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Breed
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Heavier breeds such as draft horses, ponies and mules, stock horses and warmbloods are more susceptible than other breeds, although any horse can be affected. The disease is also seen often in Thoroughbreds, Standardbreds and Arabians, as well. Other breeds that may be predisposed are Andalusians, miniature horses, Morgans, Paso Finos, Saddlebreds, Tennesse Walkers and Welsh ponies.
Loss of muscling
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One of the most obvious symptoms of muscle wasting is a lack of muscling that is symmetrical over the horse's topline (the topline is the top of the horse's neck, the horse's back and the top of the hindquarters). These horses fail to develop rounded muscle in these areas even when exercised and fed properly.
Tying Up
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Horses that have muscle wasting are prone to tying up (other names include exertional rhabdomyolysis, azoturia and Monday morning disease). Tying up is when a horse develops muscle cramps and stiffness, and passes dark-colored urine.
Movement Resistance
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Horses that have untreated muscle wasting usually exhibit exercise intolerance, or may not want to perform certain movements such as cantering or backing, and have stiff gaits or stiffness in the hindquarters. They also may not want to lift their legs for farrier work. Your horse may also exhibit lameness that does not exist in just one leg.
Other Diseases
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Occasionally muscle wasting is confused with other diseases because of the similarity in symptoms. These other diseases include joint soreness, anemia, tying up (which can exist as a problem of its own separate from EPSM), colic, Equine Protozoal Myeloencephalitis or EPM, neurological disorders, and Lyme disease.
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Equine Muscle Wasting Symptoms
Muscle wasting in horses is more commonly known as EPSM or PSSM, which stands for equine polysaccharide storage myopathy. In this disease, the affected horse is less able to properly digest and utilize carbohydrates for energy. Horses with this disease need to be switched from carbohydrate-based diets to diets high in fat.