Neonatal isoerythrolysis (NI) is the most common alloimmune disease in neonatal foals (median age, 2.5 days).15

NI is character­ized by hemolytic anemia, icterus, and hemoglobinuria. Clinicopathologic abnormalities include anemia, high serum indirect and direct bilirubin concentrations, and sorbitol dehydrogenase (SDH) activity.

NI is caused by a blood group incompatibility between the foal and dam. The foal inherits and expresses, from the sire, an erythrocyte antigen (alloantigen) that is not present in the mare. The hemolytic syndrome is mediated by maternal antibodies generated against foal erythrocytes (alloantibodies) absorbed from colostrum. Blood factors associated with NI in horse foals are Qa, Qb, Qc, Aa, Pa, and Dg and donkey factor in mule foals.¹⁵ The most common antigens are Qa and Aa. Foals with NI may also present with alloimmune thrombocytopenia. For a complete description of NI, see Chapter 53.

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