foal failure of passive transfer

Foals might be able to walk shortly after they enter the world, but they can’t protect themselves from disease. Born with a naive immune system, they’re designed to get the antibodies (immunoglobulin G, or IgG) they need from their dam’s colostrum—the first milk she produces. Sometimes, however, that doesn’t happen.

In these cases—called failure of passive transfer (FPT) of immunity—the earlier the foal can receive treatment the better. But, one researcher said, current diagnostic testing options are either expensive and technically difficult or take a significant amount of time to produce results. That’s why an international team of researchers teamed up to evaluate whether digital and optical refractometry—evaluating the foal’s serum IgG concentrations—could help detect FPT quickly, reliably, and inexpensively.

They found that “digital and optical refractometers are simple, rapid, and cost-effective methods for assessing FPT in foals with moderate to good accuracy,” said Ibrahim Elsohaby, DVM, MVSc, PhD, a postdoctoral researcher at the University of Prince Edward Island’s Atlantic Veterinary College, in Charlottetown, and the Zagazig University Faculty of Veterinary Medicine, in Egypt.

“Refractometers can produce a reading in less than 15 seconds once a serum sample is obtained and require minimal equipment and technical expertise,” he said.

Breeders could even do the tests themselves—as long as they can obtain a properly prepared serum sample from their veterinarians, he added.

FPT 101

Timing is critical in FPT cases because the foal’s gut absorbs most of the antibodies from the dam’s colostrum during the first 12 to 18 hours of life. At 18 to 24 hours postpartum, however, the gut begins to mature and can no longer absorb the antibodies that will help protect him from disease.

Elsohaby said current testing is either fast but less accurate—leaving foals at risk of undiagnosed FPT—or relatively slow but accurate.

“The current reference testing method, radial immunodiffusion (RID), takes 24 hours to provide results, and that precludes it from early detection of FPT,” he said.

If veterinarians can confirm FPT before the foal’s gut loses its ability to absorb antibodies, they might be able to rectify the situation by administering oral high-quality colostrum, Elsohaby said.

“On the other hand, if a foal is over 24 hours old when FPT is diagnosed, oral supplementation is no longer an option, and intravenous (IV) plasma transfusion is required,” he continued.

Given those two options, colostrum supplementation is better tolerated by the foal, easier to manage, and much less expensive for the owner than IV plasma, Elsohaby said.

Study Findings

In their recent study, Elsohaby and colleagues from Canada and New Zealand ran digital and optical refractometry and gold-standard RID testing on the serum of 230 newborn foals, younger than 2 days old. The refractometry gave results were less accurate at diagnosing FPT than the RID, but the former took seconds to complete instead of hours.

“A second confirmatory test with higher specificity (such as an RID) could be used to confirm FPT positive foals” if refractometer tests are borderline, he said. While that test is being run, however, the foal should receive colostrum if he’s less than 24 hours old as a precaution, he added.

The study, “Usefulness of digital and optical refractometers for the diagnosis of failure of transfer of passive immunity in neonatal foals,” was published in the Equine Veterinary Journal.