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  Home > Neuromuscular Diagnostic Laboratory > Seasonal Pasture Myopathy > What should I do if I think my horse has SPM/AM? - Owner Information

What should I do if I think my horse has SPM/AM? - Owner Information

What is seasonal pasture myopathy?

  • A toxic, highly fatal condition in pastured horses in the spring and fall that causes break down of postural muscles, breathing muscles, and heart muscle.
  • It can affect one or several horses on the pasture and often resembles colic or founder.
  • The condition resembles “atypical myoglobinuria/atypical myopathy” observed in northern Europe.
  • In North America, it is caused by the ingestion of box elder tree seeds that contain the toxin hypoglycin A.

What are the signs?
Because signs can resemble severe colic or founder, a careful examination is necessary so as not to miss this diagnosis:

  • Horses appear weak, stiff, may tremble and will lay down frequently or for long periods of time.
  • If the heart and breathing muscles are affected, the gums become dark and horses will have difficulty breathing.
  • Horses often have dark colored urine.
  • The disease progresses quickly and most affected horses die within 24-72 hours after onset of signs.

Which horses are at risk?

  • Risk is highest in:

o Horses exposed to box elder tree seeds
o Horses pastured for over 12 hours daily
o The fall season
o Horses that do not receive supplemental hay while they are out on pasture, including horses of normal or excess body condition
o Horses on sparse pasture with short grass
o Horses on pasture with many trees and dead wood on the ground
o Weeks when there has been heavy wind or rain
o Horses that are on a pasture for their first season, such as a young horse or a horse that has recently moved to a farm

When does it occur?
We investigated weather conditions in the Midwest for 14 previous cases of SPM.

  • 13 cases occurred from September to November, and 1 case during a particularly wet, cool May (mean temperature, 54oF).
  • 4 horses developed SPM 2 to 4 weeks prior to the first snowfall, 2 horses were evaluated 2 to 3 days after the first snowfall, and the remaining horses were evaluated 4 days to 6 weeks after the first snowfall.

What should I do if I think my horse has SPM?

Step 1:

  1. Call your veterinarian immediately if you think your horse is suffering from SPM or is otherwise unwell; the earlier the diagnosis is made, the better chance of saving your horse.
  2. A blood sample submitted by your veterinarian can also detect muscle damage by showing very high serum creatine kinase (CK) activity.


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