THE NATURE OF THE THREAT
The danger posed by wilted or dried red maple leaves has long been recognized: Horses who consume them may sicken or die within hours or days. Toxins in the plant damage the hemoglobin in the horse’s red blood cells, so they can no longer carry oxygen. Affected red cells may rupture, clogging the kidneys with waste products, and the liver and spleen will remove damaged cells from the bloodstream faster than they can be replaced by the bone marrow, which results in severe anemia.
Starved of oxygen, other tissues and organs begin to fail. Outward signs of red maple poisoning include lethargy, poor appetite, colic, pale yellowish gums that progress to dark muddy brown, increased heart rate, faster respiration and distinctive dark red to black-colored urine.
The speed and severity of red maple toxicosis depends on how many wilted leaves have been eaten relative to the horse’s body weight. As little as half a pound of wilted leaves can kill a small pony or miniature donkey; a pound or two can be a fatal dose for an average adult horse.
“If you see your horse eating fallen red maple leaves, first remove the horse from the source of the maple leaves, and call your veterinarian immediately,” says Knight. Do not wait for signs to appear---your horse’s best chance of survival depends on
The speed and severity of red maple toxicosis depends on how many wilted leaves have been eaten relative to the horse’s body weight.
The study revealed
another piece of the puzzle: The equine
digestive tract contains microbes that can turn gallic acid into an even more damaging substance
the earliest possible intervention.
There is no specific antidote to red maple toxicosis, but supportive treatment including intravenous fluids and possibly blood transfusions may help a horse survive long enough for the toxins to clear his system. “Some studies have shown that large amounts of vitamin C [a potent antioxidant] can counter the oxidative effect of the toxins,” says Knight, “but that treatment has to be given very early to be effective.”
Once signs appear, a horse’s odds of survival drop significantly, even with hospitalization. One 2006 study from North Carolina State University reviewed the cases of 32 horses with red maple poisoning admitted to referral hospitals in the Southeast. Of those cases, 29 horses were anemic, 24 had serious systemic inflammation, 12 had kidney dysfunction, nine had laminitis and 13 had colic---19 (59 percent) died.
The exact mechanism of poisoning has not been fully documented. Studies from Cornell University have uncovered significant clues. Compounds extracted from red, sugar, silver and Norway maple leaves incubated with equine blood samples caused oxidation, hemolysis (the breakdown of red blood cells) or other types of damage to the cells. Several chemicals were identified as toxic to the equine red blood cells, most notably gallic acid. “While gallic acid was a major player, most likely it is not the only player,” says Jeanelle Boyer, PhD, who conducted the study. “Quite likely there is a synergistic effect in combination with other chemicals.”
In the next phase of the research, Boyer analyzed extracts from silver maple, sugar maple and Norway maple leaves and found that all contained some level of gallic acid. The extracts derived from the silver and sugar maples did less damage to the equine red cells than did the red maple extracts, but the changes were significant enough to be potentially harmful to a horse. The extracts drawn from the Norway maple were far less toxic. In fact, Boyer speculated that horses would most likely not be able to ingest enough Norway maple leaves to cause themselves any serious harm.
In a separate study, Cornell researchers Karyn Bischoff and Karan Agrawal investigated the effects of gallic acid and tannins from red maple leaves that were incubated with samples of digestive fluids drawn from the equine ileum, the lowest portion of the small intestine. Their findings revealed another piece of the puzzle: The equine digestive tract contains microbes that can turn gallic acid into an even more damaging substance called pyrogallol.
Maple toxicosis is unique to horses and other equids---it does not occur in sheep, cattle or other farm animals. The reason, says Knight, is that “horses do not have a digestive system like that of ruminants [such as cattle, sheep and goats] that can break down gallic acid into harmless components.”