Meet “George”: A Polysaccharide Storage Myopathy (PSSM) Horse

I’m proud to introduce you to George, a “Heinz-57” draft cross, and my current equine partner.  He is the result of a ½ Thoroughbred, ½ Percheron (dam) x ½ Hanoverian, ½ Paint horse (sire). I’ve had the privilege of knowing George since he was a weanling, and bought him as a yearling.  It wasn’t until I started him under-saddle as a three and a half year old that I started noticing behavioral changes (crankiness – not like George), non-specific muscle soreness, and a transient, almost undetectable gait abnormality, all of which happened to be associated with new hay delivery.  I won’t mention how much I’ve spent having him worked-up, imaged, adjusted, fitted and many more things to get to the bottom of what his body was trying to tell me.  We were coming up empty handed and frustrated.

It wasn’t until after I returned from an equine nutrition symposia that it occurred to me to have him tested for polysaccharide storage myopathy (PSSM). PSSM causes the horse’s muscle cells to store energy (glycogen) in excess, which can result in a variety of symptoms, the most severe of which is tying up after aerobic exercise.  Nearly all of the classic signs were there, short of a bad tying up episode.  Wouldn’t you know it, he came back positive for Type-1 PSSM a.k.a. EPSM, tying- up syndrome/exertional rhabdomyolysis/Monday morning disease, set fast or azoturia.  There is more than one version of PSSM (Type-1 is most common) and the diagnostic tests for each are unique.

Recent advances in equine genetics have made testing a blood, muscle, or hair follicle sample possible.  As it turns out, three of the four breeds that George represents have been identified as prone to carrying the genetic mutation responsible for PSSM.  Unlike some other recessive genetic diseases, PSSM is inherited as a dominant gene; in other words having just one copy of the mutated gene means the horse has the disease.  Horses lucky enough to inherit 2 copies of the gene can be more severely affected.   The good news is, with a little diligence, these horses can be managed and go on to have a good quality of life and successful athletic careers; both of which I want for George.

Diagnostic information can be found at the University of Minnesota Neuromuscular Diagnostic lab website: http://www.cvm.umn.edu/umec/lab/home.html