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Neonatal isoerythyolysis (or NI) is a blood condition found in newborn foals. It manifests itself within the first few days of birth and is the result of a discrepancy between the mare’s blood and that of the foal, whereby the mare develops antibodies to the foal’s blood type. This becomes a problem when the foal drinks the colostrum (first milk) of the mare, which contains these antibodies. These maternal antibodies against the foal’s own blood type then destroy the foal’s blood cells, causing severe, life-threatening anemia and other complications.
As previously stated, a discrepancy between the blood type of the mare and the foal is the cause of neonatal isoerythyolysis. This condition occurs in roughly 1–2% of all horse foals, and at a slightly increased rate of almost 7% in mule births. To occur, a few things must happen. First, the foal must inherit from its father a specific blood type (Aa or Qa). Secondly, the mother must become sensitized to the foal’s blood type. This is thought to most commonly occur via transplacental hemorrhage during a previous pregnancy. It can also happen if the mare has ever had a whole blood transfusion. If both of these events occur, then the mare develops antibodies to her current foal’s blood type. The foal then becomes exposed to these antibodies when it drinks its mother’s colostrum. These antibodies then begin to destroy the foal’s own blood cells.
A presumptive diagnosis of NI can be made for a foal less than four days old that is exhibiting the above-mentioned signs. Other more specific laboratory tests can be done to confirm the presence of maternal antibodies on the foal’s red blood cells, but this takes time and sometimes time is of the essence with this disease.
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