~Equine Genetic Disorders/Diseases Explained~
also known as AQHA's Panel 5+, IMM and MYH1 Myopathy, Lethal White Overo (LWO), etc..
PSSM / EPSM
Equine Polysaccharide Storage Myopathy (abbreviated as EPSM outside the stock horse community) is an inherited autosomal dominant disease resulting in the excess storage of sugar in skeletal muscles, estimated to affect 6-10% of Quarter Horses. It is often the cause of the common form of tying-up in horses. Affected individuals can have such minor symptoms to go unnoticed or multiple episodes of tying up, to severe colic and recumbency. Two types of PSSM have been identified. Yet, a simple DNA test currently only exists for Type 1. Horses with a single dominant gene EXPRESS the disorder (at some point) AND can pass it to their offspring 50% of the time. Horses with two dominant genes will always produce afflicted offspring. NOTE: Horses on forage based, low carb diets (little to no grain) may NOT exhibit symptoms since the conditions are related to sugar storage. For that reason AND the bloodline source(s) is still unknown, this is a "best to test" genetic disease.
HERDA / HC
Hereditary equine regional dermal asthenia (HERDA) or hyperelastosis cutis (HC) is an inherited autosomal recessive connective tissue disorder (skin disease). About 4% of all Quarter Horses are carriers (28% in cutting horses) of HERDA with the genetic link to the popular POCO BUENO, and his full brother OLD GRANDDAD through both sire and dam. Early on, researchers named Dry Doc, Doc O'Lena, Great Pine, and Zippo Pine Bar as carriers after producing at least one afflicted foal. Horses with a single recessive gene do not express the disorder but have a 50% chance of passing it to their offspring. Horses must have two recessive genes to suffer from this disorder. Breeding two carriers allows for a 25% chance of producing an afflicted offspring with two recessive genes.
Glycogen Branching Enzyme Deficiency is an inherited autosomal recessive disorder with approximately 8% of Quarter Horses as carriers. GBED causes late term abortion, stillborn foals,or weak foals with contracted tendons on all four legs. In all cases, a foal is born lacking the ability to store sugar correctly. Live foals can appear healthy for a short time before becoming weak with elevated respiration and eventually seizures and death as skeletal muscle, heart muscle and the brain tissue die. This disease is genetically linked to the Quarter Horse stallion, KING through his sire Zantanon. Horses with a single recessive gene do not express the disorder but have a 50% chance of passing it to their offspring. Horses must have two recessive genes to suffer from this disorder. Breeding two carriers allows for a 25% chance of producing an afflicted offspring with two recessive genes.
Hyperkalemic periodic paralysis is an inherited autosomal dominant muscle disease affecting 1.5% of Quarter Horses (60% of halter horses) that affects the sodium channels in the horse’s muscle and the potassium levels in the blood. This disorder has been traced to the stallion, IMPRESSIVE. The disease causes sporadic attacks of muscle contractions and convulsions. These range from mild episodes of involuntary muscle twitching, to a full body collapse and an inability to breathe. It is often confused with seizures or fainting, yet unlike seizures and fainting horses are conscious during HYPP attacks. Things such as a change in feed/supplements or external factors that cause stress can trigger an HYPP attack. Horses with a single dominant gene EXPRESS the disorder (at some point) AND can pass it to their offspring 50% of the time. Horses with two dominant genes will always produce afflicted offspring.
MH / EMH
Malignant Hyperthermia is an inherited autosomal dominant disease identified in less than 1% of Quarter Horses (and other stock type breeds) that can cause severe typing up and even death when horses are subjected to anesthesia. A gene mutation causes a dysfunction in skeletal muscles resulting in excessive release of calcium inside the muscle cells. This results in a hyper metabolic state and/or death. Symptoms include fever, excessive sweating, elevated heart rate, irregular heart rhythm, shallow breathing, muscle rigidity, and even death. Horses with MH have been linked to two specific bloodlines that have not been publicly announced, as of February 2012. Horses with a single dominant gene EXPRESS the disorder (at some point) AND can pass it to their offspring 50% of the time. Horses with two dominant genes will always produce afflicted offspring.
Lethal White Overo. Horse breeding programs specializing in overo have particular challenges compared with programs for other white patterns such as tobiano. Not only is there the possibility of producing a solid dark foal without the overo pattern but there is also the risk of producing an all-white foal that dies of complications from intestinal tract abnormalities (ileocolonic aganglionosis). As far as we are aware, overo horses themselves have no specific health risks. While breeding evidence shows that some overos are heterozygous for a gene that is lethal in the homozygous condition, it has not been easy to identify which horses have the overo gene that is associated with the lethal white foal syndrome. Occasionally even solid-colored horses without obvious body spotting patterns have been reported to produce lethal white foals. Clearly the spotting pattern classified as overo is phenotypically and genetically heterogeneous. Breeders can test horses for this mutation to avoid producing lethal white foals and to identify new pedigree sources of the overo gene that may be useful in their breeding programs. The gene appears to be associated with horses often characterized as "frame-overos" in Paints and Thoroughbreds, but is also present in some tobiano/overos, some solid-colored (breeding stock Paint) offspring from overo matings, some tobianos and Quarter Horses without obvious evidence of the overo pattern. The gene has also been identified in an overo Miniature Horse.
IMM and MYH1 Myopathy
Immune Mediated Myositis is a genetic disorder in Quarter Horse related breeds: Quarter Horses, Paint, and Pintos. It is an autoimmune disorder. When triggered, the body's immune system attacks mutated proteins in skeletal muscles, resulting in a rapid loss of muscle mass along the topline. The disease has been recognized for nearly 20 years, but not until 2017 was it recognized as a genetic disorder, myosin heavy chain 1 myopathy (MYHN). It is estimated that 7% of all Quarter Horses carry the mutated gene. Reining, cow, and halter horses are thought to be the largest group of carriers with 16%-22% affected. The disease lays dormant until the immune system is compromised. Streptococcus infections are seen as the leading trigger of IMM episodes, but certainly not the only trigger. Veterinarians are able to treat the clinical symptoms and manage episodes. Complete recovery, however, will take several months as the process of regaining muscle mass is a slow one.
Genetic testing is available and should be considered if you intend to breed. Horses will be classified as:
- N/N (no copy of mutation)
- N/My (one copy of mutation, 50% of offspring will be affected)
- My/My (two copies of mutation 100% of offspring will be affected)
Dwayne and Sunday Blue
Gordon, TX 76453
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