Equine Blood Transfusions

A veterinarian and professor of equine surgery explains the unique challenges of blood transfusion in horses.


Q: My horse was hospitalized recently after he sustained a serious injury, and part of his treatment included a blood transfusion. (He’s now making a full recovery.) My wife and I regularly give blood to the Red Cross blood bank, and it made me wonder about equine blood donations. When veterinarians need to administer a transfusion to a horse, where do they get the blood? Is there a “blood bank” for horses? Can people volunteer their healthy horses as donors?

A: Equine red blood cells do not store well, and so unfortunately, there are no big blood banks for horses as there are for people and small animals. Generally, veterinarians do not draw blood from a donor horse until the transfusion is needed. Another complicating factor is that horses have a lot more blood types than people do, and there are no true universal donors.

A “blood type” is how blood is classified based on the type of antigens0 that appear on the surface of the red cells. In people, there are four major blood types---A, B, AB and O---with an additional classification, positive or negative, based on the Rhesus (Rh) factor, which refers to a different set of antigens that might be found on the cells. (In addition, the International Society of Blood Transfusion currently recognizes more than 30 different blood classification systems, based on hundreds of other antigens a person might have that can complicate blood transfusions, but these are rare.) If a person receives a transfusion that is a poor match for her own blood, her antibodies0 will identify the new red cells as “foreign” and destroy them; this can create a potentially serious systemic allergic reaction.

Horses have eight major blood types---A, C, D, K, P, Q, U and T (the first seven of which are internationally recognized)---with more than 30 different factors for which they can be positive or negative. These can combine in a number of different ways, designated by an upper-case letter for the type, combined with a lower-case letter for the factor, such as A (a,b,c,f,g), Ca, D (a,g,k), Ka, P (a,b), Q (a,b,c), etc. All of these possibilities create about a 1-in-400,000 chance of finding a 100 percent perfect match between two horses. The two most important and problematic antigens we consider when selecting equine blood donors are Qa and Aa.

Usually blood transfusions are administered to horses with really low red blood cell counts, most commonly because of hemorrhage. The average horse has about 40 liters of blood and can lose up to a quarter of that without having severe signs of shock. More than 10 liters of blood loss may require a transfusion.


When a veterinarian needs to administer a transfusion, she will typically test several available donor horsesfor compatibility. These tests, called cross-matching, are done by mixing blood samples from the donors with the patient’s red blood cells, then looking for signs of clotting or other potentially serious effects. Reactions to transfusions are more likely if the horse has had a previous transfusion (or treatment with other equine products originating from blood), which caused him to build up antibodies to the foreign blood.

In some situations where testing is not possible, such as in a rural practice, it may be decided to pick the best candidate and just use him as a donor. Certain breeds---Quarter Horses, Standardbreds, Morgans---are more likely to be negative for the Qa or Aa antigens. Unrelated geldings are better choices than mares who have foaled, especially more than once, because they are more likely to be carrying antibodies against different red blood cells resulting from their pregnancies. Horses who have previously received transfusions are not good candidates either, also because they’ve created antibodies to the foreign blood products. The donor horse must be healthy. He should be checked for equine0 infectious anemia (EIA) and be free of other diseases. Larger horses can donate more blood than smaller ones. We typically take less than 20 percent of their total volume of blood.

Most university referral hospitals keep a population of horses they can use as blood donors. Other places may rely on their veterinarians’ personal horses. If you have a horse who might be a good candidate and you’re interested in volunteering him, I’d suggest contacting your local veterinary hospital and asking about their policies.

Julie Cary, DVM, MS, DACVS
Washington State University
Pullman, Washington