Congenital defects in newborn foals of mares treated for equine protozoal myeloencephalitis during pregnancy.

: Three weak, recumbent neonatal foals with skin lesions, including a thin wooly coat, were born to mares being treated for equine protozoal myeloencephalitis. Mares received sulfadiazine or sulfamethoxazole-trimethoprim, pyrimethamine, folic acid, and vitamin E orally. Foals were anemic, leukopenic, azotemic, hyponatremic, and hyperkalemic. Serum folate concentrations in the 3 foals and 2 mares were lower than those reported in the literature for clinically normal brood mares. Treatment was unsuccessful. For each foal, necropsy revealed lobulated kidneys with thin cortices and a pale medulla, and the spleen and thymus were small. Histologic examination revealed marked epidermal necrosis without inflammatory cells, thin renal cortices, renal tubular nephrosis, lymphoid aplasia, and bone marrow aplasia and hypoplasia. These observations indicate that oral administration of sulfonamides, 2,4-diaminopyrimidines (pyrimethamine with or without trimethoprim), and folic acid to mares during pregnancy is related to congenital defects in newborn foals.