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Finding the Ideal Blood Donor Horse in an Emergency

large animal

Blood transfusions are often unplanned, and therefore, things are often chaotic. The purpose of this handout is to help you decide which horse is best to use as a donor, screening tools to consider, and knowing how much blood to collect/how to safely acquire blood in a field setting. Hopefully this will give you more confidence the next time you need to do this in an emergency!

Ideal blood donor characteristics:
  • Geldings
    • Why not mares? Any possibility of previous pregnancy means there is the potential for anti-RBC antibodies in their blood. If you know for certain that a mare has never been bred, and there is no stallion on the premises, a mare would also serve just as well.
      • If all you have are mares that have previously been bred, just check them for anti-RBC antibodies before you use them as a blood donor!
    • When would a gelding NOT be ideal? If they’ve previously received a blood transfusion!
  • Middle-aged
    • Older horses (>20 years old) often have lower red blood cell counts, or are more likely to have other comorbidities that will make them a less than ideal donor. That being said, if you know a horse > 20 year old that has a solid packed cell volume and is the picture of health, you can use them as well.
  • Breeds of choice:
    • Standardbreds, Friesians, Morgans, Peruvian Pasos, Tennessee Walking Horses (Quarter Horses are also acceptable but prioritize these other breeds)
    • Why these breeds?
      • Aa and Qa are the most antigenic blood types in horses
      • Horses from these breeds are most likely to be negative for Aa/Qa based on current research; this means less chance of transfusion reactions/problems with incompatible blood types
  • Vaccines: please ensure your donors are current!
    • Core vaccines (EWT + WNV) +/- risk-based vaccines depending on your region and the horse’s environment

Pre-transfusion screening: Ideally performed annually for designated blood donor horses-
  • Equine parvovirus
  • Equine infectious anemia
  • Anti-RBC antibodies
  • These are not conditions you’ll be able to test for in an emergent setting, if you haven’t already tested your donor horse(s). If you don’t know your donor’s status with regards to these conditions, though, warn your clients about the possible adverse effects! The presence of any of these conditions can be variably problematic (even fatal) for your patient.
    • Equine parvovirus can cause severe acute hepatitis
    • Equine infectious anemia is essentially a death sentence for positive animals (either that or a lifelong quarantine)
    • Anti-RBC antibodies increase the risk of a severe transfusion reaction including, but not limited to, hemolysis post-transfusion.

How much volume to take?
  • Up to 20% of your donor’s blood volume (this corresponds with 8L in a 500kg horse)
    • 8L is considered the ‘universal’ blood donor volume
  • However, if you are truly removing 20% of the donor’s blood volume (ie not a giant draft horse), this is a sufficient volume to cause mild clinical signs from blood loss
    • IDEAL situation is to replace the blood volume taken with isotonic fluids post-transfusion; this will negate any adverse effects from this amount of blood loss
  • *ALWAYS check a PCV/TS before taking 8L blood from your donor! If they’re borderline anemic, you will want to use a different animal.

What to do with the blood as you collect it:
  • Have your sterile fluid bag(s) prepped with 10% ACD (acid citrate dextrose) as an anticoagulant (this corresponds with 90mL ACD for 900mL whole blood)
  • Alternate anticoagulants that will also work: CPD (citrate phosphate dextrose) or CPD-A (citrate phosphate dextrose-acetate)
    • If needing to store blood and not give right away (up to 3 weeks in the fridge), use CPD-A


About the Guide: Sarah Humphreys, DVM, DACVIM (LAIM)

Dr. Sarah Humphreys is a large animal internal medicine specialist, and as part of her training she completed a rotating internship at University of Pennsylvania’s New Bolton Center, a rotating fellowship at Oregon State University, and her residency at University of California Davis. Sarah started Humphreys Veterinary Consulting in 2022 to offer teleconsulting services for large animal veterinarians with internal medicine cases, and  she loves teaching veterinarians and students alike.



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