Hyperkalemic periodic paralysis (HYPP), which is caused by an autosomal dominant gene linked to the stallion Impressive. It is characterized by uncontrollable muscle twitching and substantial muscle weakness or paralysis among affected horses. Because it is a dominant gene, only one parent has to have the gene for it to be transmitted to offspring. There is a DNA test for HYPP, the AQHA requires testing and is now limiting registration of some horses who possess the gene.

Hereditary Equine Regional Dermal Asthenia (HERDA), also known as hyperelastosis cutis (HC). This is caused by an autosomal recessive gene, and thus, unlike HYPP, HERDA can only be transmitted if both parents carry the gene. When a horse has this disease, there is a collagen defect that results in the layers of skin not being held firmly together. Thus, when the horse is ridden under saddle or suffers trauma to the skin, the outer layer often splits or separates from the deeper layer, or it can tear off completely. It rarely heals without disfiguring scars. Sunburn can also be a concern. In dramatic cases, the skin can split along the back and even roll down the sides, with the horse literally being skinned alive. Most horses with HERDA are euthanized for humane reasons between the age of two and four years. The very hotly debated and controversial theory, put forth by researchers at Cornell University and Mississippi State University is that the sire line of the great foundation stallion Poco Bueno is implicated as the origin of the disease. As of May 9, 2007, Researchers working independently at Cornell University and at the University of California, Davis announced that a DNA test for HERDA has been developed. Over 1,500 horses were tested during the development phase of the test, which is now available to the general public through both institutions.

Glycogen Branching Enzyme Deficiency (GBED) is a genetic disease where the horse is lacking an enzyme necessary for storing glycogen, the horse's heart muscle and skeletal muscles cannot function, leading to rapid death. The disease occurs in foals who are homozygous for the lethal GBED allele, meaning both parents carry one copy of the gene. There is a DNA blood test for this gene.

Lethal White Syndrome. Although "crop-out" Quarter Horses with Paint markings were not allowed to be registered for many years, the gene for such markings is a recessive and continued to periodically appear in Quarter Horse foals. Thus, it is believed that some Quarter Horses may also carry the gene for Lethal White Syndrome. There is a DNA test for this condition.

http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/22206.htm

http://en.wikipedia.org/wiki/American_Quarter_Horse
Colitis - X: This peracute, fatal disease of horses is characterized by sudden onset of profuse, watery diarrhea and development of hypovolemic shock. Many affected horses have a history of stress. The cause of colitis-X is unknown, although multiple causes have been proposed, including peracute salmonellosis, clostridial enterocolitis, and endotoxemia.
Clinically, there may be a short febrile period, but body temperature soon returns to normal or subnormal. Tachypnea, tachycardia, and marked depression are present. An explosive diarrhea develops, followed by extreme dehydration. Hypovolemic and endotoxic shock are manifest by poor capillary refill time, purplish mucous membranes, and cold extremities. Death may occur within 3 hr of onset of clinical signs. In less acute cases, death occurs within 24-48 hr. The mortality rate approaches 100%. At necropsy, edema and hemorrhage in the wall of the large colon and cecum are pronounced, and the intestinal contents are fluid and often blood-stained.
Typically, the PCV is >65% even shortly after the onset of clinical signs. The leukogram ranges from normal to neutropenia with a degenerative left shift. Metabolic acidosis and electrolyte disorders are also present.
Disease onset is often closely associated with stress, eg, surgery or transport. Signs are similar to those of other diarrheal diseases, including peracute salmonellosis, toxemia caused by Clostridium spp , Potomac horse fever, experimental endotoxic shock, and anaphylaxis. A similar condition may be seen after administration of tetracycline or lincomycin to horses. Colitis-X is the term reserved for those cases in which no definitive diagnosis can be made and the horse dies.
Treatment for colitis-X usually is not effective but would be similar to that for salmonellosis (see Salmonellosis: Introduction). Large volumes of IV fluids are needed to counter the severe dehydration, and electrolyte replacement is often necessary. Flunixin meglumine may help block the effects of toxemia.

 

 

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