How Many Blood Types Do Horses Have?

Horses are pretty amazing animals. They’re strong enough to pull almost double their body weight without any problem, and up to six times their weight in short bursts. They also have an instinct for swimming and can run for long bursts.

But it’s not only their physical prowess that’s amazing. They are also unique when it comes to their blood and their blood types. Their blood grouping is so unique that they have the most blood types of all animals studied to date. If you want to learn more about the blood types horses have, continue reading below.

The 8 Blood Group Systems

There are a total of eight different red blood cell groups for horses. They are A, C, D, K, P, Q, U, and T. Sometimes, it’s said horses only have seven blood types, and T is excluded. This is because T is still being studied thoroughly and there’s not much known about it.

Each of these major groups produce their own antibodies. Then, there are minor blood factors, which are the cell membrane proteins. The minor blood factors a, b, c, d, e, f, and g are the most common, but over 34 different blood factors have been identified.

Each blood type includes one of the major groups and at least one of the blood factors. Some examples include D(a, g, k), C(a), A(a, b, c, f, g), and Q(a, b, c). Additionally, like with humans, some of the major groups may combine. For example, you could have a horse with both A and Q major blood groups, in addition to all the blood factors.

Horse Blood Groups Compared to Humans

In comparison, humans only have three blood types, which are A, B, and O. This makes it a little easier to transfer blood as some types are compatible with other types, and there are even universal donors. Humans will have either one or a combination of two of the above blood types, along with a positive or negative marker. In total, there are seven human blood types. That means that humans have the same total blood types that horses have in just major types.

Are Some Blood Types More Common Than Others?

There have not been many studies done on horse blood types, so it’s hard to tell if some blood groups or blood factors are more common across all horses. However, it has been found that the percentages of different blood types are more common across certain breeds.

For example, in Thoroughbreds, if they have a Q blood group and an (a) blood factor, there’s an 82 percent chance that their blood type is a Qa+. Meanwhile, in Standardbred horses, there’s a 100 percent chance that the blood type is Qa-.

How Many Blood Types Do Horses Have?

Because of the numerous combinations that are available, horses can have a huge number of blood types. In fact, that there are about 400,000 different blood types for horses. This includes all the possible combinations of major blood groups and blood factors.

Based on the information scientists currently have, this means it’s very likely that they are the animals with the most number of blood types. Cows have about 800, while dogs have about four, and cats have about three.

Do Horses Have Universal Donors Like Humans Do?

Horses don’t have a universal donor. Blood transfers for horses are one of the most difficult medical procedures for these big beasts. It’s possible to do partial matches, but nearly impossible to have a complete match in blood types for horses.

Usually, this isn’t a problem. In humans, what causes rejections of different blood types is something called alloantibodies. These are antibodies that attack foreign blood types and can lead to someone becoming severely sick if they get the wrong blood type.

Horses, on the other hand, don’t usually have these alloantibodies. Less than 10 percent of horses in the world have them. So the first blood transfusion a horse receives doesn’t have to be a match. However, after that, a horse starts to develop antibodies against the antibodies they receive.

This means that blood matching becomes more and more essential with every transfusion. It’s also important to keep track of all the blood work and transfusions done on horses to prevent any problems.

Struggles With Blood Transfusions

Even when the blood types are compatible, the red blood cells only survive a maximum of four days in the horse getting the transfusion. It’s more likely to destroy the red blood cells immediately. Additionally, horse blood can only be stored for a month, so it’s impossible to have horse blood banks like you would with people.

If the blood types don’t match well, the half-like is only five days, instead of a month. On top of that, there are the chances for complications, however rare, that include infections and fevers.

The Most Compatible Blood Type

Though there aren’t universal donors, some blood types are more compatible than others. In domestic horses, blood types without Aa, Ca, and Qa antigens are generally the best to use. That’s because they are the most common in some form in horses, so a partial match at least is likely. The more likely of the three blood types change based on the breed of horse.

This is important to know as the process of cross-matching blood types takes up to 24 hours, and blood type results often take up to three weeks. If a horse is in desperate need of a blood transfusion, they don’t often have the chance to wait around this long.

Issues That Come From Unique Blood Types in Horses

It’s common for even closely related horses, such as siblings and parents, to have vastly different blood types. Sometimes, this can cause a problem between a mother and a foal. Certain blood types of the mother don’t interact well with the blood types of the foal.

The condition is known as neonatal isoerythrolysis (NI) and is very likely to be fatal to newborns. This only occurs in about two percent of the total horse population, though Thoroughbreds, Quarter Horses, and Standardbreds are more likely to develop this condition. It also appears that horses without Aa and Qa blood types and antigens develop a higher risk of having a foal with NI.

For this condition to occur, a few conditions that must be met. For one, the blood type of the foal must have been fully inherited from the stallion, and be different from the mare’s. Also, the mare must have developed antibodies to the blood type of the stallion (which occurs due to blood transfusions and exposure to foreign blood).

These antibodies are the problem. The foal absorbs these antibodies when it drinks its mother’s milk. These antibodies then attach to the foal’s blood cells and start to destroy them, which can cause organ failure and even death.

More studies are being done on horses, their blood types, and transfusions every year and people fight to find ways to give their horses the best possible care. As more people start to pay to find out their horse’s blood type, and more blood transfusions are being done, more information is being found out about the most compatible types, and the negative effects that come with unmatched blood transfusions.

The photo featured at the top of this post is © Christopher Crosby Morris/Shutterstock.com