Cover Story | Foal Health

REHIP Has Little Effect on Foal Electrolytes

By Marie Rosenthal MS

There is no vaccine to protect young foals against Rhodococcus equi bronchopneumonia, so R. equi hyperimmune plasma (REHIP) is often used for immunoprophylaxis.

However, there are concerns about marked variation in concentrations of sodium, potassium, chloride and albumin from commercially prepared plasma. Devynn M. Volding, DVM, and her colleagues at Texas A&M University, College of Veterinary Medicine, in College Station, wanted to know if there were variations and whether they could affect electrolyte concentrations when transfusing foals. And if there were variations, should these electrolyte concentrations be monitored?

R. equi, a gram-positive intracellular bacterium, is endemic on breeding farms throughout the United States, and often causes pneumonia in foals from 3 weeks to 6 months of age. It is associated with severe illness, high costs for veterinary care, decreased performance of affected foals as adults, and lost income from deaths in several cases, according to Dr. Volding, DVM, an internal medicine resident at Texas A&M.

“The objectives of our study were to describe serum concentrations of sodium, potassium chloride, albumin, globulins and total protein before and after R. equi hyperimmune plasma transfusions in foals,” she said. The team also looked at individual lots of R. equi hyperimmune plasma (MG Biologics).

“We hypothesized that serum concentrations of sodium, potassium chloride, albumin, globulins and total protein would not change significantly after transfusion,” she explained at the 2025 AAEP Annual Convention, held in Denver, and those concentrations would not change regardless of the volume transfused.

The study focused on results after 1 L and 2 L of REHIP were transfused. Starting in 2022, the team looked at 363 Thoroughbred foals from 13 breeding farms in New York with a history of R. equi pneumonia. All the foals were born healthy and had immunoglobulin (IgG) measurements greater than 800 mg/dL. “They received care from 1 of our co-authors and received a transfusion with R. equi hyperimmune plasma from the same manufacturer within 48 hours after birth,” Dr. Volding explained.

Foals were excluded for various health and other reasons, such as a lack of a paired sample or the absence of a REHIP transfusion. Of the 355 foals enrolled in the study, 116 were transfused with 1 L of REHIP and 239 with 2 L of plasma, Dr. Volding explained.

An IV catheter was placed in a jugular vein, and blood was drawn and labeled before and after transfusion. The researchers refrigerated the blood until it could be shipped to the Equine Infectious Disease Laboratory at Texas A&M University, where the samples were aliquoted and frozen at –80° C.

Additionally, 90 aliquots from independent lots of REHIP made by the same manufacturer from 2022, 2023 and 2024 were collected and tested for sodium and potassium chloride on a Beckman Coulter DXC 700AU analyzer. Concentrations of albumin and total protein in both serum and plasma were determined with polymetric assays, and concentrations of globulins in serum and plasma were also determined using a Beckman Coulter DXC 700AU analyzer.

Results showed that the infusions led to statistically significant increases in serum sodium, albumin, globulin and total protein concentrations, and decreases in chloride and potassium concentrations (P<0.05). However, none of the values were clinically significant, according to
Dr. Volding. Serum protein concentrations were significantly greater among foals given 2 L of REHIP, but not in those given 1 L. The researchers saw no adverse events among the foals that received REHIP, she added.

When investigators examined the individual aliquots of the REHIP concentrations of electrolytes and protein, they observed little variation, even when comparing REHIP collected from different years, except for sodium, which showed a small but significant difference from year to year. Most of the values were above reference ranges, except for albumin, which tended to be below reference ranges.

Dr. Volding admitted there were limitations to the study, including that the REHIP was made by a single manufacturer and that it was frozen, but that did not appear to have a significant effect on the products. In addition, the study was done on healthy foals, rather than sick ones. “It would be valuable for future studies to also include ill foals since they might have a reduced ability to buffer shifts in electrolyte concentrations,” Dr. Volding said.

“As for our take-home messages, transfusion of R. equi hyperimmune plasma resulted in small changes of serum sodium, potassium chloride, albumin, globulin and total protein in healthy foals,” she said. “The R. equi hyperimmune plasma samples did not vary greatly among manufactured lots or years, and the co-efficients of variation were generally less than 10%.” MeV

FOR MORE INFORMATION
Volding DM, Flores-Ahlschwede P, Cramer MJ, et al. Transfusion of rhodococcal hyperimmune plasma to newborn foals does not markedly alter serum electrolyte or protein concentrations. JAVMA. 2025;267(7):859-865.
https://avmajournals.avma.org/view/journals/javma/263/7/javma.25.02.0115.xml?tab_body=pdf