Horses have seven main blood type systems (A, C, D, K, P, Q and U) which are of clinical interest. These blood types can occur singly or in combination within a horse – for example, a horse may have blood type A or ACQ. Interestingly, blood type prevalence varies between breeds of horses. Because Thoroughbreds have had a closed stud book for over 200 years and have been bred to select for one overriding genetic trait, they have very little variation among their blood types. The vast majority of Thoroughbreds (98 percent) have the A blood type while 84 percent have the Q blood type. On the other hand, 0 percent of Standardbreds have the Q blood type. Even among Standardbreds, there is variation; 97 percent of Standardbred trotters have the A blood type while only 78 percent of Standardbred pacers do. Breeds with less stringent stud books or that have been bred for multiple traits tend to have more heterogenicity among their blood types; 53 percent of Shetland ponies have the A blood type and 52 percent have the Q blood type.

While blood types in adult horses may seem inconsequential since blood transfusions are rare, they are very important in broodmares and foals due to a disease called neonatal isoerythrolysis. In this disease, the foal has a different blood type from his dam and ingests anti-red blood cell antibodies with the colostrum (first milk produced by the dam). In order for the dam to produce anti-red blood cell antibodies, she must have been exposed to a foreign blood type previously – usually through a previous pregnancy, although it could also be from a previous blood transfusion. These anti-red blood cell antibodies then attack the foal’s red blood cells and can lead to hemolytic crisis and death. Because of the relatively small size of foals (and their lack of blood volume), veterinarians may use blood transfusions more commonly in these cases. Clinically, the A and Q blood types have been shown to be the most likely to cause a problem because of their high antigenicity. Because the A blood type is so common in Thoroughbreds, broodmares without the A blood type are at an especially high risk of producing a foal with neonatal isoerythrolysis.

Fortunately, there are several methods of screening mares and foals for neonatal isoerythrolysis that your veterinarian may choose to employ. The easiest is to obtain a blood sample from the dam about 14 days prior to her foaling date and testing the blood for anti-red blood cell antibodies. If the test is negative for anti-red blood cell antibodies, the foal can be allowed to nurse normally. If the sample is positive, one option is to test the stallion’s blood type; if he and the mare have the same blood type, the resulting foal will as well. Comparing blood types of the mare and stallion (without running an antibody screen) can also be utilized as a preventative method. Finally, after foaling (but before the foal is allowed to ingest colostrum), a test can be performed to determine if there are antibodies in the colostrum that will bind to the foal’s red blood cells.