Neonatal Isoerythrolysis in Foals

A Silent Killer

NEONATAL ISOERYTHROLYSIS IN FOALS

Horses have been foaling without a lot of interference from us for thousands of years. Which is as it should be.

It is a very straightforward process and once the foal is safely delivered, everyone can breathe a sigh of relief. I like to teach people the 1-2-3 rule which is: The foal should stand within an hour, nurse within two hours and the mare should pass the placenta within three hours. However, there is one very important instance where you should NOT let the foal nurse its mother for at least 36 hours.

My mother was the neighbourhood horse midwife when I was growing up. We had a small Arabian breeding program and raised a handful of foals each year. I would help with our own foals and as soon as the foal stood and nursed, that was the signal that we could relax and hopefully soon be in our own beds. The sooner the foal nursed and took in the colostrum that would provide the protective antibodies needed to survive until its own immune system kicked in, the better. Growing up I never knew that colostrum could sometimes be deadly to the foal.

It wasn’t until I was foaling out over 100 mares/year for a large breeding farm that a few cases came up that changed the protocol on every foal from that point forward. Especially after the advent of embryo transfer and the use of recipient mares.

Most people are familiar with the term ‘Rh factor’ when it comes to discussing human babies. Babies inherit a protein that attaches to the red blood cells. If the baby has the protein, they are Rh positive; if they don’t have the protein, they are Rh negative. So if the baby was, say, blood type ‘O positive’, it would have the blood type O and the protein on their red blood cells. An incompatibility can occur if the baby is Rh positive and the mother is Rh negative. Even though there is usually not exchange of blood between the baby and the mother during pregnancy, sometimes there may be some leakage or there could be blood contact between baby and mother with an incompatibility at birth. When that happens, the mother’s immune system builds antibodies to the baby’s blood type. It will not be a problem for the current birth, but the mother’s immune system is now sensitised and if she has another Rh positive pregnancy, the antibodies will cross the placental barrier and attack the baby’s red blood cells, causing severe anemia in the baby.

AN NI FOAL SHOWING THE MUZZLE REQUIRED TO STOP FEEDING FROM ITS MOTHER

PC CINDY REICH

In horses there is something similar called ‘Neonatal Isoerythrolysis’ (NI) which simply means a condition whereby a newborn’s (neonatal) red blood cells (erythrocytes) are destroyed (lysis) by antibodies present in its own mother’s colostrum. In humans, it is a simple Rh factor. In horses, there are several ways the mare can develop antibodies against her foal.

It can happen when a foal inherits a blood type from the stallion that is foreign to the mare (there are actually seven different blood groups in horses). Mares that have had multiple foals might have had some leakage from the placenta that allowed some blood from the foal to enter the mare’s circulatory system and sensitise against the foal. Mares that received blood products early in life may have a sensitised immune system as well. Also, recipient mares are a risk factor as they are not the same blood type as the foal they are carrying.

It is important to note that in cases whereby the mare’s blood group is not compatible with the foals due to the sire being of a different blood group, the first foal from this type of mating will usually not be affected as the antibodies are activated during the birth process. Thus, a healthy foal does not mean subsequent foals from the same mating will be healthy as well…quite the opposite in fact. The initial healthy foal may have activated the antibodies in the mare, and they will be present in the colostrum for her next foal.

What happens in an NI foal is that the antibodies that the mare has formed against the foal’s blood type concentrate in her colostrum. That is the function of colostrum – to convey antibodies to the foal to protect it from disease until it develops its own immune system at 5–6 months of age. However, not all antibodies are helpful, and in the case of NI foals, the antibodies will seek out and destroy the foal’s red blood cells. Affected foals will become weak and jaundiced (yellow mucous membranes, yellow eye sclera) at around 12–24 hours and can die. Because what is happening is that the antibodies in the colostrum are attacking the foal’s red blood cells, making it severely anemic. Unfortunately, by the time foals are showing signs of being an NI affected foal, it is often too late to intervene however if your foal is presenting as affected, it is essential that the foal be muzzled and not allowed to nurse from the mare until 36–48 hours after birth.

The mare should be milked regularly and the colostrum discarded, and the foal will require supplemental colostrum if colostrum has been banked from other mares. Otherwise, a foal milk replacement should be given for the first 36–48 hours. The foal will also require a plasma transfusion if it does not receive colostrum, in order to assure it has antibody protection from disease.

From a management perspective, it is a good idea to feed the foal from a bottle with the foal next to the mare’s flank and the bottle held near the udder. If you take the foal to a different part of the stall to bottle feed it, it may not make the association between nursing and the mare’s udder.

MAKE SURE YOU ARE FAMILIAR WITH THE SIGNS – AN NI AFFECTED FOAL BORN IN AUSTRALIA

After 36–48 hours, the muzzle can be removed from the foal and it can nurse from the mare safely. It is wise to have the foal’s immunoglobulin status checked with an IgG test after 48 hours to check that it acquired adequate protection from either banked colostrum or a plasma transfusion.

HOW DO I KNOW IF A MARE IS AT RISK FOR PRODUCING AN NI FOAL?

It is possible to test mares before foaling for the presence of NI. It has to be done within 2–3 weeks of foaling to be accurate. Only specialty labs can do the testing, so if you think you might have a mare that is suspect, it is important to do your homework in advance and have a lab identified.

WHAT SHOULD I DO IF I SUSPECT NI?

If you have not had a test on the mare, there is a quick stall-side test that might give you more information. We do this on all foals born at our farm. If you are present for the foaling, collect some blood from the foal’s umbilical cord as soon as it breaks. Mix a drop of the foal’s blood on a clean slide with a drop of the mare’s colostrum. If the mixture clumps or clots, the foal is a possible NI case and your veterinarian should be consulted. This is a rough, stall-side test and is not considered definitive, but could give you an indication that you need to investigate further with your veterinarian before allowing the foal to nurse.

WHAT MARES ARE AT RISK?

Embryo transfer recipients are at risk, as they will be a different blood type generally than the foal, as she is not related to the foal, genetically. Any mare that has produced an NI foal in the past should be tested prior to foaling. Maiden mares are always an unknown and could benefit from testing. While the chances of having an NI Arabian foal are small, it is a devastating thing to occur for any horse owner. I was asked to write this article after an Arabian mare in Australia of well known bloodlines, when bred to a Straight Egyptian of well known bloodlines, produced her eighth foal, a filly who was diagnosed with NI at two days old, and subsequently died, even with all the veterinary care available to her. It is important to note that this mating was the first pairing of the two horses, however it is likely that a previous foal produced by the mare had been of an incompatible blood type and instigated the antibodies that killed her filly.

What was more important was the breeder, and others they were close to, including the breeders of both parents, were unaware of NI being prevalent in the Arabian horse.

The good news is that if caught early, after 36–48 hours, the foal can be allowed to nurse from the mare and grow into a normal, healthy individual.

A HEALTHY RESULT – AN NI AFFECTED FOAL’S BLOOD WILL CLOT WHEN MIXED WITH ITS MOTHER’S COLOSTRUM

DRAWING BY CINDY REICH

Close monitoring of the foal after birth, and having your veterinarian do a neonatal exam on all foals at birth are both good management practices. Any sign of weakness or jaundice (yellow membranes) should be cause for immediate veterinary intervention.

PC CINDY REICH Author Cindy Reich Images courtesy of Cindy Reich & Sharon Meyers