Q: Two days ago I lost a horse to red maple leaf poisoning, which was a shock because he had been in the same fields on my farm for years. We have been trying for years to identify these maples, but there are a ton of varieties. The department of agriculture told us that poisonous leaves are no danger in fall when they come off the tree but, rather, when they are green and wilted, like after a branch falls. The night before this happened we experienced extremely cold temperatures. There was one tree that was full that night and completely bare by morning. We had no trees down, but autumn leaves were blowing and falling. I can’t get any clear information on the topic and I am nervous to lose my other horses. Exactly when are these leaves lethal?

—Larissa, via e-mail

A: I’m so sorry to hear about your recent loss. How maple leaves affect horses isn’t entirely clear to us. We know that wilted leaves (which can come from downed tree limbs or occur, as you discovered, after a frost) are dangerous, and most poisoning cases occur in the autumn. We also know that it doesn’t take much to poison a horse—less than a pound of dry leaves could be fatal for a pony.

Based on some work that a student did in my laboratory, we know that red maple leaves contain gallic acid. Intestinal bacteria can convert gallic acid from leaves into pyrogallol, which can damage red blood cells and, we believe, could be responsible for the clinical picture of maple poisoning (signs appear within 12-24 hours and include depression, poor appetite, yellow or brown gums and membranes, dark red or brown urine, colic, and fast heart and respiratory rates; abortion, sudden death, and kidney failure might also occur). If this is true, then individual horse factors are important as well: The horse would have to have the right intestinal bacteria to produce the toxic pyrogallol from the leaves. Our work thus far has been done predominantly in test tubes, and we still need blood samples from accidentally poisoned horses to verify that our hypotheses are correct