Seasonal Pasture Myopathy in Horses: Signs, Prevention and What To Do

Seasonal pasture myopathy is rare, but when it happens, it is a true emergency.

By Dr Duncan Houston

Seasonal pasture myopathy is one of those conditions horse owners should know about before they ever see it.

The disease can appear suddenly in horses that were grazing normally, often in autumn or early spring. Affected horses may become weak, stiff, reluctant to move, develop dark urine, struggle to stand, or deteriorate within hours. The frustrating part is that by the time the signs are obvious, the muscle damage may already be severe.

The cause is now much better understood. In North America, seasonal pasture myopathy has been linked to ingestion of hypoglycin A in box elder tree seeds. In Europe, the closely related disease is usually called atypical myopathy or sycamore myopathy and is linked to sycamore maple seeds, leaves, or seedlings. Both diseases damage the horse’s ability to use fat as an energy source, leading to severe muscle breakdown affecting skeletal muscles, breathing muscles, and sometimes the heart.

The good news is that prevention is possible. The hard truth is that treatment is difficult once clinical signs begin.

Quick Answer

Seasonal pasture myopathy is a severe and often fatal muscle disease in grazing horses caused by ingestion of hypoglycin A, most often from box elder seeds in North America or sycamore maple seeds and seedlings in Europe. It causes rapid muscle damage, weakness, stiffness, dark urine, difficulty standing, breathing problems, and sometimes sudden death. It is an emergency, and affected horses need immediate veterinary care, bloodwork, supportive treatment, and often hospitalisation. Prevention depends on identifying risky Acer trees, limiting access to seeds or seedlings, avoiding overgrazed pastures, and providing adequate hay during high-risk seasons. University of Minnesota reports that about 75% to 95% of horses affected by seasonal pasture myopathy die. (University of Minnesota Extension)

What Is Seasonal Pasture Myopathy?

Seasonal pasture myopathy, or SPM, is an acute muscle disease of horses grazing on pasture. It is closely related to atypical myopathy, which is the term more commonly used in the UK and Europe.

The disease causes rapid breakdown of muscle tissue, also called rhabdomyolysis. It can affect:

• Postural muscles

• Limb muscles

• Respiratory muscles

• Swallowing muscles

• Heart muscle

Merck Veterinary Manual describes pasture myopathy or atypical myoglobinuria as a sporadic disease of horses kept on pasture, usually with no supplemental feeding, occurring most often in autumn and sometimes in early spring. It is linked to ingestion of seeds from Acer species, especially box elder in North America and European sycamore maple in Europe. (Merck Veterinary Manual)

This is not ordinary tying-up.

This is a toxic muscle disease with a high fatality rate.

What Causes Seasonal Pasture Myopathy?

The key toxin is hypoglycin A.

Hypoglycin A is found in certain seeds, seedlings, and plant material from Acer species. After ingestion, it is metabolised into toxic compounds that interfere with energy production inside muscle cells.

In a major North American study, box elder seeds were found on all seasonal pasture myopathy farms studied, hypoglycin A was identified in those seeds, and affected horses had serum and urine abnormalities consistent with acquired multiple acyl-CoA dehydrogenase deficiency, known as MADD. Horses with seasonal pasture myopathy also had longer turnout, more overgrazed pastures, and less supplemental feeding than control horses. (PubMed)

Merck explains the mechanism clearly: hypoglycin A metabolites interfere with enzymes needed for metabolism of short and medium-chain fatty acids and branched-chain amino acids. The result is an energy crisis in muscle cells. (Merck Veterinary Manual)

In plain English: the horse’s muscles cannot fuel themselves properly, and they start to fail.

Seasonal Pasture Myopathy vs Atypical Myopathy

The names vary by region.

