Foals are born without an antibody protection. Unlike humans, antibodies are not transferred through the placenta from the mother to the fetus. The foal is dependent on receiving colostrum to obtain the antibodies to protect them from infectious diseases and bacteria.
Colostrum is produced in the last 1 to 2 weeks gestation. Antibodies are transferred from the mare’s blood and concentrated in the mammary gland. Colostrum is the mare’s first milk and an essential source of immune components and nutrients for newborn foals. The most important of the immune system are antibodies, often referred to as immunoglobins (IgG). There are specialized cells lining the small intestine of the foal that facilitate the absorption of antibodies and transfer them to the foal’s blood. These cells are most effective during the first 6 to 8 hours after birth. Approximately 24 hours after birth these cells lose the ability to absorb antibodies.
Failure to absorb sufficient antibodies is referred to as “Failure of Passive Transfer” (FPT) and occurs in 10 to 20% of newborn foals. Two main reasons for FPT include poor quality colostrum and premature lactation. Failure to properly vaccinate the mare may impact the quality of the colostrum as can poor prenatal nutrition. Premature lactation is seen in mares that drip or “leak” milk for several hours to a day so before foaling (picture 1). In this situation you should hand milk the mare and save the colostrum for the foal. Strain the colostrum through a gauze filter and place it in a labeled plastic bottle. It can be refrigerated or frozen if feeding doesn’t appear near (see video or blog on signs of foaling). Another cause of FPT is the failure of colostrum production. Mares grazing on endophyte-infected fescue often do not produce colostrum or milk, a condition referred to as agalactia. Foals that do not nurse in a timely manner (see 1, 2, 3 Rule with Dr. Longworth at www.equiuniversity.com), born prematurely, failure to absorb antibodies when ingested are other reasons for FPT. Poor mothering behavior in the mare, e.g. rejection of the foal, will contribute to FPT.
Antibody levels in the newborn foal can be determined and be used to identify potential cases of FPT, allowing for early treatment. A blood sample is obtained from the foal and the circulating IgG level determined. This may be done about 12 post-foaling, before the closure of the small intestine to antibody absorption. An IgG concentration of less than 400mg/dl in the blood is considered FPT and suggest the foal receive additional colostrum or a commercial colostrum substitute be given. IgG levels between 400mg/dl and 800mg/dl may have partial FPT and may benefit from supplemental colostrum or a commercial colostrum substitute. Environmental conditions often dictate where or not supplemental colostrum is required. Foals born into a clean environment with “good” management practices may not require additional colostrum. Whereas a foal born into a “pathogen rich” environment and at risk of developing infections may benefit from additional colostrum. IgG levels above 800mg/dl at 12 hours indicate adequate transfer of antibodies and under most circumstances additional colostrum is not needed.
Testing after 24 hours of life will determine the extent of antibody absorption. Low IgG at this time requires intravenous administration of plasma or a commercial equine IgG product to successfully increase the blood antibody concentration. Since the ability of antibodies to cross the intestinal epithelium at this time is negligible, oral supplementation is of very little use.
Testing for IgG content may be performed with several different methods. One most often used is call a “SNAP Foal IgG Test” (pictures 2 & 3). This test uses whole blood, plasma, or serum mixed with a reagent. Test results can be read in about 10 minutes. Talk with your veterinarian about this and other tests they might recommend.
Early testing (12 hours) of the newborn foal allows oral supplementation of colostrum or a commercial colostrum substitute when the intestinal tract can absorb antibodies. At either time frame, knowing the IgG concentration in the foal will allow early medical intervention and minimize life-threatening health issues.