Ingestion of wilted or partially dried leaves from red maple (Acer rubrum) trees is potentially fatal to horses, causing severe oxidative damage to the red blood cells--often leading to methemoglobinemia and hemolytic anemia. Consumption of 1.5 - 3 grams of leaves per kg of body weight (0.7 - 1.5 kg for the average 450-kg horse) will cause horses to develop hemolytic anemia.

The toxins present in red maple leaves are gallic acid and tannins, which convert to pyrogallol within the horse's stomach. The amount present in the leaves varies depending on the stage of decomposition of the leaves, red maple tree variety (there are many kinds, including hybrids), and environmental conditions. Research studies have found that the older the leaves, the more fast acting the poison. Thus, leaves that have wilted mid-September and onward are going to be more toxic to horses than leaves undergoing the early stages of wilting in August. There have also been incidents reported of horses getting poisoned by ingestion of red maple leaves in the spring as well, however, infrequently. Wilted leaves remain toxic for several weeks.

Many poisoning incidents have been associated with the occurrence of a strong storm 3 to 4 days prior to the onset of clinical signs of illness. During storms, high winds are more likely to knock down tree branches in horse pastures.

Summary of Reported Cases of Red Maple Toxicosis in Horses in the United States

# Horses	Location	Season	Parts	History	Ref
32	southeast	autumn	wilted leaves	32 horses were confirmed to have red maple toxicity in referral hospitals in the southeast region of the United States. 19 of the horses died. Horses that were treated with a corticosteroid had a significantly increased likelihood of death	A Alward et al., 2006
2	northeast	autumn	wilted leaves	Horses recovered 36 hours after receiving blood transfusions, IV fluid therapy, and high doses of vitamin C (ascorbic acid) for treatment of heinz body anemia, marked methemoglobinemia, and severe tissue anoxia.	R McConnico et al., 1992
4	southeast	autumn	wilted leaves	Signs of acute hemolytic anemia developed in horses 3 to 4 days after the ingestion of wilted leaves from cut red maple trees. Three of the four horses effected, died 5 to 6 days after ingestion of the leaves. Gross pathologic changes included generalized icterus, splenomegaly and swollen, black kidneys. Microscopic changes including tubular nephrosis with hemoglobin casts, vacuolization of centrilobular hepatocytes, and sequestration of erythrocytes in splenic sinusoids.	L George et al., 1982; T Divers et al., 1982
33	northeast	autumn	fallen branches	Affected horses had access to outside paddocks or fields containing a variety of native grasses, weeds, and trees. On 2 farms, hemolytic anemia developed after the horses were observed browsing fallen branches of red maple trees. Red maple leaves and bark were obtained from 1 of these farms, and approximately 1 kg of a leaf and bark mixture was fed to each of 2 ponies. Within 48 hours, both ponies became ill. The syndrome was indistinguishable from that observed in clinical patients and was characterized by methemoglobinemia and intravascular hemolysis. The ponies died 5 and 6 days after which time the packed cell volumes were 6% and 7% respectively.	B Tennant et al., 1981
3	northeast	autumn	fallen branches	Red maple toxicity occurred in a miniature horse and a pony. Both animals were treated with a blood transfusion, intravenous crystalloid fluids, nasogastrically administered mineral oil, and activated charcoal. Horses also received ultrapurified bovine hemoglobin (PBHg) (16 and 11 ml/kg, respectively) when the hematocrit dropped below 10%. Both animals recovered and were discharged 15 to 17 days later.	R Vin et al., 2002
2	northeast	autumn	trimmed branches	An 8 year old, female Tennessee Walking horse had a history of depression and anorexia for 1 day. Another horse on the same pasture had colic for 2 days and subsequently died on the farm. The owner noted that 3 to 4 days prior to the onset of illness in the horses, the trees in the pasture had been trimmed, and thrown into the pasture where the horses were kept.	W Reagen et al., 1991
2	southeast	autumn	dried leaves	Two Percheron mares aborted and developed fatal hemolytic anemia; clinical signs included depression, lethargy, loss of appetite, red-brown urine, and brown mucus membranes.	E Stair et al., 1993
1	southeast	spring	fallen branches	An 11-year-old American Quarter horse mare presented with a 3-day history of lethargy, anorexia, abdominal pain, brown icteric mucous membranes and weakness that progressed to recumbency. A couple days prior, there had been thunderstorms in the area that had resulted in leaves and branches from nearby red maple trees, falling into the same pasture as the horse. The referring veterinarian had treated the horse with flunixin meglumine (1.1 mg/kg bwt IV) and vitamin K.	D Witonsky et al., 2001
1	southeast	autumn	fallen branches	A 14 year-old Tennessee Walking horse mare had a sudden onset of vulvar bleeding (which later was found to be the result of the red-colored urine), depression and lethargy,	M Ernst et al., 2016