03/04/2026
Here’s a great simplified breakdown of one of the more commonly tested genetic diseases in our industry, IMM/MYHM
𝐈𝐌𝐌 / 𝐌𝐘𝐇𝐌 𝐢𝐧 𝐇𝐨𝐫𝐬𝐞𝐬 - Understanding the Disease, Clinical Risk, and Long-Term Nutritional Management
Immune-Mediated Myositis, commonly referred to as IMM, is now more accurately described under the broader term MYHM (Myosin Heavy Chain Myopathy). Both conditions are associated with a mutation in the MYH1 gene, which affects a specific fast-twitch muscle protein (type 2X myosin).
While the topic can feel overwhelming in breeding and performance circles, the most important thing to understand is this:
A positive genetic test does not automatically mean a horse will become sick. MYHM represents a genetic susceptibility, not a guaranteed outcome.
𝐖𝐡𝐚𝐭 𝐈𝐬 𝐌𝐘𝐇𝐌?
MYHM is caused by a missense mutation in the MYH1 gene.
A missense mutation is a type of genetic change where one single DNA “letter” is switched, and that change causes the body to insert a different amino acid into a protein.
Think of DNA like a recipe book.
Each three-letter DNA “word” (called a codon) tells the body which amino acid to use.
If one letter changes, the word can change.
When the word changes, the amino acid can change.
When the amino acid changes, the shape or function of the protein may change.
That is a missense mutation.
This mutation alters a contractile protein within skeletal muscle fibers. Under certain circumstances, often involving immune stimulation, the body may inappropriately target muscle tissue, leading to inflammation and muscle damage.
Two clinical syndromes are associated with this mutation:
𝟏. 𝑰𝒎𝒎𝒖𝒏𝒆-𝑴𝒆𝒅𝒊𝒂𝒕𝒆𝒅 𝑴𝒚𝒐𝒔𝒊𝒕𝒊𝒔 (𝑰𝑴𝑴)
Characterized by:
Rapid, often dramatic muscle atrophy
Symmetrical topline and hindquarter muscle loss
Stiffness and weakness
Decreased appetite or dull attitude early in onset
Muscle loss can occur quickly and may appear shocking in severity.
𝟐. 𝑵𝒐𝒏-𝑬𝒙𝒆𝒓𝒕𝒊𝒐𝒏𝒂𝒍 𝑹𝒉𝒂𝒃𝒅𝒐𝒎𝒚𝒐𝒍𝒚𝒔𝒊𝒔
This presentation resembles severe tying-up without exercise trigger:
Extreme muscle pain
Elevated CK and AST
Stiffness or difficulty rising
Dark urine in severe cases
Some horses show atrophy. Others primarily show painful muscle breakdown.
𝐇𝐨𝐰 𝐋𝐢𝐤𝐞𝐥𝐲 𝐈𝐬 𝐚 𝐏𝐨𝐬𝐢𝐭𝐢𝐯𝐞 𝐇𝐨𝐫𝐬𝐞 𝐭𝐨 𝐒𝐡𝐨𝐰 𝐒𝐲𝐦𝐩𝐭𝐨𝐦𝐬?
Genetic test results fall into three categories:
N/N – No increased susceptibility
N/My – Heterozygous; may develop disease
My/My – Homozygous; higher risk and will pass the mutation to all offspring
𝑹𝒆𝒔𝒆𝒂𝒓𝒄𝒉 𝒔𝒖𝒈𝒈𝒆𝒔𝒕𝒔:
Approximately 20% of heterozygous (N/My) horses develop clinical signs.
Approximately 80% of homozygous (My/My) horses develop clinical signs.
However, this still means:
Many heterozygous horses never show symptoms.
Even homozygous horses may live years without incident depending on triggers and management.
This is called variable penetrance, not every horse with the gene expresses the disease.
𝐊𝐧𝐨𝐰𝐧 𝐓𝐫𝐢𝐠𝐠𝐞𝐫𝐬
Clinical episodes are frequently associated with immune stimulation, including:
Recent respiratory infection
Exposure to Strangles
Pigeon fever infection
Recent vaccination
Not every horse experiences a clear trigger, but immune system activation appears to play a key role.
