By Dr. Martin Adams, PAS Equine Nutritionist for Southern States

Polysaccharide Storage Myopathy (PSSM), better known as "tying up disease" is a metabolic muscle problem found in stock and draft horse breeds. Horses with PSSM have symptoms including exertional rhabdomyolysis or tying up, muscle tremors and gait abnormalities. Recent research has shown that a mutation in the GYS1 gene can cause unregulated and excessive glycogen formation in the muscle cells of horses with PSSM.

PSSM is generally classified into two types. Type-1 is caused by the GSY1 mutation and occurs mainly in Quarter Horse, Paint, Appaloosa, Morgan, Tennessee Walker and draft purebreds and crosses. About 75% of Quarter Horses and 85% of draft horses affected with PSSM have the GSY1 mutation. Type-2 mainly affects most Warmbloods, Arabians, Thoroughbreds and Standardbreds. There is a low incidence of the GSY1 mutation found in Warmbloods showing symptoms of PSSM, and the cause for Type-2 has not yet been discovered.

Most horses affected with PSSM respond well to a high-fat and low-starch diet. Southern States has feeds that can be used for PSSM horses, and all our fixed-formula horse feeds are analyzed for starch and sugar content. To find a low soluble carbohydrate feed for a horse with PSSM, use the starch plus ethanol soluble carbohydrate (ESC) values available at www.equussource.com under the Nutrition and Health section. Select a feed with less than 15% starch plus ESC, and add more oil to the diet if symptoms do not totally subside. Recommended feeds for PSSM horses include Triple Crown Low Starch, Triple Crown Senior and Legends Performance Low Carb Formula Pelleted Horse Feeds.

A third type of PSSM, found only in 0.5% of Quarter Horses, is caused by a genetic mutation of the RYR1 gene and can exist in horses with and without the GSY1 mutation. The RYR1 mutation is a life-threatening condition, which does not respond to a high-fat, low-starch dietary treatment. The condition can cause malignant hyperthermia and sudden death.

Horse breeds with the GSY1 mutation can be tested for Type-1 PSSM by using hair root or blood samples. Genetic testing is also available for Quarter Horses with the RYR1 mutation. A muscle biopsy test can also be used for diagnosis of Type-1 and Type-2 PSSM. Consult with your equine veterinarian about testing if you suspect your horse has tying up disease.