Although a transfusion with the wrong blood type won’t necessarily put a horse’s life in jeopardy, it’s still a good idea to seek a match, according to a study from the University of Pennsylvania’s New Bolton Center.
In people, there are eight different blood types, determined by group (A, B, O and AB) and antigens0 (known as the Rh factor). If a person receives blood with incompatible antigens, the resulting immune response can be deadly. For this reason, people are usually only given blood that matches their own type, and a test called cross-matching is done using the blood of both the donor and recipient prior to a transfusion.
By comparison, the procedure for administering blood transfusions to horses is simpler because mismatched blood types do not usually lead to severe adverse reactions. “Horses do not generally have the naturally occurring alloantibodies [antibodies against different blood types] the way humans and cats do,” explains Rose Nolen-Walston, DVM. “The conventional wisdom was that they [only] needed to be cross-matched after the first transfusion because it was assumed that they would [then] make antibodies against the proteins on the transfused blood.” There are eight major equine blood groups (A, C, D, K, P, Q, U and T) and nearly 30 equine antibody factors, making for a total of nearly 400,000 different combinations.
In a study to investigate the potential long-term impact of matching blood types for equine transfusions, Nolen-Walston first took blood samples from 20 healthy horses and determined their blood types. Then various samples were cross-matched to see whether clumping of blood cells or other signs of adverse reactions occurred. The degree of incompatibility was scored on a 0 to 4 scale. Based on this test, the horses were then paired as donors and recipients for two compatible and eight incompatible transfusions.
After the transfusions were performed, the horses were monitored for signs of reactions for five days. Among the eight who received transfusions with an incompatible blood type, seven developed mild signs of adverse reactions, including a low fever, increased heart rate and hives. None of the complications were life-threatening.
After any transfusion, a recipient’s body eventually destroys the donor’s blood and replaces with it with its own type. To determine how quickly this occurred in the study horses, the researchers drew blood from the recipient horses one hour after the transfusion and periodically for the next 35 days. They then tested each sample to determine how much donor blood was left.
“Horses can regenerate blood pretty quickly,” says Nolen-Walston. “Within a week they are already mounting a good regenerative response. If a horse is given incompatible blood, it’s possible that the blood would be destroyed so rapidly that the horse would develop dangerous anemia before the body had caught up and produced enough red blood cells to carry oxygen.”
The data showed that in incompatible pairs, the new blood was destroyed seven times faster than in pairs that were compatible, with half of the transferred red blood cells absent three to five days later. In horses given blood that was matched, the decline was gradual, suggesting a lifespan of around 70 days for the transfused cells.
This, Nolen-Walston says, indicates that matched blood types result in longer benefits and sheds light on the potential risks of incompatible matches. She notes that “other studies have described horses having fatal reactions to incompatible blood, so it is possible, though rare. In this study, most of the incompatibilities were horses receiving blood type ‘Ca,’ whereas the more life-threatening reactions are more likely to be against type Aa and Qa blood.”
Reference: “Survival time of cross-match incompatible red blood cells in adult horses,” Journal of Veterinary Internal Medicine, November 2015
This article first appeared in EQUUS issue #461, February 2016.