Red maple toxicosis

Attention! This is a potentially life-threatening condition for your horse. Time is of the essence, contact your veterinarian immediately.Find a Vet

Red Maple Toxicosis

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

# HorsesLocationSeasonPartsHistoryRef
32southeastautumnwilted leaves32 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 deathA Alward et al., 2006
2northeastautumnwilted leavesHorses 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
4southeastautumnwilted leavesSigns 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
33northeastautumnfallen branchesAffected 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
3northeastautumnfallen branchesRed 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
2northeastautumntrimmed branchesAn 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
2southeastautumndried leavesTwo 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
1southeastspringfallen branchesAn 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
1southeastautumnfallen branchesA 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


Dark red urine
Muddy brown mucous membranes
Colic signs
Loss of appetite
Decreased abdominal sounds


  • History
  • Clinical signs
  • Physical Exam
  • Labatory tests



Intravenous crystalloid fluidsUsually requiredR Vin et al., 2002; R McConnico et al., 1992
Blood transfusionsAre usually requiredR Vin et al., 2002; R McConnico et al., 1992
Vitamin CHigh dosesR McConnico et al., 1992
Ultrapurified bovine hemoglobin (PBHg)aided in recoveryR Vin et al., 2002
Intranasal oxygenMay be of benefit in some cases
Acetylcysteine (sterile nebulization solution)Mixed with IV fluids
Nasogastrically administered mineral oilR Vin et al., 2002
Activated charcoalR Vin et al., 2002


  • Do not plant red maple trees in or near horse pastures.
  • Routinely walk pastures following storms
  • Identify all trees that are planted nearby or in horse pastures


Poor, it is fatal 60 percent of cases.

Scientific Research

General Overviews

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Risk Factors

  • Strong storm recently occurred (within the past 3 or 4 days)
  • Hungry horses
  • Red maple trees present in or in the vicinity of horse pastures
  • Not knowing what tree species are in horse pastures
  • Drought - can cause the leaves to wilt