Up to 6% of apparently healthy Quarter Horses could have subclinical (inapparent) polysaccharide storage myopathy (PSSM), and that percentage could be even higher in populations used specifically for breeding. Researchers at the University of Minnesota said that inadvertently breeding horses that have subclinical PSSM will perpetuate the disease in the Quarter Horse breed.

Polysaccharide storage myopathy is a common genetic cause of exertional rhabdomyolysis (tying-up) in Quarter Horses. Until now, researchers knew that 48% of Quarter Horses with signs of neuromuscular disease have PSSM, but they didn't know how prevalent the disease was in apparently healthy breeding and performance Quarter Horses.

Molly McCue, DVM, MS, Dipl. ACVIM, and her colleagues took muscle biopsies from 164 apparently normal Quarter Horses on six ranches in six states, and she presented the study findings at the American College of Veterinary Internal Medicine's Forum on June 1 in Louisville, Ky.

Five of the ranches that were primarily used for breeding and one performance horse ranch participated in the study. Two of the breeding ranches had previously produced PSSM horses. Those previously diagnosed horses were excluded from the study.

Of the horses the group sampled, 117 were breeding stock and 47 were performance animals. "On the four ranches without a recognized history of PSSM, 6.1% (8/132) of horses had PSSM," wrote the researchers in their abstract. "On these ranches, 4.8% (4/84) of breeding stock, and 8.5% (4/47) of performance horses had PSSM. The prevalence of PSSM on the two breeding ranches with a history of producing PSSM horses was 20% (1/5) and 40.7% (11/27)."

According to McCue, "The results of this study suggest that the prevalence of PSSM in the general population of apparently healthy Quarter Horses is approximately 6%. However, an unexpectedly high percentage of horses in certain breeding operations may have subclinical PSSM."

She said these subclinical cases might not be apparent because the horses might be kept on pasture under management conditions that include low-starch diets, extensive pasture turnout, and/or consistent exercise, all of which can stifle clinical signs of PSSM.

The researchers caution that inadvertently using horses with subclinical PSSM for breeding "may perpetuate this heritable myopathy within the breed," and the disease might "become a clinically significant disorder when offspring that inherit PSSM are dispersed and kept under different management conditions." Therefore, testing for the disease prior to breeding is recommended.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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