Acute hemolytic anemia, methemoglobinemia, and heinz body formation associated with ingestion of red maple leaves by horses.

From June 1975 through June 1979, acute hemolytic anemia developed in 11 horses from 7 New York farms. Of the 7 horses that died, 6 had methemoglobinemia. In the 4 horses that recovered, methemoglobinemia was not observed. but Heinz body formation was seen in 3 of the 4. On 2 of the premises involved, frank methemoglobinemia was observed concurrently with Heinz body formation, suggesting a relationship between the pathogenesis of methemoglobinemia and Heinz body formation in the hemolytic process. In addition to the 11 cases described, 22 clinically similar cases were reported to us during the period of this investigation by practicing veterinarians from New York, Pennsylvania, and the New England states. All 33 cases of hemolytic anemia occurred between June and October of each year, and affected horses had access to outside paddocks or fields containing a variety of native grasses, weeds, and trees. On 2 farms, hemolytic anemia developed after the horses were observed browsing fallen branches of red maple trees (Acer rubrum). Red maple leaves and bark were obtained from 1 of these farms, and approximately 1 kg of a leaf and bark mixture was fed to each of 2 ponies. Within 48 hours, both ponies became ill. The syndrome was indistinguishable from that observed in clinical patients and was characterized by methemoglobinemia and intravascular hemolysis. The ponies died 5 and 6 days after which time the packed cell volumes were 6% and 7% respectively. It was concluded that many cases of hemolytic anemia in horses in northeastern states may be related to ingestion of leaves or bark from red maple trees. The studies did not, however, define the factors that predispose to poisoning and did not exclude the possibility that other environmental toxins may have been involved.