Neonatal Isoerythrolysis in Foals

Prevention is key to avoiding neonatal isoerythrolysis, or destruction of a newborn foal’s red blood cells.

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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

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Written by:

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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