Equine Blood Types: Biochemistry, Typing & Related Medical Conditions – [Guide]

Blood types in horses refer to the classification of immune markers known as antigens that are present on the surface of red blood cells. ^[1]

Red blood cells are abundant in the blood of all mammals, and are responsible for transporting oxygen throughout the tissues. The proteins that make up the outermost layer of red blood cells vary between individuals, referred to as variance in expression of surface factors. ^[1]

This variance is genetic and defines different blood types. ^[1] Horses have over 30 major blood types, compared to the 8 major blood types seen in humans. ^[1]

Blood types are important because the horse’s immune system recognizes which surface factors are the body’s own and will not target those red blood cells for destruction. ^[1] Introduction of a foreign blood type, such as during a blood transfusion, can result in the immune system attacking the foreign red blood cells.

Understanding blood types and how the immune system reacts to each type is an important part of equine medicine, particularly in emergencies. Blood typing is also important in the development of neonatal isoerythrolysis, a condition affecting newborn foals.

Blood Types in Horses

Knowing a horse’s blood type is important in scenarios where they are exposed to other blood types, such as during a blood transfusion. If the recipient’s blood type is different from the donor horse, the recipient’s immune system will target the foreign red blood cells for destruction.

A horse’s blood type is determined by its genetic code, which dictates what antigens are produced by the red blood cells.

Antigens are proteins that cover the surface of the red blood cell, and act like a fingerprint: the shape and contours of a blood cell’s antigens identify the cell to the rest of the immune system.

Antigens are classified by group and factor, which establish the horse’s blood type. There are 30 major combinations of blood groups and factors, producing the 30 major blood types in the horse.

Blood Groups

Horses have 7 major blood groups, which refer to specific locations in the horse’s genetic code. ^[1] At each location in the genetic code, different DNA sequences result in the production of different red blood cell proteins.

Depending on what alleles (variants) are present at these locations, the red blood cells produce different factors to express on their membrane surface. ^[1]

The alleles for each blood group factor work as an off/on switch, activating the production of certain set of factors. Depending on the allele present, red blood cells may produce multiple factors within that particular blood group. ^[1] For example, a horse can express A group factors a, b, d, and f at the same time. ^[1]

Horses that do not have any active factors within a blood group are negative for that group (e.g., A-). ^[1]

Each blood group can produce at least 1 factor: ^[1][2]

Blood Testing

If seeking a blood type profile for a horse, a full blood type panel describes the factors present for each blood group. ^[1] However, most laboratories focus on the most antigenic blood factors, or the factors that stimulate the strongest immune system reaction. ^[3]

Commonly tested equine blood factors include: ^[3]

• Group A: factors a, b, and c
• Group C: factor a
• Group K: factor a
• Group P: factors a and b
• Group Q: factors a, b, and c
• Group U: factor a

Use in Blood Transfusions

Blood typing is particularly important for blood transfusions, because introducing a foreign blood type can activate the immune system and trigger a transfusion reaction. ^[4] To avoid transfusion reactions, veterinarians cross-match donor blood with the patient, to ensure that an immune reaction will not develop. ^[4]

Horses receive blood transfusions to treat severe blood loss or anemia (low red blood cell count). ^[5] Causes of anemia in horses include: ^[6]

• Equine infectious anemia
• Piroplasmosis
• Immune-mediated hemolytic anemia
• Drug reactions
• Toxins, such as onions, oak, and red maple
• Liver disease
• Some types of tumors
• Chronic kidney disease

Transfusion Reactions

Transfusion reactions typically occur if the horse has pre-existing antibodies against factors present in the donor horse’s blood. ^[4] These antibodies target the donor’s red blood cells for destruction and removal in a process called hemolysis. ^[5]

Most horses do not have antibodies against other blood factors, as their immune system has never been exposed to those molecules. ^[4] Potential reasons a horse may have pre-existing antibodies include: ^[4]

• Previous blood or plasma transfusion
• History of pregnancy, where the foal’s blood type may have leaked into the mare’s bloodstream
• Accidental exposure to another blood type from contaminated medical equipment

