- Horses that eat enough boxelder seeds can develop seasonal pasture myopathy (SPM).
- SPM is a muscle disease usually fatal in horses.
- Signs of SPM include stiffness, dark urine and difficulty walking.
- You can prevent SPM by not overgrazing pastures, trimming or removing boxelder trees and limiting grazing during high risk periods.
Identifying boxelder trees
Usually consists of three to seven leaflets attached to a common stem.
Oval-shaped that comes to a pointed end.
Leaves drop in the fall.
Winged similar to other maples.
Grouped in clusters.
Seeds drop between September and March.
In the U.S. boxelders grow from the midwest and northeast states south to northern Florida and west through Texas.
Boxelder trees grow in lower, moist areas such as:
On river floodplains.
Along lakes and streams.
In some hardwood forests.
Boxelder seeds, and possibly other maple seeds, can cause seasonal pasture myopathy (SPM) in horses. SPM is a muscle disease with a high fatality rate. About 75 to 95 percent of horses affected by this disease die. The toxin responsible for SPM is hypoglycin A, which is found in boxelder seeds.
SPM occurs most often in the fall and isn’t normally seen when snow is present. Horses don’t naturally eat boxelder seeds. Horses pastured for over 12 hours daily on sparse, wooded pastures without supplemental hay may eat boxelder seeds. Having multiple boxelder trees in your horse pasture is the final key risk factor for SPM.
Herdmates of an affected horse are at risk due to the presence of seeds on pastures. Not every horse on a pasture with boxelder seeds will develop SPM. Many horses on pastures with boxelder trees have remained healthy for years. This is likely from horses having other preferred sources of feed or because the toxin levels in seeds in those pastures are very low.
Signs of illness
Contact your veterinarian promptly if your horse shows any of these signs:
Eventually, rapid breathing and recumbency
Diagnosis and treatment
If you suspect your horse has SPM, rapid diagnosis and treatment are essential to their survival.
Blood tests can detect muscle damage and hypoglycin A toxin levels in your horse. Horses with SPM will have high toxin levels in their blood and urine. These levels indicate horses have eaten boxelder seeds.
Horses with SPM need aggressive medical therapy. Hospital care provides the best odds for survival.
Don’t allow overgrazing.
Provide hay to horses on overgrazed pastures.
Limit turnout to less than 12 hours a day when grazing pastures with boxelder trees during high risk periods.
Don’t introduce horses to a new pasture with boxelders right before or during high risk periods.
Trim low-hanging boxelder tree branches or remove trees from the pasture to reduce the amount of seeds present.
Reviewed in 2021