[b]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.
System Thoroughbred Arabian Standardbred Quarterhorse Morgan
Aa- 15% 18% 44% 51% 43%
Qa- 39% 79% 100% 83% 99%