Blood Groups and Blood Transfusions in Horses

Horses have 8 blood groups: A, C, D, K, P, Q, U, and T (although T is not internationally recognized).  Each group corresponds to a gene that produces surface molecules on red blood cells (RBCs) called factors. These factors represent surface protein markers containing antigenic sites. The blood group is a combination of the group and the factors present. For example, horses with blood group A can have factors a, b, c, d, e, f, or g, or a combination of these. There are 400,000 possible combinations of groups and factors, meaning that giving a “matched” transfusion is all but impossible. However, unlike many other mammals, horses do not typically have preformed anti–RBC antibodies. When these exist, they are typically acquired by exposure (usually through pregnancy; see neonatal isoerythrolysis) or by repeated blood transfusions over an extended period.

AntiCa (blood group C, factor a) antibodies can naturally occur and cause mild agglutination reactions; however, these are not usually clinically important in vivo. As horses age, they seem to develop more alloantibodies, possibly due to cross reaction with ingested dietary proteins.

All donkeys possess the RBC antigen known as donkey factor, and some horses express anti–donkey factor antibodies. This is relevant when selecting equine blood product (either whole blood or plasma) donors for donkey or mule patients. Mule foals also can suffer from neonatal isoerythrolysis when alloantibodies in mule's maternal blood are passed to these foals in colostrum; this occurs when mule mares and stallions have different blood types and foals inherit the stallion's blood type.

No commercial blood banks provide whole blood or erythrocyte products for horses, and most erythrocyte-containing transfusions are of whole blood obtained from donor horses, either from the hospital or provided by the owner. Due to the logistical difficulties of centrifuging large volumes of blood needed for adult horses, most transfusions are of whole blood.

Packed RBCs are useful in patients that require transfusion as a result of hemolysis (for example, from red maple leaf ingestion) and cannot tolerate high-volume transfusions. Washed maternal RBCs are the product of choice for foals with neonatal isoerythrolysis but are rarely administered—again, due to the logistical difficulties of washing a high volume of blood without large centrifuges. 

Concentrated platelet products are not used in equine transfusion medicine, and it is difficult to administer sufficient volumes of whole blood to replenish platelets to clinically relevant levels. Platelets are inactivated by refrigeration; therefore, fresh, uncooled blood must be administered if platelets are to remain active.

Commercial equine plasma is readily available, often with specific antibodies against infectious agents such as Rhodococcus equi, Clostridium botulinum, and bacteria causing endotoxemia (J5). Plasma with high levels of immunoglobulins is typically used for foals with failure of transfer of passive immunity. If plasma transfusion is to be used for replenishment of coagulation factors, specific fresh frozen plasma (FFP) must be used. When administering plasma to a donkey or mule, a veterinarian can check with the company manufacturing the plasma to confirm whether the donor of the unit is negative for donkey factor.

Horses have minimal naturally occurring anti–RBC antibodies, and most transfusions can usually be performed without crossmatch if necessary, especially if the horse has not been sensitized by previous transfusions. Transfusion without crossmatch is almost always preferable to forgoing blood transfusion in horses in which it is indicated.

Major and minor crossmatch procedures in horses are similar to crossmatching in small animal and human patients; donor's erythrocytes are washed and incubated with recipient's serum, and donor's serum is with washed recipient's erythrocytes. One important difference in horses is that in addition to agglutination reactions, hemolytic transfusion reactions can occur in horses because they have endogenous hemolysins. Complement must be added to the reaction mixture to assess for hemolysins. These tests are technically quite difficult and usually only performed in referral hospitals or laboratories.

Because many equine hospitals maintain a blood donor herd, samples from these donors could theoretically be stored in the laboratory so that crossmatches could be performed without phlebotomizing horses each time; unfortunately, however, crossmatch results are not stable on aliquots of equine blood stored for a week or more.

There is no such thing as an equine universal donor. However, in a blood donor herd, donor horses should ideally be Qa and Aa antigen- and antibody-negative. Tests to determine this can be run at specific laboratories, ideally with antibodies checked annually.

The Qa/Aa negative blood type is most commonly found in Standardbreds and Quarter Horses. Desirable donors are typically large, quiet geldings (or maiden mares) with a PCV > 35%. Donors need to be fully vaccinated and undergo annual testing for bloodborne diseases, including equine infectious anemia and equine viral arteritis, as well as the equine hepatitis viruses parvovirus and hepacivirus, as follows:

• Horses that test positive for parvovirus can fluctuate for years above and below the limit of detection of the PCR assay. A conservative approach would be to remove them permanently from the donor herd.

