This disorder results from the incompatibility of blood types between a mare and her newborn foal.
The foal inherits a red blood type (antigen) from its sire, and the mare has antibodies against this antigen. The foal ingests these antibodies from the mare in the colostrum (first milk), and they bind to the foal’s red blood cells, causing them to be targeted by the immune system and destroyed.
The foal develops life-threatening anemia, and severe jaundice (yellow gums) within 1-5 days. The jaundice results from the elevation in blood levels of bilirubin, a product of the breakdown of hemoglobin (oxygen binding pigment in red blood cells) by the liver.
How the mare is exposed to the foal’s blood leading to the development of antibodies is not definitively known, but may occur via placental blood leakage, placentitis, blood transfusion, or birthing trauma. Somehow the mare’s blood must first be exposed to the antigen. Then she forms the antibodies.
This condition can be diagnosed at a post-foaling vet exam. Ensure that your vet performs a thorough examination on your foal within 12-24 hours after birth.
Treatment consists of preventing or ceasing the foal’s intake of colostrum by muzzling the foal, and supportive nursing care. Severely affected foals will require a blood transfusion. If the foal does not ingest the mare’s colostrum, they must get colostrum from another nursing mare, or they will not gain sufficient antibody levels (Failure of Passive Transfer). In these cases, plasma may be given intravenously.
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Prognosis & Relevant Factors
The prognosis is poor without early aggressive treatment, which usually requires hospitalization. Early recognition and the prevention or reduced intake of colostrum is key. NI is more common in mares that have had multiple foals.
The incidence of disease is low in horses, about 1-2%, but much higher in mule foals (10- 25%). This is because donkeys possess unique red blood cell markers ("donkey factor") that is not present on mare blood cells.
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