause of pneumonia in foals 1 to 6 months old and has a mortality rate ranging from 1% to 20%. Current antibiotic treatments for infected foals are administered orally and are prolonged and expensive.

There is no approved vaccine against R. equi. Farms with a history of R. equiinfections often use ultrasound screening and aggressive antimicrobial treatments to detect and treat infected foals. However, this approach has contributed to the appearance of antimicrobial resistance in R. equi.

Another preventive approach is to administer HIP to foals at birth with a second dose at 4 to 8 weeks. The HIP contains antibodies against R. equi, which are thought to provide protection to the foal. While the exact mechanism of this protection is unknown, Cesar focused on those antibodies directed against virulence-associated protein A (VapA), an important pathogenic factor of R. equi.

Prior field studies of HIP produced conflicting results. Studies from the 1990s indicated HIP administration was an effective preventive method, yet more current studies from the 2000s showed HIP as being ineffective.

“There are a couple of potential reasons for this (conflicting result),” Cesar said. “The age at which the foals were administered HIP varied, and each study had no definitive diagnosis.” Variations between products and individual foals could also play a role.

In the first study, researchers compared foals on Thoroughbred farms in Lexington that were routinely administered HIP to foals on control farms that did not use HIP. They collected and evaluated serum samples for the presence of antibodies to VapA in the foals and their mares. The research team compared four different commercial HIP products: EquiplasREA, PneumomuneRE, ReSolution, and ImmunoGlo. Because none of the companies that produced the HIP products funded the study, there was no conflict of interest.

The results showed that the amount of antibodies to VapA each product contained varied. Further, the amount of VapA antibodies in each of the foals that received the HIP varied significantly. In fact, some treated foals had fewer antibodies to VapA than did the untreated foals. This could be due to the product variation in antibody composition and/or natural variations within foals.

In a second study to determine if HIP administration could prevent infection, one group of foals received HIP and a second group received no treatment. Both groups were exposed to R. equi, and the researchers monitored their clinical response to the disease.

The treated foals showed no adverse reaction to the HIP. While administering the HIP did not prevent foals from becoming infected with R. equi, those foals receiving HIP had milder disease and more rapidly resolved their infections.

“(It) appears that while infection was not prevented, natural regression was enhanced,” Cesar said.

Take-Home Message

Administering HIP seems to have a positive effect on limiting R. equi infections. In previous studies, if a foal did become infected, HIP decreased its duration and severity. HIP is a safe treatment for foals, but it does not have 100% efficacy in preventing infection.

“Further work is needed, in the field and in controlled conditions,” Cesar said, ”to determine which components of HIP were involved in this protection.”

Jackson Wells is an equine science and management undergraduate student and communications intern for UK Ag Equine Programs.