Anemia in Newborn Foals Leave a comment

Equine Neonatal Isoerythrolysis

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. 


  • Lethargy
  • Anemia (PVC < 20%)
  • Rapid heart rate
  • Yellowing of the eyes and mucous membranes (also called jaundice or icterus)
  • Dark urine



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.


If this condition is diagnosed when the foal is less than 24 hours old, it should be prevented from nursing its mother.  Nutrition should be given in the form of milk replacer via a nasogastric tube. A majority of the mother’s antibodies are gone from her milk after 24 hours, so if this condition is noticed after this time point, the foal does not need to be prevented from nursing any longer.

Other treatment will include IV fluids to help support the foal’s circulatory system and kidney function, supplemental oxygen if needed, and systemic antibiotics to prevent the foal from secondary bacterial infections. If the foal’s anemia is severe, a blood transfusion may be required.

Living and Management

If neonatal isoerythyolysis is caught early enough and the foal is not severely compromised at the initiation of treatment, the prognosis is much more favorable than with a foal who has been severely affected days before treatment is started.  Supportive care will be required for a period of time, as the foal slowly begins to produce healthy red blood cells to replace the ones it has lost.


Prevention of neonatal isoerythyolysis is far more successful than treatment. If you know your mare has ever had a blood transfusion, or has previously had a foal with NI, do not allow any of her subsequent foals to nurse her within 24 to 48 hours of birth. Some breeds such as Arabians and Thoroughbreds are more genetically prone to carrying the Aa and Qa blood types.  If you are planning to breed either of these, it is sometimes recommended to find stallions that are negative to these blood types for your breeding program.


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