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Here’s some good news for owners of horses with polysaccharide storage myopathy (PSSM) type 1: studies have found no evidence of heart problems in horses with this muscle disorder.

Polysaccharide storage myopathy has two types that manifest with various signs in horses. In humans with a metabolic muscle disease similar to PSSM type 1, cardiac muscle cells may be affected along with skeletal muscle. In individuals with this problem, heart rhythm and other functions may be abnormal.

A recent study led by scientists from the Royal Veterinary College in England looked at blood samples from 125 draft-breed horses. Although some horses with the PSSM type 1 mutation did show slightly higher levels of cardiac proteins, ECG readings did not reveal evidence of heart muscle injury or arrhythmia in horses with either one or two copies of the gene mutation that causes PSSM type 1.

Even without the worry of heart disorders, owners of horses with PSSM type 1 have other concerns. Common in at least 20 breeds including Belgians, Percherons, and some warmblood horses, PSSM type 1 is a dominant genetic trait that can appear in horses when one or both parents are carriers.

Affected horses may seem normal in their early years, but by two or three years of age they begin to show gait abnormalities, painful muscle cramping, weakness, sweating, skin twitches, and reluctance to move out freely when exercised. The problems are caused by an irregularity in glycogen synthase, an enzyme that helps produce glycogen.

The best diagnostic techniques are genetic testing and microscopic examination of muscle tissue samples.

Dietary and exercise modifications have been extremely helpful in keeping affected horses more comfortable and allowing them to move normally. Nutritional management includes replacing much of the horse’s grain with a low-starch, high-fat feed. Grass hay with a nonstructural carbohydrate level below 24% is recommended. By reducing ingested carbohydrates, glycogen synthase activity is better regulated, thereby reducing muscle pain. However, the resulting drop in dietary energy must be offset by providing calories from another source, and adding fat is a helpful approach. Feed products designed for horses with PSSM type 1 contain fat from various sources, and once horses have been gradually introduced to the change in diet, they have no trouble adapting to the use of this energy source.

Another dietary concern is supplying adequate amounts of vitamin E and selenium. These nutrients work together as antioxidants that prevent muscle damage, so they are especially important for horses with muscle disorders.

Horses with PSSM type 1 should be on a program of regular exercise unless muscle soreness prevents this. Generally, full turnout is preferable to stalling.

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