Term	Usually used where	Main tree risk
Seasonal pasture myopathy	North America	Box elder, Acer negundo
Atypical myopathy	UK and Europe	Sycamore maple, Acer pseudoplatanus
Atypical myoglobinuria	Veterinary description	Acer species with hypoglycin A

The diseases are clinically very similar. Both involve hypoglycin A exposure, severe muscle damage, dark urine, weakness, recumbency, and high mortality. The Royal Veterinary College describes atypical myopathy as a severe and often fatal muscle disorder caused by ingesting sycamore seeds, leaves, or seedlings, with fatality around three quarters of affected horses. (Royal Veterinary College)

The practical point for owners is simple:

If horses are grazing near risky maple species in autumn or spring, take the risk seriously.

Why Box Elder Trees Matter

Box elder, also known as Acer negundo, is a maple species found widely across North America. University of Minnesota notes that box elders grow from the Midwest and northeastern United States south to northern Florida and west through Texas, often in lower moist areas such as river floodplains, lake and stream edges, and some hardwood forests. Box elder seeds are winged, maple-like “helicopter” seeds, grouped in clusters, and may drop between September and March. (University of Minnesota Extension)

Horses do not normally seek out box elder seeds when there is good forage available. The risk rises when pasture is sparse, overgrazed, wooded, or horses are turned out for long periods without supplemental hay.

That is why seasonal pasture myopathy is a pasture management disease as much as a toxic plant disease.

The tree is the source.

The empty pasture is what often pushes horses toward eating the seeds.

Which Horses Are at Risk?

Any horse with access to toxic seeds or seedlings may be at risk, but risk increases when several factors stack together.

Higher-risk situations include:

• Autumn seed drop

• Early spring seedling emergence

• Pasture with box elder or sycamore maple trees

• Overgrazed or sparse pasture

• Limited supplemental hay

• Turnout for more than 12 hours per day in risky pasture

• Wooded paddocks

• Windy or stormy weather that drops seeds

• New turnout onto a pasture with risky trees

• Multiple horses grazing the same seed-contaminated area

University of Minnesota identifies key risk factors as fall season, sparse wooded pasture, horses turned out more than 12 hours per day, lack of supplemental hay, and multiple box elder trees in or near pasture. It also notes that herd mates are at risk when an affected horse is found because the seeds are present in that same pasture. (University of Minnesota Extension)

Not every horse exposed to seeds becomes sick. Some horses graze near box elders for years without disease. That may be due to seed toxin variation, less ingestion, better forage availability, or individual susceptibility. The Royal Veterinary College also notes that some horses appear more susceptible than others, possibly because of genetic differences. (Royal Veterinary College)

This uncertainty is exactly why prevention matters.

Signs of Seasonal Pasture Myopathy

Seasonal pasture myopathy can progress quickly.

Early signs may include:

• Stiffness

• Reluctance to move

• Weakness

• Muscle trembling or fasciculations

• Depression

• Low head carriage

• Difficulty walking

• Sweating

• Colic-like signs

• Normal or preserved appetite despite severe illness

More severe signs include:

• Dark brown, red, or coffee-coloured urine

• Difficulty standing

• Recumbency

• Rapid breathing

• Difficulty breathing

• Choke or swallowing difficulty

• Reduced gut sounds

• Reduced manure production

• Fast heart rate

• Collapse

• Death

Merck notes that clinical signs are sudden and rapidly progressive, and that affected horses may be reluctant to move, weak, fasciculating, recumbent, tachycardic, and may have pulmonary oedema, metabolic and respiratory acidosis, increased cardiac troponin, very high CK and AST, and myoglobinuria. (Merck Veterinary Manual)

University of Minnesota lists stiffness, difficulty walking, dark urine, rapid breathing, and recumbency as key signs that should trigger immediate veterinary contact. (University of Minnesota Extension)

Why Dark Urine Is a Red Flag

Dark urine occurs because damaged muscle releases myoglobin into the bloodstream. The kidneys filter this pigment into the urine, creating brown, red, or coffee-coloured urine.

Myoglobinuria is serious because it means muscle damage is significant. It can also place stress on the kidneys.

A horse with dark urine, weakness, stiffness, or recumbency after grazing near box elder or sycamore maple trees should be treated as an emergency.

Do not assume this is a mild tying-up episode.

How Worried Should You Be?

Low Risk

This is more likely when:

• There are no box elder or sycamore maple trees nearby.

