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Equines, like humans, have more than one blood type. In some cases, the antibodies in the dam’s blood are incompatible with the blood type that the foal received from its sire. Under these circumstances, the foal would be born perfectly healthy as the antibodies are prevented from reaching the foal by the placenta. The foal doesn’t get ill until it ingests the antibodies that are concentrated in the specialized milk called colostrum that is produced in the first 12-24 hours of the foal’s life.
Neonatal Isoerythrolysis is a reaction to antibodies in the mother’s colostrum related to conflicting blood alleles that affects newborn foals in their first few days of life.
Foals are born healthy, but sometime within the first week, they become progressively weak and lethargic. Mucous membranes will lose their color and later become jaundiced, and their urine may take on a distinct orange tinge. If the disorder progresses, labored breathing and rapid heart rate will develop, followed by possible convulsions, coma, or death. Foals that are untreated will generally succumb to the disorder within 48 hours from developing symptoms. In severe cases, the disease progresses quickly, and a foal may develop shock and die within just six to eight hours after ingesting the colostrum.
Humans have just a few blood types; A, B, AB, and O with three alleles. Equines, on the other hand, have eight possible blood types. These include A, C, D, K, P, Q, T, and U. All but blood type T are recognized internationally, blood type T is used primarily as a research designation. Each of the blood types listed can be broken down into multiple alleles, which can create hundreds of thousands of possible combinations. This can make blood transfusions in horses more complicated than in humans, and in most cases, cross-matching is required before transfusions take place. Universal donors are the exception to that rule, and the ideal universal donor is gelding of a breed other than thoroughbred with negative factor Aa, Ca, or Qa blood.
Equines get blood types and alleles from both parents. In some cases, a foal will acquire their blood type from the sire, but the antibodies in the dam’s blood are incompatible and cause severe anemia. In equines, the antibodies that are passed down from the dam to the foal are not able to cross the placenta and are instead bestowed to the foal in the colostrum, a specialized type of milk that is produced in the first twelve hours after giving birth. The troublesome antibodies are no longer present in the milk after approximately two to three days. Foals who have a horse for a dam and a donkey for a sire, known as mules, have a much higher probability of developing this disorder.
Foals that are afflicted by neonatal isoerythrolysis generally begin to show symptoms within a few hours to a few days after birth, although it can take as long as a week for the symptoms to appear. Preliminary diagnosis will be confirmed by further testing to determine the compatibility of the antibodies to the foal’s red blood cells. One of the more commonly used tests is called a jaundiced foal agglutination test. This test is done by mixing the foal’s red blood cells with colostrum from the mother to see how quickly it clots. This test is only effective the first two or three days after the foal is born, however, and a Coombs test is generally used at that point. A Coombs test is an antiglobulin test which detects if there are antibodies stuck to the outside of the red blood cells. Unfortunately, the Coombs test is not specific to neonatal isoerythrolysis and is prone to false negatives.
Foals that are in medical distress are likely to be hospitalized as soon as possible to manage the symptoms, including the administration of intravenous fluids to prevent dehydration and correct any imbalances. If the foal is less than 24 hours old, it should be muzzled to keep it from drinking the dam’s milk and fed a supplemental milk. If you have stored colostrum from a different mare, you may be able to give this to the foal once it is tested for compatibility. If the anemia is severe, a blood transfusion from the dam may be required.
In order to make the blood from the dam safe for the foal it will need to be washed. If this is not possible, or if the dam is unavailable for the transfusion, blood from a suitable blood-typed donor may be used. The patient’s liver function will need to be continually monitored during recovery, and in order to ensure that the mare’s milk does not dry up, she will need to be milked every few hours.
Prognosis for this disorder depends on how much of the antibodies were concentrated in the colostrum. Foals who receive lower concentrations have a better prognosis than those who receive higher levels. The best treatment for this disorder is prevention, however. Testing to ensure that the mare and stallion have compatible alleles often prevents this disease. Many horse owners also test the mare’s serum for antibodies a week or two before she is scheduled to foal to ensure compatibility. Mares who have produced a foal that experienced neonatal isoerythrolysis are more likely to have subsequent foals who are afflicted with this disorder.
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