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The native red maple (Acer rubrum), also called swamp or soft maple, is a potent killer of horses and ponies. Red maple is a tree native to the eastern half of North America.

The toxic ingredient in red maple leaves is believed to be gallic acid (1). Gallic acid causes methemoglobinemia and is plentiful in both water and methanol extracts of red maple, sugar maple and silver maple, and in the extract fractions from these species that oxidize blood cells (1). Ingestion of wilted or partially dried red maple leaves from fallen or pruned branches causes lysis of the red blood cells with the subsequent development of a hemolytic anemia, which can be deadly (2). The problem can occur from June to October. Older wilted leaves, e.g., those collected after September 15, cause faster poisoning than wilted leaves of early summer growth. This indicates that the amount of toxin increases in leaves during the summer. Wilted leaves remain toxic for a few weeks or more. Ingestion of fresh leaves does not appear to cause disease. The ingestion of 1.5-3 gm of leaves per kilogram of body weight (0.7-1.5 kg for the average 450-kg horse) will cause hemolytic disease.

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