Top Genetic Muscle Conditions in Horses: What to Know and Why Testing Matters

Genetic muscle diseases are a significant concern in the equine world, affecting performance, health, and overall quality of life for horses. Fortunately, with genetic testing, breeders, owners, and trainers can make informed decisions to better manage or even prevent these conditions. Etalon Equine Genetics offers testing for several key muscle disorders, each with unique symptoms, inheritance patterns, and genetic causes. Let’s dive into the basics of these top genetic muscle conditions: Hyperkalemic Periodic Paralysis (HYPP), Myosin-Heavy Chain Myopathy (MYHM), Glycogen Branching Enzyme Deficiency (GBED), Polysaccharide Storage Myopathy Type 1 (PSSM1), Malignant Hyperthermia (MH), and Myotonia (MYT).

Hyperkalemic Periodic Paralysis (HYPP)

What It Is

HYPP is a disorder that affects muscle function, leading to episodes of muscle tremors, weakness, and paralysis. It’s mainly found in Quarter Horses and related stock horse breeds.

Common Symptoms

Affected horses can experience trembling, collapse, and severe muscle weakness. These episodes can be triggered by high-potassium feeds, stress, or excitement.

Gene Mutation

HYPP is linked to a mutation in the SCN4A gene, which affects sodium channels that play a role in muscle contraction.

Genetic Inheritance

HYPP is also autosomal dominant, so even one mutated gene from either parent can result in the disease. Heterozygous horses, which are horses with one copy of the mutation, (HYPP/n) experience more moderate symptoms that can usually be managed through diet and exercise. However, homozygous horses, horses with two copies of the mutation (HYPP/HYPP) are much more severely affected, and some breed registries will not accept these animals.

Myosin-Heavy Chain Myopathy (MYHM)

What It Is

MYHM, previously known as Immune-Mediated Myositis (IMM), is an immune-mediated myopathy where the immune system attacks muscle tissue, causing severe muscle wasting and stiffness.

Common Symptoms

Horses with MYHM may experience sudden and extreme muscle loss, especially around the top line, and can show muscle stiffness and pain.

Gene Mutation

MYHM is linked to a mutation in the MYH1 gene, which is involved in muscle structure and function.

Genetic Inheritance

This condition has a complex inheritance pattern and is influenced by multiple factors, though studies suggest MYHM is autosomal dominant. This means that horses with one copy of the mutation (MYHM/n) may still be at risk, though horses with two copies (MYHM/MYHM) are likely to experience more severe symptoms. Because MYHM is thought to have variable penetrance, not all horses with the mutation will show symptoms.

Glycogen Branching Enzyme Deficiency (GBED)

What It Is

GBED is a fatal condition affecting foals, which cannot store or use glycogen properly due to enzyme deficiency. GBED most commonly occurs in stock breeds such as Quarter Horses and Paint Horses.

Symptoms

Affected foals may exhibit weakness, low energy, and difficulty standing. Most foals with GBED are stillborn or die shortly after birth.

Gene Mutation

The mutation associated with GBED affects the GBE1 gene, which is essential for proper glycogen storage.

Genetic Inheritance

GBED is an autosomal recessive condition, meaning a foal must inherit two copies (GBED/GBED) of the mutated gene to be affected. Carriers (GBED/n) have no known issues.

Polysaccharide Storage Myopathy Type 1 (PSSM1)

What It Is

PSSM1 is a muscle disorder that causes muscles to store excessive glycogen, leading to muscle stiffness, pain, and sometimes reluctance to move. It’s common in stock breeds like Quarter Horses, Paint Horses, and Appaloosas, as well as draft horses and some Warmbloods.

There is a secondary form of PSSM, known as PSSM Type 2 or PSSM2, that presents with similar symptoms but has yet to successfully be linked to a specific gene, therefore a valid commercial test is not currently available.

Common Symptoms

Horses with PSSM1 often display symptoms like sweating, muscle cramping, reluctance to exercise, and “tying up,” especially after rest.

Gene Mutation

This condition is associated with a mutation in the GYS1 gene, which impacts glycogen synthesis in muscles.

Genetic Inheritance

PSSM1 is inherited in an autosomal dominant manner, meaning just one copy of the mutated gene can lead to symptoms. Horses with one copy of the PSSM1 gene (PSSM1/n) tend to have milder symptoms that can often be managed with diet and exercise, whereas horses with two copies (PSSM1/PSSM1) often have more intense symptoms, making them more challenging to manage. Additionally, studies have shown that horses with both PSSM1 and malignant hyperthermia (MH) can experience even more severe symptoms.¹

Malignant Hyperthermia (MH)

What It Is

MH is a life-threatening condition that causes a dangerous increase in body temperature, typically triggered by stress or certain anesthetics.

Common Symptoms

Horses with MH may experience sudden fever, muscle rigidity, and heart issues during anesthesia or stressful events.

Gene Mutation

MH is associated with a mutation in the RYR1 gene, which affects calcium release in muscle cells and can cause extreme reactions under certain conditions.

Genetic Inheritance

MH is inherited in an autosomal dominant pattern, so a single mutated gene (MH/n) can lead to the condition. One study has shown that horses with both MH and Polysaccharide Storage Myopathy Type 1 (PSSM1) have more severe clinical symptoms.¹

Myotonia (MYT)

What It Is

Myotonia is a rare genetic condition that causes prolonged muscle contractions, leading to stiffness and delayed muscle relaxation, especially after movement. This condition can affect a horse’s performance and overall quality of life. Myotonia is thought to be a relatively recent condition found in the New Forest Pony breed.

Common Symptoms

Horses with Myotonia may show stiffness, difficulty moving, and muscle twitching. The muscles often remain tense for longer than normal after use, causing a "frozen" appearance, especially in the legs.

Gene Mutation

This disorder is often linked to a mutation in the CLCN1 gene, which affects chloride channels in muscle cells, interfering with normal muscle relaxation.

Genetic Inheritance

Myotonia is believed to be an autosomal recessive condition, meaning a foal must inherit two copies (MYT/MYT) of the mutated gene to be affected. Carriers (MYT/n) are currently known to exhibit symptoms of the disease.

Conclusion: Why Genetic Testing is Essential

Understanding these genetic muscle diseases allows for better management and informed breeding choices. Genetic testing with Etalon Equine Genetics can help determine if a horse is a carrier or affected by any of these conditions, allowing owners and trainers to create care plans that improve a horse's health and well-being. If you are looking for DNA testing for just the genetic muscle conditions covered in this blog (excluding myotonia), we would recommend choosing our Essentials Package. For the most complete picture of your horse’s genetic health, including all of our available health condition and health risk tests, we recommend choosing our Health Panel. Whether for breeding, training, or overall management, knowing a horse's genetic status for these muscle diseases can make a world of difference in their quality of life and in your peace of mind.

References

¹ McCue, M. E., Valberg, S. J., Jackson, M., Borgia, L., Lucio, M., & Mickelson, J. R. (2009). Polysaccharide storage myopathy phenotype in quarter horse-related breeds is modified by the presence of an RYR1 mutation. Neuromuscular disorders : NMD, 19(1), 37–43. https://doi.org/10.1016/j.nmd.2008.10.001