Neonatal Isoerythrolysis in Foals

Neonatal Isoerythrolysis in Foals

Photo: Anne M. Eberhardt/The Horse

This potentially deadly disease is easy to prevent, but it can be challenging and expensive to treat

Neonatal isoerythrolysis (NI): This mouthful of a term literally means the destruction of a newborn foal's red blood cells. The debilitating condition, in which a mare's antibodies attack her foal's red blood cells, can be lethal for a foal due to the resulting liver damage and other problems. The pregnancy might appear completely normal, without complication: a happy mare swelling larger with foal each day. But the foal's blood type could be incompatible with his dam's antibodies when they mix during the foal's first meal (colostrum).

The dam's antibodies do not affect the fetus in utero because they are too large to slip through the placental barrier. However, for the final three weeks of gestation, the mare concentrates antibodies she's developed via natural exposure to diseases, from vaccination, or by other means, in her colostrum. Consuming this antibody-rich meal generally would provide a foal with good protection against any disease he might encounter, but with NI it can be deadly.

Since the foal's ability to absorb antibodies into his bloodstream and lymphatic system is greatest during the first few hours of life--then gradually diminishes as the intestinal lining closes--he is most at risk of developing NI the first day. After his intestine can no longer absorb antibodies, and after the mare's regular milk comes in, it's safe for him to drink her milk.

Possible Causes

If a foal inherits a blood type or components from his sire that don't match the mare's, and if these reach her bloodstream, her immune system considers the foal's red blood cells "foreign." In such a case she creates antibodies against them. Or, if there's abnormal blood transfer between dam and fetus (such as placental hemorrhage during pregnancy), the mare is exposed to the foal's red blood cell antigens, triggering her immune system to attack and destroy them and priming her system to release targeted antibodies in the colostrum.

Other ways a mare might become exposed to a blood type different than her own is if she were treated with blood or blood-based products, according to Kristina Lu, VMD, Dipl. ACT, theriogenologist at Hagyard Equine Medical Institute, in Lexington, Ky. "If the mare received a blood transfusion at some point, she could become exposed," she explains. "If you purchased the mare you may not know her medical history."

Yet another circumstance in which the mare might become exposed to a foal's blood type is during foaling--if blood from the foal/placenta accesses her bloodstream (for example, birthing trauma). This would not affect that particular foal, but the mare could develop antibodies that might affect her next foal.

Nathan Slovis, DVM, Dipl. ACVIM, an internist at Hagyard, says NI foals can occur in any breed but Thoroughbreds, along with Standardbreds, Paints, Quarter Horses, and donkeys, seem to be more affected than other breeds. "Mule foals have higher incidence due to what we call 'donkey factor' (a difference in blood types between horses and donkeys)," he says.

It's important to have a veterinarian examine each foal within 12-24 hours after birth. "Not only do we take a blood sample and measure antibody levels (to see if the foal received adequate disease protection from the dam via passive transfer), but we can do a red blood cell count," says Slovis. "This gives a clue if the foal has a problem with NI, and you can catch it early."

Testing the Mare

Veterinarians can draw blood from the mare during the last two to four weeks of gestation to help determine whether she is at risk for delivering an NI-affected foal. They test it for antibodies that indicate to what blood types she's been exposed.

"We generally don't know the stallion's blood type, but the blood types commonly involved (against which the mare would develop antibodies) are Aa and Qa." says Lu.

Slovis says there are more than 10 blood types, and the screening test only checks for antibodies against the four most common. "So if you get a negative test this doesn't mean your foal is safe; it just means there's low risk," he explains.

When presented with a high-risk foal, "We manage that foal (often with a nylon muzzle) to make sure he does not nurse the mare," says Lu. "We provide colostrum from mares that don't have dangerous antibodies. At Hagyard we have a colostrum bank. People donate colostrum from mares that are NI negative (after testing). Or you can give a colostrum substitute, but mare colostrum is best for the foal."

After milking the mare frequently for 12-24 hours, the veterinarian performs another test to ensure there are no more harmful antibodies present in the mare's colostrum and that it is safe for the foal to begin nursing. The testing should be repeated if antibodies are still present at high levels.

Testing the Foal

Another test to help determine whether precautionary management is needed, or if it's safe to let the foal nurse, is a jaundiced foal agglutination (JFA) test. This involves mixing a sample of the foal's blood with a sample of the mare's colostrum. "If there are many antibodies against the foal's red cells, this mix will clot at a certain ratio, creating clumps," Lu says. "If clotting ratio is 1 to 16 or higher, this is considered positive, and the foal should not ingest that colostrum."

