New Research on PSSM Prevalence, Heritability in Progress

Researchers with the University of Minnesota have identified polysaccharide storage myopathy (PSSM) in 6-12% of overtly healthy Quarter Horses on six farms involved in a new study. They also said the disease could be more common within certain breeding populations.

The study, published in the Sept. 1 edition of the Journal of the American Veterinary Medical Association, explores one of many new findings concerning the disease, according to Molly McCue, DVM, MS, PhD, Dipl. ACVIM.

The disease stems from a genetic abnormality in muscle cell metabolism that leads to exertional rhabdomyolysis (tying-up). Horses with PSSM make and metabolize glycogen abnormally. This glycogen can build up in muscle, and when it cannot be burned as fuel it causes cramping and pain. There is no cure for the disease, but management changes, including elimination or reduction of concentrated feeds containing high levels of starch and sugar, and increasing exercise and turnout (such as keeping the horse on pasture), can minimize clinical signs.

Along with exploring PSSM in Quarter Horses, the researchers have also identified the genetic defect responsible for the disease, and they hope to have a genetic test commercially available within the next six months. They have also investigated the heritability of PSSM, which previous studies have shown is also common in draft horses and Warmbloods.

"We have identified the genetic defect that accounts for a large percent of PSSM cases in several breeds," McCue said. "PSSM is inherited in a dominant fashion. A horse with one copy of the mutant gene will have PSSM and pass PSSM on to its offspring 50% of the time. A horse with two copies of the mutant gene will have PSSM and will pass PSSM on to its offspring 100% of the time."

More information on these new conclusions will become available once the study is published in a scientific publication.

These findings impact horse breeders, in that horses that are maintained on pasture might never show signs of the disease, but they will be able to pass it to their offspring.

The study, "Estimated prevalence of polysaccharide storage myopathy among overtly healthy Quarter Horses in the United States" was published September 1, 2007 in the Journal of the American Veterinary Medical Association. Contributing authors were McCue and Stephanie Valberg, DVM, PhD, Dipl. ACVIM.

About the Author

Erin Ryder

Erin Ryder is a former news editor of The Horse: Your Guide To Equine Health Care.

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