Genetic Health Testing for Stock Horses

Stock horse breeds, which is a group of North American-bred horses that includes Quarter and Paint horses, are some of the most popular in the United States. Despite their popularity and general athleticism, they are also known to be prone to certain genetic health conditions. Some of the health conditions you may want to consider DNA testing your stock horse for, whether you intend to breed them or not, include:

Hyperkalemic Periodic Paralysis (HYPP)

Hyperkalemic Periodic Paralysis (HYPP) is a hereditary muscle disorder prevalent in descendants of the Quarter Horse stallion, Impressive. It is a dominant genetic disorder resulting from defects in sodium channels in the muscle. Affected animals experience episodes of muscular weakness or tremors, which can lead to paralysis, collapse, and even sudden death. Heterozygous horses (HYPP/n) experience more moderate symptoms that can usually be managed through diet and exercise. However, homozygous horses (HYPP/HYPP) are much more severely affected, and some breed registries will not accept these animals.

Glycogen Branching Enzyme Deficiency (GBED)

Glycogen branching enzyme deficiency (GBED) is a lethal recessive disorder that affects the horse's ability to store and utilize glycogen, a vital source of energy. Foals born with GBED typically exhibit signs of muscle weakness, seizures, respiratory failure, and may suffer from sudden collapse or death. Many affected foals do not make it full term and are aborted or stillborn. Carriers (gbed/n) have no known issues. Studies have indicated that between 4-11% of registered American Quarter Horses and American Paint Horses carry the GBED mutation.

Hereditary Equine Regional Dermal Asthenia (HERDA)

Hereditary equine regional dermal asthenia (HERDA) is a degenerative skin disease that primarily affects the American Quarter Horse breed. Loose skin is often an early indication of the disease, and severe seromas, hematomas, and ulcerations usually develop around 1.5 years of age and progressively worsen. Horses affected by HERDA often face significant challenges in performing basic activities and are prone to skin injuries. There is no cure, and the majority of affected animals have to be euthanized within 2-4 years. This disease follows an autosomal recessive mode of inheritance, so for a foal to be affected, both the sire and the dam must carry the allele. This also means that horses can appear normal but be carriers for the condition. If two carrier horses mate, there is a 25% chance that the foal will have HERDA. Studies estimate that ~3.5% of Quarter Horses are carriers. It's recommended that both carriers and clinically affected horses with HERDA be removed from breeding programs.

Malignant Hyperthermia (MH)

Malignant hyperthermia (MH) is a muscle disorder in which anesthesia, stress, or extreme exercise can trigger a hyperthermic state. Symptoms include high temperature, increased heart rate, high blood pressure, sweating, acidosis, and muscle rigidity. If symptoms are not immediately resolved, death is likely to occur. As this condition is autosomal dominant, even horses with a single allele (MH/n) exhibit symptoms. One study has shown that horses with at least one copy of both MH and polysaccharide storage myopathy (PSSM) variants have more severe clinical symptoms.

Polysaccharide Storage Myopathy Type 1 (PSSM1)

Polysaccharide Storage Myopathy (PSSM) is a muscle disorder characterized by abnormal glycogen storage in muscle cells, leading to episodes of muscle stiffness, pain, and exercise intolerance, as well as chronic episodes of exertional rhabdomyolysis (“tying-up”). Acute symptoms are an unwillingness to move and muscle damage following exercise. Horses with PSSM1 often struggle with performing strenuous activities and may require specialized management strategies to alleviate symptoms. Heterozygous horses (PSSM1/n) experience more moderate symptoms that can usually be managed through a low-sugar/starch diet and exercise. However, homozygous horses (PSSM1/PSSM1) are generally more severely affected and are thus harder to manage. Additionally, one study has found that horses with PSSM1 and malignant hyperthermia (MH) exhibit more severe clinical symptoms.

Myosin-Heavy Chain Myopathy (MYHM)

Myosin-Heavy Chain Myopathy (MYHM) is a genetic muscle disease found most commonly in stock-type horses that can result in two different presentations, Immune-Mediated Myositis (IMM) and Nonexertional Rhabdomyolysis “tying up,” which are both characterized by muscle damage or loss. Both presentations are associated with the same genetic variant. Horses with an MYHM variant may exhibit one or both presentations at different times in their lives, although some horses carrying the variant might not display any symptoms at all.

Lethal White Overo (LWO)

Lethal White Overo (LWO), also known as Overo Lethal White Syndrome (OLWS), is a gene variant that produces a white spotting pattern. LWO is a genetic condition primarily found in Paint Horses with overo coloring patterns, as the name implies, however, it has been found in related breeds such as the Quarter Horse. It is an example of incomplete dominance, as horses that inherit one copy have a different phenotype than horses with two copies. Heterozygous horses (LWO/n) tend to have patches of white bordered (“framed”) by normal pigmentation, usually with blue eyes. In the typical expression, white patterning generally does not cross the topline. Homozygous horses (LWO/LWO) are born completely white and with megacolon, which leads to severe digestive abnormalities and necessitates immediate euthanasia on humane grounds. Some heterozygous horses do not have an obvious white spotting pattern but still can produce lethal white foals. Crossing two LWO/n horses has a 25% chance of producing LWO/LWO. Thus, it’s recommended to test even solid horses before breeding if there is any chance they could carry lethal white overo.

All of these tests are included with our 7-Panel test, which you can read more about here. But if you are looking to really know your horse down to their DNA using the full power of Etalon’s genetic health testing capabilities, we suggest going a step further with our Health Panel test. This comprehensive DNA test evaluates your horse for upwards of 35 health trait, risk, and condition genes to get the most complete picture of your horse’s health from their unique genetic blueprint.