Samantha called me at noon on a bright day in mid-September. She said her 10 year old mare Trixie hadn’t eaten any grain or hay that morning and now was acting “dumpy”. Banamine paste had not changed anything. I told Samantha that I would finish up at the farm that I was on and rearrange my day to see Trixie next. I arrived at her farm around 2 o’clock.
Trixie was standing along the back wall of her stall. Her head was down and she didn’t react when I walked in. Normally this mare is quite social. From the stall door I could see her breathing pattern. It was rapid and very shallow. Her heart was racing at 85 beats/minute, which was twice normal for her. When I rolled back her upper lip, I was surprised to see that her gums were dark brown instead of pink. I was sure that she must have a twisted intestine that had ruptured. This would mean intestinal contents spilling into the abdomen and a guaranteed peritonitis with certain death within a few hours. However, when I listened for gut sounds they were still there, faint but present. Usually when an intestine ruptures, all abdominal sounds stop. I knew she was in serious trouble but was baffled as to why. I pulled on a rectal sleeve and was lubing up my arm when Trixie humped up her back and urinated. The urine was pure black. We see that with “tied up” horses, but it only happens when a horse has been worked hard, and that was not the case. This left me with the only possible cause that I could think of.
I turned to Samantha and said, “Let’s take a walk around the farm.” She looked puzzled, but followed me. We walked out of the barn, through the wooden gate and along the north edge of the pasture. Sure enough, we hadn’t walked 100 feet when I spotted what I was looking for. A large limb had split off from a red maple. It was hanging across the old stone wall and into the pasture. A wind and rain storm had passed through the week before and must have taken the big branch down. We could see where leaves had been eaten off the branch. I told her that this was probably the cause of Trixie’s sickness. Samantha blamed herself. She didn’t know that the down limb could be a problem and had put off cleaning it out of the pasture until she had more time. I called the nearest equine hospital and told them that we had a probable case of red maple leaf poisoning. We got Trixie loaded and on her way within 20 minutes. Despite some heroic work including blood transfusions, Trixie died early the next morning. Blood tests and a post mortem confirmed the diagnosis.
Plant poisonings in northern New England are rare when compared to the rest of the country. The big exception is horses poisoned by eating the wilted leaves of red maples. This tree is also called swamp maple and is common in our area. Under the right conditions, just a few mouthfuls of its leaves can mean death to horses. What is unusual is that horses can eat the leaves right off of a live red maple and not get sick because those leaves haven’t gone through the wilting process. Horses can also walk through a pasture in the fall plowing through fallen leaves up to their fetlocks without harm. I have never seen or heard of a horse getting sick from those leaves that fall off in autumn.
From my experience and others it seems that the leaves are only poisonous when they are eaten off of a downed branch or a whole tree uprooted. The down tree or branch can’t provide sustenance to the leaves and they wilt, still attached. These wilted leaves are somehow attractive to horses. When they eat as little as a pound, they get very sick. Even with immediate hospitalization and intensive care, most don’t survive.
Researchers have found gallic acid to be the toxic element in the wilted leaves. When a horse eats the leaves, the toxin enters the horse’s circulation and destroys the body’s red blood cells. To compound things further, the dying red cells release hemoglobin. Hemoglobin is the compound that is carried on red blood cells. It binds with oxygen in the lungs and carries it to every living cell. When the hemoglobin is released, it is no longer capable of carrying oxygen. All of the body’s cells become oxygen starved. The released free hemoglobin itself acts like a poison and damages the kidneys, rapidly destroying them. So the horse is not only deprived of oxygen, it is also experiencing renal (kidney) failure. This explains the rapid breathing and the black urine.
All the cases I have seen, heard about, or researched seem to happen only when horses eat the wilted red maple leaves off of a down tree or split off branch. The leaves that turn colors and come off in the fall do not seem to be an issue. Maybe that’s because those dried leaves don’t taste good. Perhaps it’s like buttercup. All the books talk about buttercup being poisonous, but I’ve never seen any horse eat it. My two donkeys are chest deep in buttercup every summer. I used to whack the buttercup down, but gave up when I found that it doesn’t get eaten.
So, here is my take home about red maple poisoning. Google red maple and what its leaves look like. Walk around and see if you have any on your property. You may not be able to identify the tree itself, but the leaves are distinctive. If you do have red maple, take note, but don’t go running for the chain saw. I hate to see perfectly good trees cut down that are not a problem as they stand. It would be prudent to check after storms to see if any are uprooted or if branches are down where a horse can munch on the leaves. The wood is perfectly safe; it’s just those leaves wilting on the branches that are so toxic. They remain dangerous for a month after the tree or branch is down. If a red maple is down where horses can get to it, it’s time to get busy with a chain saw and remove any branches with leaves on. If it happens in the winter, and there are no leaves, there is no danger, and you can cut that excellent fire wood after the snow melts. Those crunchy and colorful underfoot maple leaves that drop in the fall do not seem to be a concern. If they make you nervous, go ahead and rake them up. I’m leaving mine right where they are. Be aware, and enjoy your horses and your trees.
-David A. Jefferson, DVM