• Pasture is not overgrazed.

• Horses receive adequate hay.

• There is no stiffness, weakness, or dark urine.

• Horses are not turned out long-term in wooded sparse paddocks during autumn or spring.

Action: keep monitoring pastures, identify trees, and maintain adequate forage.

Moderate Risk

This is more likely when:

• Box elder or sycamore trees are present near the pasture.

• Seeds or seedlings are visible.

• Pasture is sparse or overgrazed.

• Horses are turned out for long hours.

• Supplemental hay is limited.

• It is autumn, winter seed-drop period, or early spring.

Action: restrict access to risky areas, provide hay, shorten turnout, fence off seed zones, and consider moving horses to a safer paddock.

High Risk

This is more likely when:

• A horse on the property has signs consistent with SPM.

• Multiple horses share a seed-contaminated pasture.

• Horses are stiff, weak, reluctant to move, or trembling.

• Dark urine is seen.

• Horses have been grazing around box elder or sycamore seeds.

• The pasture is overgrazed and wooded.

Action: call your vet immediately. Remove herd mates from the pasture. Hospital care may be needed for affected horses.

Critical

Treat this as an emergency if:

• The horse is recumbent.

• The horse cannot rise.

• The horse has dark urine.

• Breathing is rapid or difficult.

• The horse is severely weak.

• The horse has choke or swallowing difficulty.

• The horse is collapsing.

• Several horses on the same pasture are showing signs.

Action: this is an immediate veterinary emergency. Do not walk the horse around to see if it loosens up. Keep the horse quiet and call your vet.

When Is Seasonal Pasture Myopathy an Emergency?

Seasonal pasture myopathy should always be treated as an emergency once signs appear.

Call your vet urgently if your horse has:

• Sudden stiffness

• Reluctance to move

• Weakness

• Trembling

• Dark urine

• Rapid breathing

• Difficulty breathing

• Difficulty swallowing or choke

• Recumbency

• Collapse

• Depression with severe muscle signs

• Signs in more than one horse on the same pasture

The Royal Veterinary College notes that vets often begin treatment immediately before official confirmation because testing can take time, and affected horses may need specialist hospital care. Horses that survive the first few days often recover, but this can take several months. (Royal Veterinary College)

The key decision is simple:

If a grazing horse becomes weak, stiff, recumbent, or passes dark urine, do not wait.

What Else Can Look Like Seasonal Pasture Myopathy?

Seasonal pasture myopathy can resemble several other serious conditions.

Important rule-outs include:

Exertional Rhabdomyolysis

Tying-up can cause muscle pain, stiffness, sweating, reluctance to move, and high CK or AST. Seasonal pasture myopathy is different because it is toxic, often pasture-associated, rapidly progressive, and may affect breathing and heart muscle.

Colic

Affected horses may show colic-like signs, but unlike many colic cases, appetite may remain present early. Merck notes that gut sounds may decrease and faecal output may reduce. (Merck Veterinary Manual)

HYPP

Hyperkalemic periodic paralysis can cause weakness, trembling, collapse, and breathing noise in Quarter Horse-related breeds. It is a different disease with different testing and treatment.

Botulism

Botulism can cause weakness, dysphagia, recumbency, and respiratory failure. Choke or swallowing difficulty can overlap with SPM signs.

Neurological Disease

EPM, wobblers, trauma, and other neurological problems can cause weakness, ataxia, or recumbency.

Severe Laminitis

Laminitis can cause reluctance to move and recumbency, but dark urine and massive muscle enzyme elevations point more toward muscle breakdown.

Toxic Plants or Other Toxins

Other plant toxicities can cause weakness, recumbency, or muscle damage. Merck notes that plants such as Senna occidentalis can also cause muscle necrosis in horses. (Merck Veterinary Manual)

Kidney Disease

Dark urine may be confused with blood or pigment from other causes. Bloodwork and urinalysis are needed.

The point is not to diagnose SPM from one sign. The point is to recognise that the combination of pasture exposure, weakness, stiffness, dark urine, and rapid deterioration is very dangerous.

