Q: My 6-year-old draft cross seems to have muscle spasms after any trailer ride lasting more than 10 minutes. He loads fine and seems to travel quietly, but when I unload him, he’s reluctant to move and his hindquarter muscles seem flexed. It doesn’t look as dramatic as tying up. If I didn’t know him as well as I do, I might not notice it at all. After about a half hour, he relaxes and can unload with no problems. I’ve started planning in this extra time on our trips to shows and clinics, but I worry about what’s causing it. Could it just be nerves? A problem with my trailer? Or is there something physically wrong with my horse? Lynn Stowers; Spokane, Washington

A: Trailering a horse actually puts more stress on muscles than you might think. Make sure your horse has enough room in your trailer to be comfortable. Even better, experiment with allowing him to travel loose to see which orientation he favors. He could still be nervous and just not showing it. Being tense will put even more stress on muscles.

However, a normal horse should not have this type of muscle spasm issue after trailering, no matter what position he is in or how nervous he is. Muscle issues in draft-related horses are often associated with an underlying “metabolic difference” that makes them
prone to equine polysaccharide storage myopathy (EPSM, also called PSSM
and EPSSM).

Many draft-related horses with EPSM do not have any changes in muscle enzyme levels in blood, and so it may not show up easily from blood testing. A genetic test is now available from the University of Minnesota that identifies many, but not all, EPSM horses. The test can be performed on pulled mane or tail hairs, and so it is very easy to do. A positive result will indicate polysaccharide storage myopathy, although a negative result will not rule it out.

If EPSM is an issue, feeding a low starch and sugar and high fat and fiber feed can make a big difference.

Beth A. Valentine, DVM, PhD
Professor, College of Veterinary Medicine
Oregon State University
Corvallis, Oregon