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