How Do Vets Diagnose Seasonal Pasture Myopathy?

Diagnosis is based on history, clinical signs, pasture risk, bloodwork, urine testing, and specialised toxin or metabolic testing where available.

Your vet may assess:

• Recent pasture access

• Presence of box elder or sycamore maple trees

• Seed or seedling exposure

• Turnout duration

• Pasture density and forage availability

• Whether other horses are affected

• Muscle enzymes such as CK and AST

• Kidney values

• Electrolytes

• Acid-base status

• Cardiac markers such as troponin

• Urine colour and myoglobin

• Acylcarnitine profile

• Urine organic acids

• Hypoglycin A or related metabolite testing where available

Cornell describes seasonal pasture myopathy as causing marked elevation in muscle enzymes, with myoglobinuria, and notes that diagnosis can be based on abnormal acylcarnitines in blood and organic acids in urine when direct hypoglycin A testing is not commercially available in the United States. (Cornell Vet College)

Merck notes that a definitive diagnosis can be made by identifying a pattern of serum acylcarnitines and urine organic acids and glycine conjugates consistent with multiple acyl-CoA dehydrogenase deficiency. (Merck Veterinary Manual)

What Does Treatment Involve?

There is no simple antidote.

Treatment is intensive supportive care.

Your vet may recommend:

• Immediate hospitalisation

• IV fluids

• Dextrose or glucose support

• Pain relief

• Anti-inflammatory treatment where appropriate

• Antioxidant support

• Vitamin E, vitamin C, riboflavin, or other metabolic support

• Electrolyte and acid-base correction

• Monitoring kidney function

• Monitoring heart rate and rhythm

• Nursing care for recumbent horses

• Respiratory support if needed

• Nutritional support

• Frequent bloodwork

Merck lists supportive treatment including antioxidants such as vitamin C, vitamin E, and riboflavin, plus IV fluids including dextrose. It also states that most horses hospitalised for severe rhabdomyolysis do not survive to discharge. (Merck Veterinary Manual)

The Royal Veterinary College notes that affected horses may need specialist hospital care and that treatment is often started before confirmation because waiting for official diagnosis can cost valuable time. (Royal Veterinary College)

The hard truth is that treatment is often unsuccessful once severe signs develop.

Prevention is far better than rescue.

What Should You Do Right Now If You Suspect SPM?

1. Call Your Vet Immediately

Do not monitor overnight. Do not wait to see if the horse warms up.

Seasonal pasture myopathy can progress rapidly.

2. Keep the Horse Quiet

Do not force exercise. Do not lunge, walk repeatedly, or trailer without veterinary advice unless transport is needed and safe.

3. Check Urine Colour

If safe, check whether the urine is dark brown, red, or coffee-coloured. Tell your vet immediately if it is.

4. Remove Other Horses From the Pasture

Herd mates may have eaten the same seeds or seedlings. Move them to a safe paddock or dry lot with hay.

5. Preserve Evidence

Take photos of trees, seeds, seedlings, pasture conditions, and any visible seed clusters. This can help your vet and extension or tree experts identify the risk.

6. Provide Hay to Herd Mates

Do not leave other horses grazing sparse pasture while waiting for answers.

7. Do Not Assume Only One Horse Is at Risk

University of Minnesota notes that herd mates of an affected horse are at risk because seeds are present on the same pasture. (University of Minnesota Extension)

Prevention Is the Main Treatment

Seasonal pasture myopathy prevention is built around three goals:

1. Identify risky trees.

2. Stop horses eating seeds or seedlings.

3. Avoid sparse pasture conditions that encourage seed ingestion.

How To Identify Box Elder Trees

Box elder leaves usually have three to seven leaflets attached to a common stem. The leaflets are oval, pointed, and toothed. The seeds are winged like other maples and grow in clusters. University of Minnesota notes that box elder leaves drop in fall and seeds may drop from September through March. (University of Minnesota Extension)

If you are not sure whether a tree is box elder, get help from:

• Your vet

• Local extension service

• Arborist

• Forestry specialist

• Plant identification app as a first screen, not final diagnosis

• University plant identification resources

Do not rely on “it looks like a maple” as your full risk assessment. Some Acer species are more concerning than others, and toxin levels can vary between trees.

