Failure of Passive Transfer

Foaling season is here! Healthy newborn foals are fun to watch and rewarding to care for. It is critical to be vigilant in the first few days to weeks of life, however, to be certain they have the best start possible.

Infection can cause serious illness in neonates. Though they are immunocompetent at birth, meaning they can mount a normal immune response, they need additional protective immunity against common environmental pathogens (disease-causing organisms). Maternal immunity does not transfer through the placenta to the fetus, so newborn foals rely on the disease-fighting antibodies present in colostrum (the mare’s first milk) for passive antibody ­protection—in other words, naturally acquired ­immunity.

Adequate passive antibody transfer, known as passive transfer, should be the cornerstone of all neonatal preventive health programs. During the first one to two months of life, foals depend on it for protection from infectious diseases. Failure of passive transfer is not a disease; however, it predisposes neonates to infection and sepsis (a systemic inflammatory response to infection) if not identified and corrected immediately.

The primary immunoglobulin (antibody) in colostrum is IgG, but significant amounts of IgM and IgA are also present. Newborn foals also need the proteins, fat, vitamins, and minerals colostrum provides for energy and sustenance. Immunoglobulins transfer into the colostrum four to six weeks before expected parturition (birth). Ideally, the foal should consume at least 2 liters of his dam’s colostrum within the first six to eight hours of life. Immunoglobulins ingested in colostrum then travel across the small intestine’s absorptive epithelial cells and travel via the lymphatic system to the blood. ­Immunoglobulin ­absorption usually reaches maximum levels at 12 to 24 hours of age in neonates, with their ability to absorb immunoglobulins starting to decline around 12 hours. Gut closure, at which point neonates can no longer absorb immunoglobulins, occurs by 24 hours.

Ideally, have your veterinarian examine the foal to make sure he’s healthy and test his IgG concentrations between eight and 14 hours after birth to assess passive transfer status. IgG is detectable in serum two hours following a colostrum feeding; concentrations of at least 800 mg/dl (­milligrams/deciliter) indicate successful passive immunity. Fewer than 200 mg/dl indicates complete failure of passive immunity, while 200-800 mg/dl points to partial failure of passive ­immunity.

Several tests are available for quantitative or semiquantitative assessment of passive transfer. The only tests that directly measure IgG concentration are the radial immunodiffusion (RID) and the enzyme-linked immunosorbent assay (ELISA). The gold standard test is the RID, but it takes about 24 hours to get results. The ELISA is a stall-side assay with similar accuracy to the RID. It gives a semiquantitative measure of IgG, is relatively cost-effective, and can be performed on whole blood or plasma. Veterinarians and caretakers can also estimate immunoglobulin levels in colostrum using a colostrometer; however, false positives can occur.

Although foals with failure of transfer of passive immunity (FTPI) have an increased risk of developing infections and sepsis, not every untreated foal becomes ill. Foals in clean environments with reduced pathogen exposure have the best chance at survival. Make your decision to treat a foal with failure of passive transfer on the farm in consultation with your veterinarian, based on economic and environmental factors. You can treat FTPI within the first 12 hours of life most cost-effectively using 2-4 liters of oral colostrum. However, owners often don’t discover these cases until after gut closure has occurred, at which point treatment with commercially available frozen plasma is most effective. Administering 1 liter of plasma intravenously typically results in about a 200-300-mg/dl increase in serum IgG in a 100-pound foal, but this amount can vary.

Foaling season is an exciting time for owners, farm managers, and veterinarians. Be sure your foals have the best possible start to life by ensuring mares have optimal perinatal care, including but not limited to vaccination boosters four to six weeks prior to anticipated foaling. Continue to monitor foals closely in the first days to weeks of life. Have a veterinarian examine new foals, and make sure you run an IgG test.