If these situations have not occurred, then most horses can safely receive a single blood transfusion without cross-matching. ^[4] However, future blood transfusions will require cross-matching, as the horse is expected to have antibodies against the previous donor horse’s blood type. ^[4]

Transfusion reactions typically occur within the first 15 minutes after blood administration. ^[4] Symptoms of transfusion reactions may include: ^[4]

• Anaphylactic shock
• Fever
• Rapid heart rate
• Rapid breathing
• Hives
• Sudden onset colic or diarrhea
• Red urine

If symptoms of a transfusion reaction develop, administration of the blood product must stop. ^[4] Emergency administration of medications to counteract the transfusion, such as steroids and epinephrine (adrenaline), may be necessary. ^[4] Transfusion reactions occur in approximately 16% of equine blood transfusions. ^[5]

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Plasma Transfusions

Rarely, plasma transfusions can also cause a transfusion reaction. ^[6] Most symptoms of transfusion reactions from plasma are mild, such as hives or swelling of the face. ^[6]

These reactions occur when the donor horse has antibodies against the recipient’s blood type. ^[6] Horses selected as plasma donors typically undergo testing to ensure they are free of antibodies against other blood types prior to plasma collection. ^[6]

Horses typically receive plasma transfusions to replace lost proteins or to provide antibodies to the patient. ^[6] Common conditions treated with plasma transfusions include: ^[6]

• Protein loss due to severe intestinal disease
• Failure of passive transfer in foals
• Coagulation disorders
• Tetanus treatment using plasma from horses with high tetanus antibody levels
• Malabsorption

Cross-Matching

Cross-matching is a relatively simple test that can reduce the risk of a transfusion reaction. ^[5] However, the test is not a guarantee of blood compatibility, so monitoring for a transfusion reaction is always necessary. ^[5]

The cross-match procedure involves collecting blood from both the recipient and the potential donor horse. ^[7] The veterinarian removes the blood plasma, the component containing antibodies, from both samples. ^[7]

They then wash the remaining blood sample containing red blood cells with saline, to ensure all of the antibodies are removed. ^[7] Once the veterinarian has plasma and red blood cell samples from both horses, they perform a major and minor cross-match.

Major Cross-Match Test

The major cross-match involves adding the recipient’s plasma to the donor horse’s red blood cells. This test checks whether the recipient’s antibodies will target and destroy the donor cells. ^[7]

If the plasma portion of the new blood sample turns red, it indicates hemolysis (red blood cell destruction) is occurring. The veterinarian may also see agglutination, or clumping of the red blood cells, indicating that antibodies are binding to the red blood cells. ^[7]

Minor Cross-Match Test

The minor cross-match mixes the donor horse’s plasma with the recipient’s red blood cells. Similar to the major test, if the red cells clump together, it indicates an antibody reaction in the plasma. ^[7]

Although donor horses are specifically selected based on no previous exposure to other blood types, horses can occasionally produce factor antibodies without previous exposure. ^[7] This test confirms whether the donor horse has pre-existing antibodies against the recipient’s blood type. ^[7]

Antibody Screening

Horses who have previously foaled or received a blood transfusion can be difficult to find blood donors for. ^[7] Antibody screening is a panel test examining for antibodies against common blood factors. ^[7] Based on the results, an appropriate donor horse can be selected based on their blood type and the results of cross-matching. ^[7]

For example, antibody screening may reveal that a recipient horse has antibodies against Ua. In this case, the donor horse must be U- to ensure that the red blood cells are not destroyed by the recipient’s immune system.

