Antibodies (immunoglobulins) are large proteins that bind to infectious agents like bacteria and viruses, and mark them to be removed by the immune system. Antibodies are a critical component of immunity, and their absence makes fatal infection of a foal likely. Antibodies are either created by the animal itself (active) or the antibodies get into the blood already formed (passive).

"Passive transfer" refers to transfer of such antibodies from mother to fetus or newborn. In some mammals, antibodies are transferred from mother to fetus through the placenta, but this is not true for horses.

Newborn foals are born with no antibodies and must get them by drinking the first milk (colostrum), within 24 hours of birth. Mares concentrate antibodies in the colostrum over the last weeks of pregnancy. If that colostrum is lost, the mare's contribution to the foal's immunity is lost.

For the first 24 hours of life a foal's intestine has "channels" that allow these large molecules to be absorbed. After 24 hours, these channels close and the foal can no longer absorb the antibodies, an intestinal process called "closure."

Successful passive transfer is assessed by your vet in the newborn foal exam, using a blood antibody test for an antibody called IgG. Most foals that nurse a healthy mother within a few hours of birth will have a high IgG level at 12-24 hours after birth. Those that are weak may not ingest enough colostrum, and so may have low levels.

There are a variety of reasons that a foal might not acquire sufficient passive transfer. 1. The mare dies prior to the foal nursing 2. the mare drips most or all of the colostrum prior to foaling, leaving none for the foal when they do nurse. 3. Mares sometimes reject foals or refuse to let them nurse 4. Colostrum is also not all created equal- some may have a low level of antibody. 5. Foals born premature may be too weak to nurse or colostrum may be insufficient at the time of birth. 5. Inability of the foal to nurse in the first 12 hours of life, for any reason is the most common reason. 6. In rare cases, the colostrum is present in adequate quantity, but it does not contain adequate antibody in it for the foal.

In foals less than 24 hours of age that have passive transfer, feeding of quality colostrum may provide the antibody needed. After 24 hours, foals must be treated with intravenous plasma in order to provide antibody directly into the blood.

Diagnosis requires antibody testing. Foals that have not nursed for whatever reason are considered to have FPT by default.

Treatment requires oral supplementation or plasma transfusion.