ProMed: Red Maple & Box Elder leaves toxic to hoses.

[Byline: Brad Pritchard] -- Communicated by: ProMED-mail [In North America, the toxic ingredient in red maple (_Acer rubrum_) leaves is believed to be gallic acid (1). Gallic acid causes methemoglobinemia [a blood disorder in which an abnormal amount of methemoglobin -- a form of hemoglobin -- is produced] 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). Horses often die within 18-24 hours of ingestion of wilted leaves. Horses that remain alive for 18-24 hours after ingestion of wilted leaves will be severely depressed and cyanotic and produce dark red or brown urine. The mucous membranes are blue to brown from poor oxygenation. They suffer intravascular and extravascular hemolysis (red blood cell breakdown). The percentage of red blood cells circulating in the blood (packed cell volume [PCV]) can drop as low as 8-10 percent, and the hemoglobin (Hb) concentration can be as low as 50 g/L. The normal PCV and Hb concentrations in horse blood are 28-44 percent and 112-169 g/L, respectively (3). Death is due to a severe lack of oxygen delivery to vital cells from hemolysis of red blood cells, anemia and the oxidation of hemoglobin to methemoglobin, which is incapable of transporting oxygen. Of 32 horses, 19 ( 60 percent) died after ingesting wilted red maple leaves. The clinical signs observed included: colic, fever, followed by laminitis and disseminated intravascular coagulation. All horses had both gross and microscopic evidence of hemoglobin in their urine (hemoglobinuria) (4). Another _Acer_ plant, is the Box elder (_Acer negundo_). The tree seeds contain a toxin, which results in severe muscular disease in horses called seasonal atypical myopathy, which is frequently fatal.