Blood Typing

Blood typing can also be an important component of donor selection, particularly for mares that may become pregnant. ^[1] Exposing mares to different blood factors increases the risk of neonatal isoerythrolysis if they become pregnant in the future. ^[1]

Ideally, the donor’s blood type matches the recipient’s blood type as closely as possible to prevent antibody formation against different factors. ^[1] However, a complete blood match is virtually impossible, as there are up to 400,000 potential combinations of blood factors.^[1]

Therefore, most blood typing comparisons aim to minimize the number of new blood factors introduced to the recipient horse. ^[1]

For example, consider a horse with the A group factors a, b, and f. An ideal donor for this horse could have any combination of these three factors, but should not have factors c, e, or g, as this would introduce new factors to the recipient. ^[1]

Use in Neonatal Isoerythrolysis

Neonatal isoerythrolysis (NI) is a specific scenario resulting from mismatched blood types between a mare and her foal. ^[1] In this condition, the mare passes antibodies against the foal’s blood type to the foal in the colostrum. ^[6] Once the foal absorbs the antibodies into its bloodstream, they target the foal’s red blood cells, resulting in hemolysis. ^[6]

Symptoms of NI in foals include: ^[6]

• Jaundice (yellowing of the skin and gums)
• Rapid heart rate
• Rapid breathing
• Difficulty breathing
• Red urine
• Seizures
• Death

Mares who have received a blood transfusion in the past have a high risk of neonatal isoerythrolysis. ^[6]

Mares who develop placental abnormalities early on in pregnancy are also at risk for NI, as the placental abnormality may allow leakage of fetal blood factors into the mare’s circulation. ^[6] This leakage is most significant if the sire’s blood type has strongly antigenic factors that were passed down to the foal. ^[6]

For these reasons, some breeders choose to perform blood typing on the stallion and mare, as well as antibody screening for the mare when selecting breeding pairs. ^[6]

Blood Typing

Blood typing allows breeders to cross-reference which strongly antigenic factors differ between a stallion and mare. ^[6] If the stallion is positive for a strongly antigenic factor and the mare is not, the breeder should withhold colostrum from the foal and provide an alternative source to prevent NI in the foal. ^[6]

Any blood factor has the potential to trigger NI in a newborn foal. However, the most common factors associated with NI are: ^[6]

• Aa
• Qa
• Donkey factor: a specific factor found in donkeys but not in horses

Mares who are negative for Aa and Qa blood factors have the highest risk of producing a foal with NI. ^[6] Ideally, these mares should be bred to stallions who are also negative for Aa and Qa. ^[6]

If they are bred to an Aa or Qa-positive stallion, antibody screening during the last month of pregnancy can alert the breeder to the presence of antibodies resulting from placental leakage. ^[6]

Donkey factor can cause NI in mule foals whose horse dams may have antibodies against this specific blood factor only found in donkeys. ^[6] Mares repeatedly bred to donkey jacks have an increased risk of producing an NI foal due to the potential risk of placental exposure. ^[6]

Jaundiced Foal Agglutination Test

The Jaundiced Foal Agglutination (JFA) test is an alternative to blood typing that is similar to a cross-match for blood transfusion. ^[8]

To perform the JFA test, the veterinarian takes a blood sample from the foal and a sample of the mare’s colostrum. ^[8] The foal’s blood sample must be collected before the foal nurses any colostrum, as it can interfere with the test results. ^[8]

After collection, the veterinarian adds a drop of the foal’s blood to the colostrum sample. ^[8] Similar to the cross-match test, they are looking for signs of agglutination, or clumping, of the red blood cells. ^[8] If agglutination is present, it increases the viscosity of the colostrum and signals that the foal requires an alternative colostrum source. ^[8]

Typically, JFA testing is only necessary for mares who have a high risk of producing an NI foal, such as those who have received a blood transfusion. However, some breeders and veterinarians routinely perform JFA testing for all foals as a preventative measure. ^[8]

Summary

Blood types refer to specific surface molecules found on red blood cells in the horse’s body that the immune system may react to.

• Horses have over 400,000 possible blood types, based on combinations of over 30 different surface molecules
• The immune system recognizes surface molecules from other horses as foreign, and can produce an anaphylactic response
• Veterinarians most commonly consider blood types when performing blood transfusions
• Horses may require blood transfusions due to severe blood loss or diseases that cause red blood cell destruction
• Blood types are also significant in the development of neonatal isoerythrolysis, a condition affecting foals