This online article is a supplement to “Conscious and Cautious Breeding,” a health column that appeared in the August 2009 issue of Western Horseman. Chart by Melissa Cassutt • Photograph by Sebastian Gerhard There are many genetic equine diseases currently being studied. The chart below details the symptoms of each of these documented diseases and the associated breeds.

Hyperkalemic Periodic Paralysis HYPP is a muscle disease that can cause muscle twitching and unpredictable paralysis attacks, which can lead to sudden death and respiratory noises. Breeds affected: Stock horse breeds Polysaccharide Storage Myopathy—Type I PSSM is commonly known as “tying up,” causing symptoms such as muscle soreness, reluctance to engage the hindquarters, muscle atrophy, weakness, and difficulty in backing up and picking up hind feet. Breeds affected: It has been identified in nearly 20 breeds, but is most commonly found in stock horse and draft breeds Glycogen Branching Enzyme Deficiency GBED causes late-term abortion or death of foals. These foals might appear healthy for a time but can eventually develop seizures, become too weak to stand, or in some cases die suddenly. GEB is always fatal. Breeds affected: Stock horse breeds Hereditary Equine Regional Dermal Asthenia A disease that causes hyperextensive skin, scarring and severe lesions along the back, HERDA typically occurs by age 2. Breeds affected: Stock horse breeds Lethal White Foal Syndrome LWFS is directly related to the overo gene. One copy of the gene produces an attractive white hair color pattern, while two copies produce a white foal without a functional gastrointestinal tract. LWFS is always fatal. Breed affected: Paint Horses Malignant Hyperthermia Syndrome MHS is a rare, life-threatening circulatory collapse triggered by exposure to certain drugs for general anesthesia. Horses with both MH and Type I PSSM have more severe clinical signs of tying up and are more difficult to manage with diet and exercise. Breeds affected: Stock horse breeds Severe Combined Immunodeficiency SCID results in a complete lack of an immune system. Affected horses die within three months of birth due to opportunistic bacterial, viral or fungal infections. Breed affected: Arabians Junctional Epidermolysis Bullosa JEB causes sloughing of the skin in newborn foals, resulting in serious bacterial diseases and death. Breeds affected: Belgians and Saddlebreds

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