Benner had nice home in the paddock he shared with his stablemate. One stormy night brought him a fateful snack. The high winds blew a tree down into his paddock. Benner did not know this particular tree was not good for him — in fact it was highly toxic. The tree that was blown down was a red maple which is very toxic to horses. The most toxic part is the wilting leaves. So, about a day after this tree was blown down, Benner began to not feel so good. He began exhibiting signs of mild colic. The following day, he was very sick. He had definite symptoms of colic. What was even more concerning to his owners was his coffee colored urine.

I was called to examine Benner. He was exhibiting classic signs of colic. He was very uncomfortable. He was not running a temperature. He was very depressed. His gum color was abnormal with a yellowish tinge. A sample of urine was present on the ground which was thick and black. Blood work showed a packed cell volume of 10 (Normal values 35-45). The diagnosis was straightforward with the fateful branches in the paddock and the clinical signs.

The exact toxic principle behind red maple poisoning is unknown. What we do know is there is a substance in wilting red maple leaves which causes the breakdown of red blood cells.benner This inhibits the body’s ability to supply nutrients to its tissues especially oxygen. In addition, the hemoglobin in red blood cells is broken down into myoglobin. Myoglobin is a very large molecule which must be processed by the kidneys. It is so big that it can actually interfere with kidney function and lead to renal failure.

Treatment for Benner consisted of corticosteroids to help stabilize the red blood cells and decrease the rate of destruction. He was also started on intravenous fluids to help with blood volume and to flush the kidneys to try and avoid renal failure. An IV catheter was placed and his owners were instructed on how to run the fluids. Initially, 20 liters of fluids were dispensed to be given continuously.

I was called back out several hours later. Benner had become very painful. He was down in the mud and rain. We were able to get him up only with considerable effort. He was unwilling to stand quietly. He had a very elevated heart rate. He was in so much pain that it forced discussions about the possibility of euthanasia as referral was not an option. Rectal palpation revealed a very enlarged and distended bladder. A few minutes of walking encouraged him to urinate. His urine was still black with some protein masses associated with it. Fortunately, urinating appeared to make him more comfortable and we were able to resume treatment.

Over the next 24 hours, Benner received 60 liters of IV fluids. Slowly his attitude began to improve and his urine began to clear. Two days later, he was eating and his urine had returned to a normal appearance. Blood work was repeated. His hematocrit while still very low, had begun to improve. Even better, there was no elevation in CPK or creatinine. These values would be elevated if renal failure had occurred. All in all, Benner was much improved. He has recovered and is back enjoying the company of his paddock mate. Fortunately, Benner did not require a blood transfusion. However, red maple poisoning may result in a severe drop in hematocrit requiring one or more blood transfusions.

Red Maple poisoning occurs from the decaying leaves. It is not uncommon as in this case of one horse to eat the leaves, while others leave them alone. It appears some horses will develop a taste for them. The best way to prevent problems is to remove red maples trees from horse pastures. These trees are most easily identified during the Fall. As leaves change colors, the stem of the red maple leaf will become bright red. Red Maple is the only species of maple that is known to cause this problem. Other varieties such as Japanese and Silver Maple are not known to cause problems. Red Maples may be more common in some areas associated with newer construction. The maples tend to be fast growing and are frequently planted with new construction as they will more quickly grow to provide shade.