Polysaccharide Storage Myopathy is an autosomal dominant disease caused by a mutation in a gene. About 11% of Quarter Horses are affected, as well as other breeds that have utilized Quarter Horse bloodlines. It is also found in 20 other breeds unrelated to Quarter Horses.
PSSM is a genetically caused form of “tying up”. The mutation in the gene causes an unregulated flow of glycogen, which results in excessive sugar in muscle cells which leads to muscle stiffness, pain, sweating, weakness, and a reluctance to move. It is very painful for the affected horse. Occasional changes in the horse’s gait, mild colic and muscle wasting may also occur.
Most horses that have tested positive, however, have had no history of symptoms usually associated with PSSM. Not all cases of tying up are PSSM related either.
Hereditary Equine Regional Dermal Asthenia is a genetic skin disease found in the Quarter Horse, as well as in breeds such as the Paint and Appaloosa that have utilized Quarter Horse breeding. It is most prevalent in particular lines of cutting or cow bred horses and it is thought to come from the lines of King P-234 and/or his sire Zantanon through the stallion Poco Bueno. It takes two carriers of the gene to produce a HERDA offspring.
HERDA is characterized by hyper-extensible skin, scarring and severe lesions, usually along the topline of affected horses. Approximately 3.5% of Quarter Horses are carriers of the gene.
Collagen, the connective tissue in the body that makes up bones, cartilage, muscles, skin and tissue, is defective in HERDA horses. The outer layer of skin separates from the underlying layers, and in some cases, large areas of skin slough off completely and leaves raw wounds.
Affected horses rarely show any symptoms at birth. Young horses with HERDA may appear to have an excessive number of minor wounds on their skin due to the slow healing, if any, that occurs when the skin is damaged. The condition usually becomes most evident by the age of two, generally when the horse is started under saddle.
The skin of those horses is very stretchy and will pucker and roll when pulled away from the horse. Naturally, the skin is fragile and any abrasion or pressure will cause a wound to open, which is slow to heal, if it ever does. If healed, the scarring will be extensive.
There is no cure, and since is it highly inheritable, those horses are not suitable for use as a breeding animal. Euthanasia is generally recommended.
HERDA has an autosomal recessive mode of inheritance and is found in both genders equally. Breeding a carrier horse will produces a 25% chance of a horse with the disease. Breedings between normal and carrier horses won’t produce a HERDA foal, but 50% of the offspring will be carriers.
Glycogen Branching Enzyme Deficiency is an autosomal recessive diseased caused by mutation in the GBE1 gene. In the AQHA, about 8-10% of the horses are affected and are carriers. In turn, breeds that have utilized Quarter Horse bloodlines are affect, so it is also found in Paints and Appaloosas. Breeding carriers can result in 25% of the resulting foals having GBED. It is not sex linked, so either gender can have it.
The mutation of the GBE1 gene inhibits the function of the glycogen branching enzyme so that skeletal and cardiac muscle, as well as the liver and brain, cannot store and utilize glycogen, which provides energy to the muscles. The inability to store and use glycogen leads to muscle weakness and eventually death.
It’s estimated that 3% or more of second and third term abortions are caused by GBED. It also causes stillborn, fully developed foals. Foals that survive birth usually die, with few foals surviving to four months of age. Most affected foals are euthanized. It is always fatal.
Affected live born foals will exhibit weakness and a low body temperature at birth. It will have a high respiratory rate and a weakness of the muscles used to breath. The foal will often be too weak to stand on it’s own and often will have very contracted tendons in all four legs. Pasture foals will die from sudden stopping of the heart or from seizures. GBED has undoubtedly been around for many years, but because the clinical signs can also be signs of other diseases, it wasn’t recognized until recently.
Hyperkalemic Periodic Paralysis is an autosomal dominant gene. It affects about 1.5% of Quarter Horses and about 56% of all halter bred horses. It is also found in Paint and Appaloosas due to the use of popular Quarter Horse bloodlines in those breeds.
HYPP is caused by a mutation in the sodium channel gene which causes a dysfunction in a specific sodium ion channel. These channels contribute to generating electrical impulses that are necessary for muscle contraction. The mutation disrupts these impulses and causes muscle tremors, and paralysis in affected horses. HYPP horses can experience sudden collapse and even death.
The disease traces it’s origins back to the very popular stallion Impressive. It was found in his offspring of both genders, and does not require two copies of the defective gene to produce a positive horse. Due to line breeding to the horses of that bloodline, the incidence and severity of the disease has increased with each successive generation. A horse can be a carrier without ever showing any clinical signs of the disease.
Some success has been attained by managing the affected horse’s diet, though stress and other factors can still cause a very debilitating and painful episode in the horse. Some affected horses have even died at horse shows while in the ring.
Testing for HYPP has been mandatory in any horse with Impressive in the pedigree for many years, though there has been no restriction on the breeding of positive or carrier horses.
Malignant hyperthermia is a genetic disorder that is believed to be an autosomal dominant trait. It is found only in Quarter Horses and in related breeds, such a Paints and Appaloosas. Less than 1% of Quarter Horses are affected and those seem to trace to two specific bloodlines.
MH usually affects mature horses of either gender. It is usually triggered by the use of anesthetic halothane, the muscle relaxant succinylcholine, and stress. Clinical signs include excessive sweating, rapid heart rate, abnormal heart rhythm, shallow breathing, hypertensions, rigid muscles, breakdown of muscle tissue, muscle protein in the urine, and even death. It can also occur in conjunction with an episode of “tying up”, usually Type 1 polysaccharide storage myopathy (PSSM).
Management of horses with MH generally requires testing for PSSM and if a dual-diagnosis is made, the horse is fed to minimize the PSSM symptoms with a diet low in starch and fermentable fiber and fat.
Das Overo Lethal White Foal Syndrom (OLWS) oder auch Lethal White Foal Syndrome (LWFS) ist eine Erbkrankheit, die aus der Verpaarung von Frame-Overo-Schecken entstehen kann. Fohlen mit einem OLWS-Doppelgen können keine Nahrung verdauen und sterben deshalb innerhalb der ersten Tage nach der Geburt. OLWS-Einzelgenträger sind zwar gesund, bei einer Verpaarung besteht aber die Gefahr, ein erkranktes Fohlen zu erhalten – ein Gentest im Vorfeld ist deshalb unbedingt notwendig.