Hemolytic anemia in horses after the ingestion of red maple leaves

Signs of acute hemolytic anemia developed in 4 adult horses from 2 Georgia farms 3 to 4 days after the ingestion of wilted leaves from cut red maple trees (Acer rubrum). Clinical findings included weakness, polypnea, tachycardia, depression, icterus, cyanosis, and brownish discoloration of the blood and urine. Blood changes included methemoglobinemia, free plasma hemoglobin, decreased pcv, and Heinz bodies in erythrocytes. These findings plus hemoglobinuria suggested intravascular hemolysis. Three of the 4 horses diet 5 to 7 days after ingestion of the leaves. Gross pathologic changes included generalized icterus, splenomegaly and swollen, black kidneys. Microscopic changes including tubular nephrosis with hemoglobin casts, vacuolization of centrilobular hepatocytes, and sequestration of erythrocytes in splenic sinusoids. A disease indistinguishable from the field cases was induced in a pony by the oral administration of dried, ground red maple leaves at a dosage of 1.5 g/kg. The findings of methemoglobinemia, hemolysis, and Heinz bodies suggested that the toxic principle of the red maple leaf was an oxidant.