In horses, these maternal antibodies are not transferred to the foal while it is still in the uterus, so the newborn is highly susceptible to disease and completely dependent on the passive immunity it receives when drinking colostrum in the first few hours after birth.
The level of Antibodies
Research has shown that the level of antibodies in the serum in young foals is directly proportional to the amount of colostrum they receive in the first few days after birth. In turn, the level of antibodies in the serum is directly related to the risk of the foal becoming infected with life-threatening infectious diseases.
During late pregnancy, and especially in the final three weeks, the mare’s udder, or mammary gland, absorbs and concentrates antibodies circulating in its bloodstream. If she aborts a foal before its due date, the udder will often
swell and colostrum will appear several days before the foetus is expelled. The amount and quality of colostrum produced by a mare depends on her age, nutritional state, health and the number of foals she has had.
Antibodies in her bloodstream, which reflect those in her colostrum, can be gained from vaccination as well as natural infection by micro-organisms in the environment. A mare moved to new surroundings shortly before foaling may not have time to develop an immunity to the pathogens in the new environment and her foal is consequently at risk. Live vaccines such as those for African horse sickness can increase the risk of a mare aborting due to the high fever occasionally caused by these vaccines.
What prevents a foal from getting colostrum?
Now and then, especially in maiden mares, the udder may not develop sufficiently and very little colostrum will be available when the foal is born. Sometimes, a mare can reject her foal or a dominant mare may adopt a newborn foal and herd it away from its true mother.
At other times, the udder swells and becomes over-full too early. The increasing level of oxytocin can stimulate the let-down reflex in the udder, allowing colostrum to flow out of the teats. Certain plants also contain oxytocin-like substances that can cause milk let-down.
How soon, how much?
The antibodies in colostrum are highly concentrated during the first eight hours after birth and gradually decrease over the next 24 to 36 hours. To receive enough protection, the average foal must drink more than 2l of colostrum within the first eight to 12 hours of its life. Unfortunately, the ability of the foal’s intestine to take antibodies from colostrum into its bloodstream is blocked once it drinks whole milk.