Although blood transfusions in horse's are rarely needed, it can be needed in certain conditions that result in a loss of a large amount of blood, according to Dr. Rose Nolen Walston, an internist at the University of Pennsylvania. Blood transfusions in horses share a lot of similarities with humans but there are some differences; while humans have three main blood types, horses have seven blood groups, and each group also has one of seven different factors. This leads us to a mathematical possibility of 400,000 different blood types in horses. Because of this possibility, giving matched blood in horses is difficult. Fortunately, although transfusion reactions in humans are a big problem, only 10% of the horses contain alloantibodies that attack the foreign blood cells. Therefore, the chance of reaction with the first transfusion of a horse is low but a second transfusion can be a problem. However, a study revealed that when unmatched blood was transfused to a horse, the red blood cells lasted less than five days compared to a matched transfusion in which the blood cells lasted 33 days. So it is certainly better to cross match and transfuse matched blood but sometimes this is impossible in an emergency.
The other difficulty with a blood transfusion in horses is collecting and storing the blood. Equine blood can only be stored for about a month and storing large amounts of blood can also be a problem. Horses that require a transfusion may need two to three gallons of blood, compared to a pint that people usually require, and that requires a lot of refrigerator space to store blood that only lasts one month.