If the foal is a few days old, however, this test is ineffective, as the mare no longer is producing colostrum. "At that point you could do a Coombs test (an antiglobulin test to detect antibodies bound to the surface of red blood cells)," Slovis suggests. "It's not specific for NI but can tell you whether there are antibodies in the foal's blood against his red blood cells."

Todd Bettin, DVM, of Breda Lake View Veterinary Services, in Lake View, Iowa, says veterinarians or foaling personnel can also perform a simple test right in the stall, immediately after the foal is born, to get a rough idea whether the mare's colostrum is safe. This test, however, should only aid in detecting very severe cases if the other tests are not an option.

"Collect a drop of blood as the foal's umbilical cord breaks and mix it with a drop of colostrum from the mare, on any clean surface or a glass slide," he says. "If the mixture clumps (clotting of the foal's red blood cells, as with the JFA test) this means there's an incompatibility. If the mixture remains creamy pink, it's probably safe for the foal to nurse the colostrum."

Clinical Signs

Neonatal isoerythrolysis can occur in varying degrees of severity. "You may see signs as early as 12 hours of age or four or five days later," Slovis explains. "It depends on the type of antibody and how much the foal ingested. He may just seem lethargic, but some foals suddenly collapse when they're only 12 hours old if they're short on oxygen. They may have kidney failure or liver disease because there's not enough oxygen getting to these vital organs."

"If the mare has only a few antibodies in her colostrum it might take longer for the foal to show signs, and they may be mild," says Lu. "It is important to have an accurate diagnosis. Other conditions that cause weakness and lethargy include sepsis from bacterial infection (of the bloodstream) and depression associated with dummy foal syndrome (perinatal asphyxia syndrome, from inadequate oxygen supply at/around the time of birth)."

Red blood cell breakdown can trigger a fever response, which, according to Peggy Marsh, DVM, a practitioner in Lexington, Ky., can confuse a diagnosis. Hence, it's important to consult with a veterinarian right away if your foal develops a fever.

Another sign is jaundice, which is why the condition is often dubbed foal jaundice. When red blood cells are destroyed they produce bilirubin. This pigment is carried around the body by the blood, causing gums, whites of the eyes, and other mucous membranes to appear yellow. "It can also make the brain yellow and disrupt normal nerve conduction if neurons die," Slovis explains. "The foal may go into seizures and die if the brain is not functioning properly."


Foals with NI should be hospitalized for treatment as soon as possible because, as mentioned, red blood cell breakdown products can affect their kidneys and brain. In severe cases foals become very anemic, as there are no longer enough healthy red blood cells to carry oxygen around the body. "Normally a foal might have a packed cell volume (hematocrit) of 30% or higher," says Marsh. "Some of these anemic foals have a value of less than 10%. That's when we become very concerned, and a blood transfusion may be needed."

Often the best donor for a blood transfusion is a gelding with no history of transfusions or treatment with blood-based products. Some equine hospitals keep geldings on hand as universal blood donors. "It's simple and fast," says Bettin. "We put the donor in the stocks, fill a collection bag with blood, and hang it on the foal."

Another option is to use the mare's blood to obtain red blood cells, but these must be separated from the plasma that contains antibodies. "It takes more time and labor than a transfusion," says Lu. "Other aspects of treatment are supportive, such as treating anemia, turning off the foal's immune system (with steroids) because it's fighting the antibodies, and possibly using antibiotics because he is vulnerable to sepsis from other sources. If the foal becomes sick he also needs fluids."


Prognosis for an NI foal depends on the severity of his condition and the speed of diagnosis and treatment. According to Slovis, retrospective studies show that about 75% of sick NI foals that come to a hospital survive. "Those are the severe cases, however, so overall survival rates are better," he adds.

Many NI foals just need time and medication to help control their fever and prevent infection. "Most will recover, with time and careful transfusions. But some don't do well," Marsh says. "We don't know whether the antibody is different and more deadly, or more long-lasting. Even if we give these foals transfusions, and no matter how carefully we cross-match blood, the antibodies tend to destroy those red blood cells, too. After a while the liver can't keep up with processing and excreting broken-down red cells, and is overwhelmed--and the foal gets liver damage. Some foals we can't save despite intensive care."

Take-Home Message

Prevention is the key to avoiding NI in foals, as owners and veterinarians can screen for disease potential prior to foaling. Another (less common) option is to blood type breeding horses to avoid breeding a mare to a stallion with a blood type that might lead to NI. If your mare has risk factors for producing a foal with NI, consult your veterinarian and take appropriate precautions to ensure arrival of a healthy foal.

About the Author

Heather Smith Thomas

Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog,, she writes a biweekly blog at that comes out on Tuesdays.

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