How To Reduce Pasture Risk

Practical prevention includes:

• Do not overgraze pastures.

• Provide hay when pasture is sparse.

• Fence off areas under risky trees.

• Remove fallen seeds where practical.

• Remove or trim risky trees where appropriate.

• Limit turnout during high-risk seed or seedling periods.

• Avoid introducing horses to new box elder pastures in autumn or spring.

• Move horses to safer pasture during seed drop.

• Check neighbouring properties because winged seeds can travel.

• Inspect after windstorms and heavy rain.

• Monitor all horses sharing risky pasture.

• Keep thin, young, or highly curious horses from seed-heavy areas.

University of Minnesota recommends not overgrazing, providing hay on overgrazed pastures, limiting turnout to less than 12 hours per day during high-risk periods when box elder trees are present, not introducing horses to new box elder pastures during high-risk times, and trimming or removing trees to reduce seed burden. (University of Minnesota Extension)

The Royal Veterinary College recommends providing supplementary forage in autumn, clearing fallen sycamore leaves and seeds from grazing areas, checking neighbouring areas because winged seeds may travel up to 200 yards, and testing for hypoglycin A where available. (Royal Veterinary College)

High-Risk Seasons

Risk is highest during:

• Autumn seed drop

• Winter seed persistence in some areas

• Early spring seedling emergence

• Windy or stormy weather after seeds drop

• Cool, wet weather patterns

• Periods when pasture grass is sparse

Merck notes that atypical pasture myopathy occurs most often in autumn but can also occur in early spring, and often follows windy or rainy weather and a cool spring. (Merck Veterinary Manual)

For owners, the practical calendar is:

Autumn and early spring are the danger zones.

But if seeds remain accessible, risk does not politely follow the calendar.

Common Mistakes Owners Make

Waiting for Dark Urine Before Calling

Dark urine is a major red flag, but some horses may be severely affected before owners notice urine colour.

Assuming Good-Looking Horses Are Safe

Some horses can deteriorate rapidly even if they looked normal earlier that day.

Leaving Herd Mates on the Same Pasture

If one horse is affected, the others have likely shared the same exposure.

Thinking Horses Will Not Eat Seeds

Horses may eat seeds when pasture is sparse, overgrazed, or hay is not available.

Only Looking Inside the Fence

Winged maple seeds can travel from neighbouring areas. RVC notes that sycamore seeds may travel up to 200 yards. (Royal Veterinary College)

Waiting Until Autumn to Inspect Trees

You should know which risky trees are present before seed drop begins.

Confusing SPM With Ordinary Tying-Up

Both can cause stiffness and high muscle enzymes, but SPM is a toxic, often fatal pasture-associated disease.

Assuming Treatment Is Usually Successful

Mortality is high. University of Minnesota reports 75% to 95% mortality for SPM. (University of Minnesota Extension)

What About Testing Trees or Seeds?

Testing may be available in some regions for hypoglycin A in plant material or for diagnostic markers in horses.

The Royal Veterinary College offers testing for atypical myopathy diagnosis and also testing for sycamore samples to identify plants containing hypoglycin A. (Royal Veterinary College)

In North America, access to direct hypoglycin A testing may vary. Cornell notes that there was no commercial assay for measuring hypoglycin A available in the United States in its described case report, but cases could be diagnosed using acylcarnitines in blood and organic acids in urine. (Cornell Vet College)

Testing is useful, but it should not delay emergency treatment in a sick horse.

Can Horses Recover?

Some horses do recover, especially if treated early and aggressively.

The Royal Veterinary College notes that if affected horses survive the first few days of treatment, they usually go on to recover completely, although recovery may take several months. (Royal Veterinary College)

Prognosis is worse when:

• The horse becomes recumbent

• Breathing muscles are affected

• Heart muscle is affected

• Muscle enzymes are extremely high

• Kidney damage develops

• Treatment is delayed

• Multiple horses are affected

• The horse cannot be hospitalised

Even in survivors, recovery can take time. Returning to work should be gradual and guided by muscle enzyme recovery, strength, comfort, and veterinary advice.

