This website is dedicated to disseminating and gathering information about the equine condition “Seasonal Pasture Myopathy” (SPM) or “Atypical Myopathy” (AM), a devastating equine muscle disease which is fatal in over 90% of cases. Characteristic signs of disease include stiffness, difficulty walking or standing, voiding dark urine, and eventually breathing rapidly and becoming recumbent before death. It is confused with colic or founder. An ounce of prevention is worth a pound of cure with this disease, but until now, veterinarians and researchers have not known what causes the disease or how to diagnose it. We are therefore excited to report that the cause of SPM has been identified through research at the University of Minnesota.
Seasonal Pasture Myopathy is most commonly seen in the fall with fewer numbers of affected horses seen in the spring and summer. It is typically not seen when snow is present. Factors that have been associated with increased risk of disease include: being pastured for over 12 hours daily; the fall season; lack of supplemental hay while horses are on pasture; sparse pasture with short grass; the presence of trees with dead wood on the ground; heavy wind or rain in the week preceding clinical signs; and introduction of a horse onto a pasture for its first season, such as a young horse or a horse that has recently moved to a farm.
A toxin in the seeds of the box elder tree (Acer negundo) has just been discovered to cause SPM in North America, making the presence of box elder seeds the final key risk factor for SPM. Ingestion of sufficient quantities of box elder seeds results in breakdown of respiratory, postural, and cardiac muscles. When one horse becomes affected, herd mates are also at risk. However, not every horse pastured near box elder seeds will develop SPM. The reason behind this is just one of the mysteries about SPM that remains to be solved. It likely has to do with the time they are exposed to seeds, the toxin level in the seeds, how many seeds blow onto the pasture and whether they received additional feed such as hay in a feeder that makes eating seeds less attractive.
The goal of this website is to assist horse owners and veterinarians in preventing and identifying cases of SPM and for us to accumulate information on as many horses with Seasonal Pasture Myopathy as possible for our research. Our research aims to develop a diagnostic test to help veterinarians to diagnose horses earlier in disease, determine which horses are most at risk for developing disease, and prevent future cases.
For this we need your help. Watch your horses closely for the signs of disease listed above (for more details, click here), and if observed, contact your veterinarian for primary care and contact the University of Minnesota by completing a short survey to determine your horse’s risk (click here). We will respond to your answers within 24 hours, Monday through Friday, and let you know whether it is a likely possibility that your horse has SPM. If it is, then we would need to contact your veterinarian for more information about clinical signs and test results. In order to make a true diagnosis of Seasonal Pasture Myopathy, we will need to obtain urine or blood samples from potentially affected horses for toxin testing.
Aggressive medical therapy is warranted in any horse that is confirmed or suspected to have SPM. Assessing your pastures and horses’ potential for toxin exposure can be done at any time. Information on minimizing your horses’ risk is available on this website (click here) as well as more detailed information on recognizing and treating SPM.
If you think your horse has experienced or is experiencing SPM, please complete the Risk Assessment Survey (click here) and a University veterinarian will contact you promptly regarding your concerns.
Thank you for your help in preventing future cases of SPM.