Approximately 25 percent of newborn foals will experience some level of failure of passive transfer.
Infectious disease is a major cause of death in neonatal foals.
The foal is born immunocompetent, meaning it probably is able to initiate an immune response to organisms to which it is exposed. However, a newborn foal lacks protective immunoglobulins at birth, and that lack of protection can allow organisms to overwhelm the foal before it can mount a suitable defense. The most important host defense in the newborn horse is the passive transfer of antibodies in the form of immunoglobulins from the mare to the neonate through colostrum. Unlike humans, in horses there is no natural transfer of antibodies through the mother's placenta. It therefore is necessary for the newborn foal to obtain antibodies through the mother's milk shortly after birth.