Neonatal Isoerythrolysis

"The results of the ones that we have had with the same problem I would say that you have about a 50/50 chance of survival. I try to do a simple test, which you might want to do in the future....take a drop of blood from the foal (I get it when the umbilical chord breaks) and mix with a drop of colostrum from the mare. If it mixes and stays mixed, everything is okay. If it separates and appears to curdle, which it will do very quickly, do NOT LET THE FOAL NURSE but get the vet there IMMEDIATELY to give the foal IGG, or an artificial colostrum or colostrum from another mare. Then keep the foal with the mare, but muzzled for 48 hours and bottle feed with another milk source.

Milk the mare so that she won't stop making milk. After the colostrum is gone the foal can start nursing without danger. It is only the antibodies in the colostrum that fights the foals immune system."

Neonatal Isoerythrolysis Everyone who foals out a mare knows that colostrum is a good thing. Critical for immune defense in the first several months of life while the foal’s own immune system builds itself up, colostrum is produced by the mare only for a short time after foaling. The foal’s GI tract can only absorb the antibodies contained in colostrum for 12-24 hours after it first nurses, so ensuring that the events of foaling pass normally and the foal stands and nurses (and continues to do so) is extremely important. Foals who fail to do so require supplemental feeding or plasma transfusion to get them through the critical period and prevent problems such as infections and sepsis.

However, there exists a situation where mare’s colostrum can be highly detrimental and even fatal to the foal: neonatal isoerythrolysis, or NI. NI is a condition in which antibodies are produced by the mare against the foal’s red blood cells causing red blood cell destruction and anemia in the foal. This is very similar to Rh factor in human pregnancy when the father passes Rh positive status to the baby and the mother is Rh negative; her body makes antibodies against the ‘foreign’ Rh factor. Almost always, NI occurs with mares who have had multiple foals, and is more likely in the mare’s second foal by the same stallion.

Horses have many blood groups, and it is not uncommon to cross a mare and stallion with different blood types and not have a problem. Certain of the blood types are more likely to cause NI than others; mares that do not have a Qa or Aa blood type, and are bred to stallions with Qa or Aa are the most likely to develop problems. If the mare is exposed to the ‘foreign’ blood type either from placental problems during pregnancy or from blood contamination at foaling, her body develops antibodies against the blood type.
If the exposure occurs at foaling, the antibody response takes long enough to build that the current foal will not be affected. The next foal by that stallion (or another stallion with that blood type) would be affected because the antibodies are already in the colostrum and absorbed through the foal’s intestines in the first hours of life.

How will you know if you have an NI foal? Usually these foals are born normal, nurse well, and then begin to go downhill within the first few days of life. At their foal check, they usually have adequate passive transfer of antibodies (IgG levels). As the anemia progresses, they become weak, uninterested in nursing, have elevated heart and respiratory rates, and their mucous membranes (eyes and gums) will become yellow with bilirubin, a byproduct of red blood cell destruction. In milder cases, supportive care may be enough to get the foal through the event.

Preventing nursing from the mare while providing supplemental feeding, avoiding dehydration, and keeping the foal as low-stress as possible is important. Antibiotics and steroids can be used to prevent infection and reduce the antibody response respectively. After 36-48 hours of life, the intestinal tract of the foal will no longer absorb antibodies from the mare’s milk, antibodies in the milk have declined dramatically, and nursing can be safely reintroduced.

If the disease causes severe anemia, a blood transfusion will be required. The ideal donor is the foal’s dam, but the red blood cells must be ‘washed’ to remove all antibodies contained in her plasma. Other donors may be used but cross-matching is important to prevent making the problem worse.

Prevention of NI is possible with a bit of planning ahead. Blood typing the mare and stallion is easily performed with a small amount of blood. If the stallion is bred to many mares, he may already be typed. The mare’s first pregnancy is usually ‘safe’ unless she has had a blood transfusion in the past. In these cases, compatibility should also be checked. If the stallion is positive for Aa or Qa blood type, the mare is negative, and this is not her first foal (a foal by the same stallion is higher risk but remember, previous stallions may have had these blood types and resulted in exposure), she should be considered ‘high risk’ for an NI foal and steps taken in accordance.

Muzzling the foal once it stands and before it nurses, and giving colostrum from another mare for 36-48 hours will protect the foal from the dam’s antibodies while ensuring it gets the colostral antibodies it needs. The foal may stay with the dam if the muzzle stays in place. She will need to be milked during this time, not only to keep her comfortable but to remove the colostrum from her udder. There are also tests that can be performed by your veterinarian with the foal’s blood and the mare’s blood or colostrum, to indicate whether there are antibodies present against the foal’s red blood cells.

NI is just one of the many things to think about when it comes time for foaling. As with many foal diseases, early recognition of a problem and quick intervention is key. Having your veterinarian out within the first 24 hours to do a foal check, and then alerting them quickly if anything seems to be abnormal goes a long way to control problems before they get out of hand. If you have any concerns about the possibility of NI in your breeding program, please contact your veterinarian or any of the veterinarians at New England Equine Medical and Surgical Center, to make a plan for prevention. If you have a foal on the ground that you are concerned about, for NI or any other reason, calling the veterinarian as soon as possible increases the chance for a happy outcome.

Credits:
Thank you to: Susan Barnett, DVM Jacqueline Bartol, DVM, DACVIM  

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