Horses have complicated blood types consisting of more than 30 factors or antigens that are grouped into systems. Factors within each system are designated with small letters. Mares that are negative for these factors are the ones most at risk for developing anti-red cell antibodies which can create problems when an incompatibility of blood types between a mare and her foal exist.
According to Dr. Kathryn Graves of the Gluck Equine Research Center at the University of Kentucky, neonatal isoerythrolysis (NI) results from an incompatibility of blood types between a mare and her foal. If a foal inherits from its sire a red cell factor (antigen) that the mare lacks, there is no harm to the foal in utero, as there is no commingling of the mare's blood with that of the fetus.
However, these antibodies will be present in the mare's colostrum and will be ingested by the newborn foal. The antibodies will attach to the foal's red blood cells, causing their destruction, resulting in a jaundiced foal and, in severe cases, death, unless quickly diagnosed and treated.
A mare's first foal is seldom affected by subsequent foals carrying the offending antigen will be at risk.
Managing mares that are at risk for producing an NI foal consists of screening these mares for anti-red cell antibodies within the last 30 days of pregnancy of after udder development has begun.
If an anti-red cell antibody is found in the mare, the foal should not be allowed to nurse the dam and should be given banked colostrum. The dam's colostrum should be stripped out over a 24 hour period before allowing the foal to nurse.
In a mare with a history of producing an NI foal, either all subsequent foals should be withheld from nursing the mare for 24 hours, or the mare should be bred only to stallions that do not possess the red cell antigen to which she is sensitized.
If there is any doubt about a mare or stallion's blood type and antibodies, NI testing and red cell typing for the most common antigens involved in incompatibilities should be done. These tests are available through university veterinary laboratories such as the one at the Gluck Equine Research Center at the University of Kentucky.