Each year a small percentage of foals are born to mares whose immune systems, in an odd twist of nature, are prepared to battle their own foals. Antibodies from the mare, delivered to the foal in the colostrum it drinks soon after birth, attack the foal’s red blood cells.

The condition that results is neonatal isoerythrolysis, more commonly known as jaundice. The occurence of NI is relatively rare. It happens in only about 1% of Thoroughbred foals and in about 2% of Standardbred foals. The relative infrequence, though, is offset by the possible severity of the condition.

Because the red blood cells in the foal are destroyed, the foal can become severely anemic and, in some cases, die. William V. Bernard, DVM, Diplomate ACVIM, of Rood and Riddle Equine Hospital in Lexington, said about 50% of the NI foals he sees require transfusions to recover. An NI-stricken foal must also be stabled to restrict activity until it has recovered. The low red blood cell count, or anemia, that results from the condition makes the foal at risk if it becomes active. With fewer red blood cells to carry oxygen, the foal can collapse and die if it tries, for example, to keep up with its’ dam in a paddock or pasture. In some cases, a foal with NI has died as it exercised in a field or paddock.

The condition can be prevented if a screening test performed in advance indicates the mare has produced the antibodies. If that is the case, the foal is then kept from the mare and her colostrum is stripped.

The condition comes about when there are certain incompatabilities in the genetic makeup of the blood of the dam and sire. While that is common, most blood groups do not give rise to the severe antibody reaction that results in NI. Almost all cases of NI result in what are called the Aa factor of the A blood group and the Qa factor of the Q blood group.

For the condition to result, several things must be present. Among them: