Sycamore trees pose a serious and sometimes fatal poisoning risk to horses, a moderate concern for certain other livestock, and a minimal direct toxicity risk to humans and most pets. The danger centers on a toxin called hypoglycin A, found primarily in the seeds and seedlings of the sycamore maple (Acer pseudoplatanus). For horses, ingesting these plant parts can trigger a devastating muscle disease called atypical myopathy, with mortality rates often exceeding 70 percent. But the word “sycamore” refers to very different trees depending on where you live, and that naming confusion matters enormously when assessing the actual risk.
Which “Sycamore” Are We Talking About?
This is the single most important thing to get right, because the common name “sycamore” applies to entirely different species on different continents. In Europe, “sycamore” almost always means Acer pseudoplatanus, a large maple tree with distinctive winged seeds (samaras) that helicopter to the ground each autumn. This is the tree responsible for widespread horse poisonings across the UK and continental Europe. It contains significant concentrations of hypoglycin A in its seeds and seedlings.
In North America, “sycamore” typically refers to Platanus occidentalis, the American sycamore or buttonwood. This is a completely different tree, not a maple, and it does not contain hypoglycin A. If you have a large, peeling-bark sycamore in your yard in Ohio or Virginia, that is almost certainly a Platanus species and is not the toxic tree discussed in equine poisoning reports. However, North America has its own hypoglycin A-carrying maple: the box elder (Acer negundo), which causes an equivalent disease called seasonal pasture myopathy in horses across the United States and Canada.1PubMed. Seasonal pasture myopathy/atypical myopathy in North America associated with ingestion of hypoglycin A within seeds of the box elder tree So the question is not really about one tree. It is about a family of maples that share the same toxin.
The Toxin and Where It Lives in the Tree
Hypoglycin A is an amino acid derivative that, once metabolized in the body, disrupts the ability of cells to burn fat for energy. When the normal fat-burning pathway is blocked, certain muscles (especially those used for posture, breathing, and heart function) lose their energy supply and begin to break down. The toxin is not evenly distributed throughout the tree. Seeds and seedlings carry the highest concentrations, while leaves and bark contain far less.
A Dutch study measuring hypoglycin A across maple species found that nearly all Acer pseudoplatanus samples contained the toxin, with seeds showing substantially higher concentrations than other plant parts. Seeds from pastures where horses had developed atypical myopathy averaged roughly 856 mg/kg of hypoglycin A, compared to about 456 mg/kg in seeds from pastures with no cases.2Wiley Online Library / Journal of Veterinary Internal Medicine. Hypoglycin A Concentrations in Maple Tree Species in the Netherlands and the Occurrence of Atypical Myopathy in Horses Two other common European maples, Norway maple and field maple, had no detectable hypoglycin A at all.
Seasonality adds another layer of risk. Seeds fall in autumn, making that the classic danger window. But a spring pilot study found hypoglycin A in sycamore maple seedlings sprouting from those overwintered seeds and even in rainwater that had washed over the seedlings overnight.3PubMed. Potential new sources of hypoglycin A poisoning for equids kept at pasture in spring: a field pilot study That means horses at pasture face risk in both autumn and spring, not just when seeds are visibly scattered on the ground. Norway maple and field maple seedlings tested negative for the toxin in the same study, reinforcing that the hazard is specific to Acer pseudoplatanus in Europe and Acer negundo in North America.
Young seedlings with their first leaf-like structures (cotyledons) are preferentially eaten by horses compared to slightly older seedlings with true leaves. One study found horses consumed about 19 percent of the youngest seedlings but only about 5 percent of more mature ones, likely because older seedlings accumulate bitter-tasting phenolic compounds that deter grazing.4Vet Rec / BMJ. Equine atypical myopathy: consumption of sycamore maple seedlings (Acer pseudoplatanus) by pastured horses is driven by seedling maturity and might be associated with phenolic compounds The youngest, least bitter seedlings happen to be the ones horses are most likely to eat, and those seedlings still carry meaningful amounts of hypoglycin A.
The Threat to Horses
Atypical myopathy in horses is not subtle. It comes on fast and is frequently lethal. Affected horses develop sudden stiffness, trembling, sweating, dark-colored urine (from muscle proteins flooding the kidneys), and profound weakness. Many become recumbent and unable to stand. In one study of 16 investigated cases, 15 horses died and only one survived.5PLOS ONE. Hypoglycin A Content in Blood and Urine Discriminates Horses with Atypical Myopathy from Clinically Normal Horses Grazing on the Same Pasture That grim ratio is consistent with broader reports across Europe, where the disease has been documented in thousands of horses since it was first recognized.
The underlying pathology is severe rhabdomyolysis, which is a rapid and catastrophic breakdown of skeletal muscle fibers. Post-mortem examination of affected horses reveals widespread muscle damage across multiple muscle groups including intercostal muscles (the muscles between the ribs used for breathing), along with distinctive microscopic fat accumulation within the destroyed muscle cells.6PubMed. Equine atypical myopathy caused by hypoglycin A intoxication associated with ingestion of sycamore maple tree seeds Because the toxin targets muscles responsible for breathing and maintaining posture, affected horses deteriorate quickly once clinical signs appear.
There is no antidote. Treatment is supportive: intravenous fluids, pain management, nutritional support, and keeping the horse standing if possible. The emphasis in veterinary guidance is squarely on prevention, because once a horse is showing clinical signs, the prognosis is poor.7In Practice. Atypical myopathy: an update Research into blood markers is advancing, though. A large-scale study analyzing blood from 263 horses found that specific acylcarnitine profiles could help distinguish poisoned horses from healthy ones and even predict survival among confirmed cases.8PubMed Central / Elsevier. Large-scale study of blood markers in equine atypical myopathy reveals subclinical poisoning and advances in diagnostic and prognostic criteria Earlier detection could eventually improve outcomes, but for now the best strategy remains keeping horses away from the seeds and seedlings entirely.
Box Elder and Seasonal Pasture Myopathy in North America
In the United States and Canada, the same disease shows up under a different name: seasonal pasture myopathy (SPM). The culprit is the box elder tree (Acer negundo), a scrubby, fast-growing maple common across much of North America. Box elder seeds contain hypoglycin A, and when horses graze pastures littered with them in autumn, the resulting muscle breakdown is clinically identical to the European version of the disease.
A landmark study confirmed the link by finding hypoglycin A in box elder seeds collected from every pasture where SPM cases occurred. The toxic metabolite MCPA was detected in both the blood and urine of affected horses, producing the same metabolic signature seen in European atypical myopathy cases.1PubMed. Seasonal pasture myopathy/atypical myopathy in North America associated with ingestion of hypoglycin A within seeds of the box elder tree Box elder seeds were found on 61 percent of control pastures too, meaning proximity to the tree is common, but not every horse that encounters the seeds gets sick. The reasons for individual susceptibility remain poorly understood, though the amount ingested and the nutritional state of the horse both appear to matter.
One practical takeaway for horse owners in North America: if your pastures border box elder trees, the risk is real and comparable to the European sycamore maple threat. Removing box elder trees from fence lines, picking up fallen seeds, and providing adequate hay so horses are not driven to forage on whatever they find on the ground are all recommended steps.
Cattle, Sheep, and Zoo Animals
Horses are the most dramatically affected species, but they are not the only animals that absorb hypoglycin A. The picture for ruminants, including cattle and sheep, is more complicated and, so far, less alarming.
Dairy cows exposed to sycamore maple seedlings absorbed the toxin and excreted it in their milk and urine, but they showed no visible signs of illness, no decline in milk production, and no obvious discomfort during the observation period. Researchers concluded that dairy cows may have a low susceptibility, possibly because their rumen bacteria break down the toxin before it can do as much damage.9Journal of Agricultural and Food Chemistry. Detection of Hypoglycin A and MCPrG Metabolites in the Milk and Urine of Pasture Dairy Cows after Intake of Sycamore Seedlings Whether the same would hold true for beef cattle, young calves, or cows under nutritional stress is unknown. The finding that hypoglycin A can appear in milk is also worth noting: what that means for human consumers of that milk has not yet been studied in any depth.
Sheep show a similar pattern of absorption without obvious disease. When researchers released sheep onto sycamore-contaminated pastures, all the sheep ate the seedlings, and hypoglycin A appeared in their blood within 48 hours. But none became clinically ill. Only one ewe showed detectable levels of the downstream toxic metabolite, and her blood hypoglycin A was the highest in the group.10Journal of Veterinary Internal Medicine. Hypoglycin A absorption in sheep without concurrent clinical or biochemical evidence of disease The suggestion is that ruminant digestive systems may offer some protection, but subclinical effects cannot be ruled out.
Zoo animals present a different concern. Two blue wildebeest (gnus) kept in an enclosure with Acer pseudoplatanus developed severe clinical signs consistent with hypoglycin A poisoning, including abnormal serum acylcarnitine profiles characteristic of the toxin’s metabolic effects.11Europe PMC. Acer pseudoplatanus: A Potential Risk of Poisoning for Several Herbivore Species That same review noted documented poisoning in Père David’s deer and two-humped camels. The common thread is that non-domesticated herbivores housed near sycamore maples may be at risk, particularly when they have limited food choices and eat the seeds or seedlings out of opportunity or boredom.
Are Sycamore Trees Dangerous to Humans?
Direct poisoning from sycamore maples is not a recognized concern for humans. People do not generally eat maple seeds or seedlings, so the main exposure route that harms horses simply does not apply. Hypoglycin A is the same toxin found in unripe ackee fruit (Blighia sapida), which has caused well-documented human poisoning in the Caribbean. But ackee has much higher concentrations in its edible aril, and people eat it as food. Nobody is eating handfuls of sycamore maple seeds.
The relevant human health concern from sycamore trees is allergic, not toxic. Research on Platanus occidentalis (the American sycamore, not the maple) found that about one in five adults with allergic rhinitis tested positive for sensitization to its pollen.12PubMed Central. Prevalence of allergic sensitization to Platanus occidentalis among adults with allergic rhinitis: A multicenter study People sensitized to sycamore pollen were also more likely to be sensitized to other tree pollens, weed pollen, and fungal allergens. The fine hairs shed from the undersides of sycamore leaves and from seed balls can also irritate the respiratory tract in some people, though this is a physical irritation rather than a toxic effect.
For children, the accidental ingestion question comes up occasionally. A toddler who chews on a sycamore maple seed is probably not in serious danger from a single seed, given the dose-to-body-weight ratio required to produce toxicity in much larger horses. But there is no formal human dose-response data for hypoglycin A from maple sources, so caution is reasonable. If a child eats maple seeds and develops vomiting, lethargy, or unusual sleepiness, seeking medical attention is prudent.
What About Dogs and Cats?
Surprisingly little research exists on hypoglycin A toxicity in companion animals. Dogs and cats are not typically grazing animals, so they do not encounter sycamore maple seeds the way horses do. A dog that picks up and chews a few samaras during a walk is probably not at serious risk, but anecdotal veterinary reports of dogs becoming ill after ingesting large quantities of maple seeds do exist, and the lack of formal study means the safe threshold is unknown.
The more established concern with maples and dogs involves red maple (Acer rubrum), whose wilted or dried leaves can cause oxidative damage to red blood cells in horses (a separate condition from atypical myopathy). Red maple toxicosis in dogs has not been well documented, but given that dogs have different hemoglobin chemistry than horses, the risk profile is probably different. In practical terms, if your dog eats a handful of any kind of maple seeds or leaves and then shows signs like vomiting, dark urine, lethargy, or refusal to eat, a veterinary call is warranted.
Regional Risk Factors for Horse Owners
Not all pastures near sycamore maples are equally dangerous. An English study examining atypical myopathy cases across counties found a significant positive correlation between the number of reported cases and a regional “fruit score” reflecting how heavily local sycamore trees produced seeds that year. Counties with the highest fruit scores had substantially more cases. Almost three-quarters of the 222 cases analyzed occurred in counties with a fruit score of 4 or 5 on a 5-point scale.13Wiley Open Access Collection. Assessment of tree‐associated atypical myopathy risk factors in Acer pseudoplatanus (sycamore) seeds and leaves In other words, years when sycamore maples produce especially heavy seed crops are years when more horses get sick.
Other recognized risk factors include sparse pasture (which forces horses to forage more broadly), wet and windy autumn weather (which brings more seeds and seedlings down), and lack of supplemental hay. Horses on lush, well-maintained pasture with plenty of hay are less motivated to eat fallen seeds. Young horses and those in poor body condition seem disproportionately affected, though the evidence base for individual susceptibility is still thin.
Practical steps for reducing the risk include fencing off areas directly under sycamore maples (or box elders in North America), regular removal of fallen seeds and seedlings from pastures, providing supplemental hay when grazing is poor, and stabling horses during peak seed-fall periods if the pasture cannot be adequately cleared. Some horse owners choose to remove sycamore maples entirely from field boundaries, though this can be expensive and controversial when the trees are mature or protected.
Subclinical Poisoning and the Horses That Do Not Show Symptoms
One of the more unsettling findings from recent research is that horses can absorb hypoglycin A and develop abnormal blood chemistry without ever showing obvious clinical signs. These “cograzers,” horses sharing a pasture with a confirmed atypical myopathy case but apparently staying healthy, often have detectable levels of the toxin and its metabolites in their blood. The large-scale blood marker study of 263 horses found that cograzers showed altered acylcarnitine profiles compared to controls, suggesting some degree of metabolic disruption even without visible illness.8PubMed Central / Elsevier. Large-scale study of blood markers in equine atypical myopathy reveals subclinical poisoning and advances in diagnostic and prognostic criteria
This raises questions about long-term effects. A horse that survives a mild or subclinical exposure may recover fully, or it may carry some residual muscle damage. The research is too young to say. But for horse owners, it reinforces the message that just because your horse looks fine after spending time in a sycamore-contaminated pasture does not mean it was not exposed. Blood testing through a veterinarian can reveal whether subclinical absorption occurred, and removing the horse from that pasture sooner rather than later is the cautious move.
Why Ruminants Handle It Better Than Horses
The emerging picture from cattle and sheep studies points toward the rumen as a protective factor. Ruminant digestive systems involve a multi-chambered stomach where microbes ferment plant material extensively before it reaches the small intestine, where most absorption occurs. It is plausible that rumen bacteria partially degrade hypoglycin A before it can be absorbed into the bloodstream in its active form. Horses, with their simple stomachs, absorb the toxin more directly.
This hypothesis fits the observed data: sheep absorb hypoglycin A but show no disease, cows excrete it without apparent illness, yet horses develop fatal muscle breakdown from similar or even smaller exposures.10Journal of Veterinary Internal Medicine. Hypoglycin A absorption in sheep without concurrent clinical or biochemical evidence of disease The wildebeest cases complicate the picture somewhat, since wildebeest are ruminants, but those animals may have ingested unusually large amounts in a confined enclosure with limited alternative forage. Whether ruminants under nutritional stress or with compromised rumen function might be more vulnerable remains an open question. Researchers have been careful to note that an absence of visible disease does not rule out subclinical metabolic effects that could matter over time.9Journal of Agricultural and Food Chemistry. Detection of Hypoglycin A and MCPrG Metabolites in the Milk and Urine of Pasture Dairy Cows after Intake of Sycamore Seedlings
Hypoglycin A in Rainwater and Unexpected Exposure Routes
One finding that surprised even researchers was the detection of hypoglycin A in rainwater collected from wet sycamore maple seedlings. After a rainy night in early spring, water that had pooled on and around germinating seedlings contained measurable levels of the toxin.3PubMed. Potential new sources of hypoglycin A poisoning for equids kept at pasture in spring: a field pilot study This means horses do not necessarily have to eat the seeds or seedlings directly. Drinking from puddles on contaminated pastures could, in theory, provide a secondary exposure route, although whether rainwater concentrations are high enough to cause disease on their own has not been established.
Sycamore maple inflorescences (the clusters of small flowers that appear in spring before the seeds develop) also tested positive for the toxin in the same pilot study. This extends the window of potential exposure beyond the autumn seed-fall and spring seedling period to include the flowering phase. For horse owners, the implication is that sycamore maples on or near pastures are a year-round management concern, not just an autumn one. The risk profile shifts with the seasons, peaking in autumn and again in spring, but the tree does not become truly safe at any point in the growing year.