• Horses that test positive for hepacivirus can be retested in 2–6 months. If they remain positive in 6 months, they are likely to remain persistently infected for life and should be removed from the donor herd.

For blood collection, donors should stand quietly and be sedated, if necessary. The PCV should be checked before donation to ensure it is > 35%. A 14-gauge (or even 10-gauge) IV catheter is placed aseptically. Some recommend placing the catheter pointing up the neck to speed collection, although this is optional.

Commercially available blood collection bags are preferable to large glass bottles, and a range of anticoagulants are available, although citrate phosphate dextrose adenine (CPDA)-1 is recommended if the blood is going to be stored. Up to 15–18 mL/kg of whole blood per donor may be collected; this equates to 7–8 L for a 450-kg horse, which is approximately 18–20 standard 450-mL blood collection bags.

Ample fresh drinking water should be provided during and after collection. Volume replacement with crystalloid fluids (10–20 L, IV) is optional for healthy donors. Horses can donate no more than every 3–4 weeks, based on recovery of PCV to > 35%.

Normal blood volume for an adult horse is approximately 8% of body weight (80 mL/kg), and thus blood deficit can be calculated using an intuitive formula: (normal PCV − current PCV)/desired PCV × 0.08 × body weight (kg). Alternatively, assuming the donor's blood has a PCV of at least 40%, every 2.2 mL of transfused whole blood/kg body weight will increase the recipient's PCV by 1%. 

Clinical signs of anemia (ie, increased heart rate, increased respiratory rate, weakness) start to become evident with the rapid loss of 20% of blood volume, with blood loss of 40% typically being fatal. Therefore, an even simpler rule of thumb is that for an equal-sized donor and recipient, take as much blood as possible from the donor, and give all of it to the recipient. During donation, the donor should be monitored for changes in heart rate, respiratory rate, or other signs of anemia, and the donation ceased if heart rate increases.

However, many horses that have had acute hemorrhage will still have a normal PCV; typically, plasma protein will drop before the PCV. In addition, horses with acute anemia will show clinical signs when their PCV is at or below around 20%, whereas horses with chronic anemia can reach a PCV of under 10% with minimal clinical signs at rest. As a result, clinical signs should be assessed as triggers for transfusion, rather than a specific numerical value of PCV. Indications for whole blood transfusion include anemia with any combination of tachycardia (heart rate > 60 bpm), colic (decreased GI perfusion), listlessness, anorexia, and hyperlactatemia (> 3 mmol/L).

Transfusion technique in horses is the same as that in small animals and humans. Blood should be administered through a filtered blood administration set that is replaced every 4 L. The transfusion should be administered slowly, if possible, for the first 15 minutes, at 1–2 drops per second for a full-sized horse and slower for foals and small equids. Monitor every 5 minutes for clinical signs of reaction (increased rectal temperature, piloerection, sweating, tachycardia, colic, diarrhea) during the transfusion. If no reaction is noted, the administration rate can be increased considerably, with vital signs monitored every 15 minutes. Treat reactions by stopping the transfusion and giving crystalloid fluids and epinephrine, if necessary (1–5 mL of 1:1,000 [1 mg/mL] epinephrine, IM, or 1–2 mL, IV, slowly, in a 450-kg horse).

The lifespan of well-matched, transfused donor erythrocytes is only 20–40 days in horses, likely because transfusions are not perfectly matched; poorly matched transfused RBCs have a lifespan of under 1 week. Therefore, a robust hematopoietic bone marrow response is essential in the recipient to maintain the higher PCV achieved through transfusion; transfusions for anemia due to decreased hematopoiesis are likely to give only transient benefit. Repeated transfusions can be administered from the same donor for up to 1 week without repeating crossmatching procedures. Equine blood is usually collected at the time of use and not banked; it can be stored for up to 28 days, although red cell half-life drops. Correct storage techniques should be followed meticulously.

• Equine Blood Type and Antibody Screen. UC Davis School of Veterinary Medicine.

• Jamieson CA, Baillie SL, Johnson JP. Blood transfusion in equids: a practical approach and review. Animals (Basel). 2022;12(17):2162.

• Also see pet owner information regarding blood groups and blood transfusions in horses.