The foal is born with an intact immune system – all of the cells needed to fight disease are present in the lymph nodes, bone marrow, thymus and spleen. In order to effectively fight disease, however, it must live in its environment for a period of time so that its immune system can be “primed” by exposure to organisms in its environment. The immune system must learn about the organisms in the environment by continuous exposure before it can mount its active immune response. Over time the foal’s cells will produce antibodies to specific organisms in order to protect itself from disease. This is known as active immunity.
When foal are born this protective antibody has not been produced yet. Nature has provided protection during this vulnerable neonatal period by providing passive immunity from the mare. It is called passive because the foal does not produce it – it is provided by the dam. The source of this passive immunity is the dam’s first milk, also known as colostrum. It is common practice to vaccinate mares approximately one month before the birth of the foal in order to boost the immune levels of the colostrum to common diseases.
Colostrum is filled with antibodies that are concentrated from the mare’s blood into the udder approximately 24 hours before the birth of the foal. A normal foal has a “window of opportunity” during the first 12 to 16 hours after birth in which it can absorb these antibodies through its intestinal wall, directly into the bloodstream. After this time, the intestinal wall loses the opportunity to absorb these antibodies – ingested colostrum will provide little or no immunity.
Some foals do not get adequate immunity from colostrum. The mare may have poor quality colostrum, or the foal, if weak, sick, or rejected by the mare, may not ingest enough during those first few critical hours. Some foals, especially premature ones, lack the ability to absorb colostral antibodies through the intestinal wall. These foals suffer from a condition called failure of passive transfer (FPT), which leaves them vulnerable to infection. FPT is the leading cause of death in young foals.
Fortunately, a simple blood teat can detect failure of passive transfer. All foals should have a thorough veterinary examination at about 8 hours of age. A simple blood test can be run at the farm to test for adequate passive transfer. If the foal has FPT it can be treated with oral or intravenous antibodies that may be lifesaving. Do not neglect this examination and test, even if your foal appears healthy – foals with FPT usually do not appear sick, and the blood test is the only way to detect this problem.
Most veterinarians no longer recommend vaccination of newborn foals. The reason for this is that the passive immunity provided by colostrum blocks the ability of the foal to respond to a vaccine with its own immune system for about three months. At three months of age the effects from the colostrum are waning and vaccination for all common diseases is indicated. Booster vaccinations should be given at four months, in order to further reinforce the immune response.