Equine blood groups

There are over 30 blood groups in horses, of which only 8 are major systems. Of these 8, 7 are internationally recognized (A, C, D, K, P, Q and U), whilst the T system is primarily of research interest. Of these, the Aa and Qa are most important for hemolytic reactions, especially neonatal isoerythrolysis (NI). Other blood groups can occasionally give NI reactions, including Dc, Ua, Ab and Pa. In addition, all horses lack a unique red cell antigen to donkeys, so they will produce antibodies (and NI) when exposed to donkey blood (such as in mule pregnancies). Natural antibodies do exist, particularly to Ca antigens, which cause weak agglutination and hemolytic cross-match reactions, however the antibodies to Ca do not appear to produce a significant hemolytic reaction in vivo. The incidence of Aa and Qa is breed-dependent. The table below gives the percentage of animals in the listed breed that are negative for the factor.

SystemThoroughbredArabianStandardbredQuarterhorseMorgan
Aa-15%18%44%51%43%
Qa-39%79%100%83%99%