Case Example: Fall Pasture Weakness Near Box Elder Trees

A 7-year-old gelding is found stiff and reluctant to move after overnight turnout in a sparse wooded paddock in late October. The owner notices dark brown urine and rapid breathing. Several box elder trees border the paddock, and winged seed clusters are visible under the fence line.

The vet is called immediately. Bloodwork shows severely elevated CK and AST, and the horse is referred for intensive hospital care. The rest of the herd is moved off the pasture, and hay is provided in a dry lot.

The key lesson is not just that the horse was treated.

The key lesson is that the pasture was recognised as a risk, herd mates were removed, and prevention became the focus for every other horse on the property.

Myth vs Reality

Myth	Reality
Seasonal pasture myopathy is just severe tying-up.	It is a toxic muscle disease linked to hypoglycin A from Acer seeds or seedlings.
Only sick-looking horses are at risk.	Horses may appear normal before rapid deterioration.
If only one horse is sick, the others are safe.	Herd mates may have shared the same seed exposure.
Good pasture grass means no risk.	Risk is much higher in sparse or overgrazed pasture, but seeds can still be present.
Treatment usually saves them.	Mortality is high, so prevention is the best protection.
Box elder trees only matter if they are inside the paddock.	Winged seeds can move from nearby areas, especially after wind or storms.
Dark urine is the first sign.	Stiffness, weakness, depression, or reluctance to move may appear before urine is noticed.

FAQs About Seasonal Pasture Myopathy

Is seasonal pasture myopathy fatal?

Yes, it is often fatal. University of Minnesota reports that about 75% to 95% of affected horses die, and RVC reports atypical myopathy is fatal for around three quarters of affected horses. (University of Minnesota Extension)

What trees cause seasonal pasture myopathy?

In North America, seasonal pasture myopathy is mainly linked to box elder, Acer negundo. In Europe, atypical myopathy is mainly linked to sycamore maple, Acer pseudoplatanus. Both involve hypoglycin A exposure. (Merck Veterinary Manual)

What are the first signs?

Early signs can include stiffness, weakness, reluctance to move, depression, muscle trembling, and difficulty walking. Dark urine, rapid breathing, recumbency, and collapse are severe warning signs. (University of Minnesota Extension)

Can seasonal pasture myopathy be treated?

Treatment is supportive and intensive. It may include hospitalisation, IV fluids, dextrose, antioxidants, pain control, electrolyte and acid-base support, and nursing care. There is no simple antidote, and survival is not guaranteed. (Merck Veterinary Manual)

How do I prevent it?

Prevent it by identifying box elder or sycamore maple trees, fencing off or removing risky areas, avoiding overgrazed pasture, providing hay, limiting turnout during high-risk periods, clearing seeds or seedlings where practical, and moving horses off risky pasture in autumn and early spring. (University of Minnesota Extension)

The Bottom Line

Seasonal pasture myopathy is rare, but it is one of the most serious pasture-associated diseases a horse can develop.

The cause is not mysterious anymore. In North America, box elder seeds containing hypoglycin A are a major risk. In Europe, sycamore maple seeds and seedlings are the classic concern. The toxin disrupts muscle energy metabolism, causing severe muscle breakdown that can affect movement, swallowing, breathing, and the heart.

Once signs appear, the situation is urgent and the prognosis is guarded.

The most effective treatment is prevention:

• Know your trees.

• Inspect pastures before autumn and spring.

• Do not overgraze.

• Feed hay when pasture is sparse.

• Fence off seed-heavy areas.

• Move horses during high-risk periods.

• Treat stiffness, weakness, dark urine, or recumbency as an emergency.

This is one of those conditions where being a little paranoid in autumn can save a horse’s life.

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If your horse is grazing near box elder or sycamore maple trees, or you are unsure whether a pasture is safe during seed drop or spring seedlings, ASK A VET™ can help you organise photos, pasture details, risk factors, and the right questions to discuss with your local veterinarian.