Horses are big, strong creatures that produce offspring capable of standing within an hour of birth. Mother Nature gave them this ability so even newborn foals would be able to flee with their dams if the herd was attacked by predators. This sometimes causes horsemen to believe neonatal foals are tougher than they really are.
Actually, newborn foals have a fragile grip on life. Even ones that are born healthy can take a turn for the worse seemingly in an instant. Nearly 6 percent of foals die during their first month of life, and half that number die within the first two days after birth.
Researchers may have an answer why sick neonatal foals require intensive veterinary care to survive the rapid decline in condition. In a study of 100 hospitalized foals, they found that 63 percent were deficient in vitamin D.
“The sicker the foal is, the greater its vitamin D deficiency, and if nothing is done about it, the foal is going to die,” said Dr. Ramiro Toribio, leader of the Endocrinology Research Program at The Ohio State University. Toribio also is board certified in internal medicine and a professor of clinical sciences.
Vitamin D's role in the neonate is complicated. Scientists don't know which comes first — vitamin D deficiency that inhibits the immune system or a weakened immune system that causes the foal's vitamin D level to drop — a situation Toribio calls “a loop.”
Vitamin D supports calcium and phosphorus absorption in the intestines. Proper calcium metabolism is essential to maintain a proper blood calcium level in the neonate. Low blood calcium can cause colic, seizures, respiratory failure, paralysis, heart arrhythmias, and even death.
Horses naturally get vitamin D from plants they graze and from sunlight, which is synthesized into vitamin D by the skin. The body then metabolizes vitamin D into products that circulate through the bloodstream to support the horse's physiology. Newborn foals get vitamin D from their mare's colostrum, the nutrient- and antibody-rich first milk.
Toribio explained his theory of what is happening in the sick foal:
“We think that when they get septic, for example, they don't nurse enough colostrum, and then the white blood cells release the inflammatory substances, and then the proteins start going down,” he said. “There's one [vitamin D-binding protein] that they have to make and they stop making it in sufficient amounts. And vitamin D is important for immunity; for example, the immune response.”
Some of the proteins that decline are antibacterials that fight disease. When these proteins, which depend on vitamin D for their synthesis, are not available in adequate numbers, harmful bacteria flourishes and the foal gets sicker. Vitamin D-binding protein (DBP) transports vitamin D to the body's organs.
Toribio and his colleagues observed that foals born closer to spring, when the mares have access to lush pasture and lots of sunlight, have a higher level of vitamin D than those born during the winter. Being born early in January is desirable for racehorses because they are physically more mature than foals born later in the spring.
Toribio said much more research needs to be done to answer crucial questions about how to resolve or prevent vitamin D deficiency in neonates. For now, the only measure he can endorse is assuring the pregnant mare has adequate vitamin D in her diet by getting her outside on pasture as much as possible and feeding a balanced ration. But he cautions against supplementing broodmares with vitamin D because of the risk of toxicity. Future research will compare foals born of mares housed inside with foals born of mares that are outside on pasture.
Complicating the research is that horses require much less vitamin D than other species, and scientists don't know why.
“If you see the vitamin D concentration in a healthy horse, the concentrations are half or less than that of a healthy cow,” Toribio said. “In nature, what does that mean? We can only speculate. The babies, for sure, the concentration drops really low, less than half what is normal, and that's something that we want to foresee in the future, basically see whether there is something we can do about it.”
An obvious solution would be to test the newborn foal's vitamin D level and supplement it either orally or intravenously, if necessary. But Toribio said many questions have to be answered and much work must be done before veterinarians can adopt that protocol. No commercial laboratory test exists, and researchers have yet to determine which route of supplementation is effective.
“This information will have clinical implications in the diagnosis, treatment, and prognosis of sick newborn foals,” Toribio wrote in his grant proposal for the study.
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