Equine Blood Groups & Transfusion

I haven't actually made a mindmap during our conversation. However, I can provide a comprehensive summary of the information contained in the provided excerpts from "Blood Groups and Blood Transfusions in Horses - Circulatory System - Merck Veterinary Manual.pdf", which covers several key aspects of equine blood transfusions.

Here's a summary of the main points:

• Equine Blood Types:

  • Horses possess eight recognised blood groups (A, C, D, K, P, Q, U, T), with T not being internationally recognised.

  • Each group is associated with a gene that produces surface molecules (factors) on red blood cells (RBCs), which act as antigenic sites.

  • There are approximately 400,000 possible combinations of groups and factors, making a "perfectly matched" transfusion virtually impossible.

  • Unlike many other mammals, horses generally do not have naturally occurring anti-RBC antibodies. When present, these are usually acquired through exposure, such as pregnancy (leading to neonatal isoerythrolysis) or repeated transfusions.

  • AntiCa (blood group C, factor a) antibodies can occur naturally, causing mild but typically not clinically significant agglutination.

  • Older horses tend to develop more alloantibodies, possibly due to cross-reaction with dietary proteins.

  • Donkeys have a unique "donkey factor" RBC antigen, and some horses have anti-donkey factor antibodies, which is relevant for blood product selection for donkeys or mules. Mule foals can also suffer from neonatal isoerythrolysis.

• Equine Blood Products:

  • There are no commercial blood banks for whole blood or erythrocyte products for horses.

  • Most erythrocyte-containing transfusions use whole blood obtained from donor horses, often due to the logistical difficulties of centrifuging large volumes for adult horses.

  • Packed RBCs are useful for patients with hemolysis who cannot tolerate high-volume transfusions.

  • Washed maternal RBCs are the preferred product for foals with neonatal isoerythrolysis, but are rarely administered due to logistical challenges.

  • Concentrated platelet products are not used, and whole blood administration is usually insufficient to replenish platelets to clinically relevant levels. Platelets require fresh, uncooled blood to remain active.

  • Commercial equine plasma is available, often with specific antibodies (e.g., against Rhodococcus equi, Clostridium botulinum, endotoxemia).

  • Plasma rich in immunoglobulins is used for foals with failure of passive immunity transfer, while fresh frozen plasma (FFP) is needed for coagulation factor replenishment.

  • When using plasma for donkeys or mules, it's advised to confirm the donor is negative for donkey factor.

• Crossmatching in Horses:
  
  

  • Due to minimal naturally occurring anti-RBC antibodies, most first transfusions can often be performed without crossmatch, especially if the horse hasn't been sensitised previously. Forgoing transfusion is almost always worse than transfusing without a crossmatch when indicated.

  • Major and minor crossmatch procedures are similar to those in small animals and humans, involving donor erythrocytes with recipient serum and vice versa.

  • A key difference is the need to add complement to assess for hemolytic transfusion reactions, as horses have endogenous hemolysins. These tests are technically challenging and usually done in referral settings.

  • Crossmatch results are not stable on equine blood aliquots stored for a week or more, making pre-storage of donor samples impractical.

• Blood Donor Selection and Collection:
  
  

  • There is no equine universal donor.

  • Ideal donors are Qa and Aa antigen- and antibody-negative. This blood type is most common in Standardbreds and Quarter Horses.

  • Desirable donors are typically large, quiet geldings or maiden mares with a PCV > 35%.

  • Donors must be fully vaccinated and annually tested for bloodborne diseases, including equine infectious anemia, equine viral arteritis, parvovirus, and hepacivirus.

  • For collection, donors should be quiet, potentially sedated, and have a PCV > 35%.

  • Commercial blood collection bags with citrate phosphate dextrose adenine (CPDA)-1 are recommended for blood storage.

  • Up to 15–18 mL/kg of whole blood (7–8 L for a 450-kg horse) can be collected per donor.

  • Donors should have ample fresh water and optionally receive crystalloid fluids.

  • Horses can donate no more than every 3–4 weeks.

• Equine Blood Product Administration:
  
  

  • Normal blood volume is approximately 8% of body weight (80 mL/kg).

  • An intuitive formula can calculate blood deficit, or as a simpler rule: every 2.2 mL of transfused whole blood/kg increases recipient PCV by 1% (assuming donor PCV > 40%).

  • Clinical signs of anemia (increased heart/respiratory rate, weakness) appear with 20% blood volume loss, with 40% loss often fatal.

  • Clinical signs (e.g., tachycardia > 60 bpm, colic, listlessness, anorexia, hyperlactatemia > 3 mmol/L) are the primary triggers for transfusion, rather than a specific PCV value, especially in acute cases. Horses with chronic anemia can tolerate much lower PCVs.

  • Transfusion technique is similar to small animals/humans, using a filtered blood administration set replaced every 4 L.

  • Administration should be slow for the first 15 minutes (1–2 drops/second), with close monitoring for reactions (increased rectal temperature, piloerection, sweating, tachycardia, colic, diarrhea). If no reaction occurs, the rate can be increased.

  • Reactions are treated by stopping the transfusion, giving crystalloid fluids, and epinephrine if necessary.

  • The lifespan of transfused donor erythrocytes is only 20–40 days (or under 1 week for poorly matched cells), meaning a robust bone marrow response is crucial for sustained benefit.

  • Repeated transfusions from the same donor can be given for up to 1 week without re-crossmatching.

  • Equine blood is usually collected at the time of use; it can be stored for up to 28 days with meticulous technique, though red cell half-life drops.