A toxin in the seeds of the box elder tree (Acer negundo) has just been discovered to cause Seasonal Pasture Myopathy in North America.
The discovery was made by researchers at the University of Minnesota Equine Center.
Seasonal Pasture Myopathy, or Atypical Myopathy is a devastating equine muscle disease which is fatal in more than 90 per cent of cases.
Characteristic signs of the disease include stiffness, difficulty walking or standing, passing dark urine, and eventually breathing rapidly and becoming recumbent before death.
It is confused with colic or founder and the researchers believe it is under-reported as a consequence.
Until now, veterinarians and researchers have not known what caused the disease or how to diagnose it.
The disease 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 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.
- The introduction of a horse on to a pasture for its first season, such as a young horse or a horse that has recently moved to a farm.
The university researchers found that a toxin in the seeds of the box elder tree caused the disease, making the presence of box elder seeds the final key risk factor.
Ingestion of sufficient quantities of box elder seeds results in the 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 the condition. The reason for this has yet to be established. 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 researchers now hope to develop a diagnostic test to help veterinarians to diagnose horses earlier, determine which horses are most at risk, and prevent future cases.
The university has placed material online to explain the research and help horse owners minimize their horses’ risk of exposure.
Information for owners who think their horses may have the condition can be found here.