In pregnant mares, unlike most other animals, antibodies do not cross the placenta into the foal’s bloodstream before birth. Therefore, when a foal is born it has no natural defense mechanisms against infection because it has no antibodies, which are the blood’s special immune proteins, with which to fight infection. Normally, the foal receives these antibodies in the colostrum (first milk) that it drinks from its mother. Failure to receive sufficient antibodies results in a condition known as ‘failure of passive transfer of immunity (FPT) and this significantly increases the risk of the foal developing life-threatening infections such as septicemia (blood infection) or septic arthritis (joint ill). Foals start making their own antibodies after three to six weeks of age but are clearly at risk prior to this.
How is colostrum formed?
Colostrum is the thick, honey-like fluid that is present in the mare’s udder at the time of foaling. It is more concentrated than normal mare’s milk and usually contains very high levels of antibodies. The antibodies come from the mare’s blood and represent her own body’s response to the disease-producing micro-organisms, with which she has come into contact, and in response to the vaccines, she has received during the weeks before the foal is born. Therefore these antibodies are relatively specific for the mare’s individual environment
How does Failure Of Passive Transfer occur?
There are several reasons why a foal may receive inadequate levels of antibody after birth. If a mare ‘runs milk’ prior to foaling, this will result in the loss of significant quantities of colostrum so that there is too little left for the foal to drink to guarantee adequate antibody intake. Some mares, either habitually or as an isolated occurrence, do not produce colostrum of sufficient quality, i.e., their colostrum has a low concentration of antibodies. In other instances, the foal may be slow to suck either due to illness or weakness. There is a finite ‘window’ of time up to about twelve to eighteen hours after birth during which these antibodies can be absorbed into the foal’s bloodstream without being digested in the intestine. Once this time is up it is no longer possible for ingested colostrum to provide the foal with useful antibody levels in its bloodstream. Conditions that result in stress to the foal, for example, a traumatic birth, death of or rejection by the mare can actually shorten the length of time that the foal’s intestines can absorb antibodies. This also reduces the number of antibodies that the foal ends up within its blood circulation.
Source: https://vcahospitals.com/know-your-pet/failure-of-passive-transfer-in-horses