Recurrence is possible. Nearly half of affected horses may experience additional episodes in their lifetime.
𝐍𝐮𝐭𝐫𝐢𝐭𝐢𝐨𝐧𝐚𝐥 𝐏𝐥𝐚𝐧𝐧𝐢𝐧𝐠 𝐟𝐨𝐫 𝐌𝐘𝐇𝐌 𝐇𝐨𝐫𝐬𝐞𝐬
Nutrition cannot change genetics. But it plays a powerful role in recovery and long-term stability.
During Recovery
When muscle atrophy has occurred, rebuilding requires:
High-quality protein with adequate essential amino acids
Balanced vitamin and mineral intake
Consistent caloric intake from forage
The focus should be on rebuilding muscle tissue gradually and safely. Simply increasing grain is not the answer. Instead, improving protein quality and ensuring micronutrient balance is critical.
𝐋𝐨𝐧𝐠-𝐓𝐞𝐫𝐦 𝐌𝐚𝐢𝐧𝐭𝐞𝐧𝐚𝐧𝐜𝐞 𝐒𝐭𝐫𝐚𝐭𝐞𝐠𝐲
For MYHM-positive horses, the goal is stability.
𝑪𝒐𝒓𝒆 𝑷𝒓𝒊𝒏𝒄𝒊𝒑𝒍𝒆𝒔:
Forage First
High-quality hay or pasture should form the foundation of the diet.
Balanced Nutrition
Use a ration balancer or properly fortified feed to prevent micronutrient deficiencies.
Avoid “hay only” programs unless professionally balanced.
Avoid Sudden Feed Changes
- Digestive and metabolic stress can compound immune stress.
- Maintain Ideal Body Condition
- Horses too thin may struggle with muscle maintenance.
- Excess fat does not protect against muscle episodes.
- Match Calories to Workload
- Avoid feast-and-famine cycles.
Nutrition works best when paired with thoughtful management and veterinary planning.
𝐓𝐡𝐞 𝐁𝐫𝐞𝐞𝐝𝐢𝐧𝐠 𝐂𝐨𝐧𝐯𝐞𝐫𝐬𝐚𝐭𝐢𝐨𝐧 𝐚𝐧𝐝 𝐇𝐢𝐬𝐭𝐨𝐫𝐢𝐜𝐚𝐥 𝐏𝐞𝐫𝐬𝐩𝐞𝐜𝐭𝐢𝐯𝐞
It is important to recognize that the MYH1 test did not exist for decades while many influential bloodlines were being developed.
One of the most discussed examples is Wimpys Little Step.
𝑾𝒊𝒎𝒑𝒚𝒔 𝑳𝒊𝒕𝒕𝒍𝒆 𝑺𝒕𝒆𝒑:
Had a highly successful show career
Won the NRHA Futurity
Became one of the most influential sires in modern reining history
He is homozygous for the IMM mutation
He competed at the highest levels and reshaped modern reining programs long before genetic testing was widely available.
𝑻𝒉𝒊𝒔 𝒊𝒍𝒍𝒖𝒔𝒕𝒓𝒂𝒕𝒆𝒔 𝒂𝒏 𝒊𝒎𝒑𝒐𝒓𝒕𝒂𝒏𝒕 𝒕𝒓𝒖𝒕𝒉:
Genetics must be understood in context.
The presence of a mutation does not automatically eliminate performance or breeding potential. However, modern testing now allows breeders to make informed mating decisions that avoid producing homozygous offspring if desired.
The goal is not panic.
The goal is informed management.
𝐂𝐨𝐧𝐬𝐢𝐝𝐞𝐫𝐢𝐧𝐠 𝐭𝐡𝐞 𝐏𝐮𝐫𝐜𝐡𝐚𝐬𝐞 𝐨𝐟 𝐚𝐧 𝐈𝐌𝐌-𝐏𝐨𝐬𝐢𝐭𝐢𝐯𝐞 𝐇𝐨𝐫𝐬𝐞
When evaluating the purchase of a horse that is N/My or My/My for IMM/MYHM, it is important to think beyond the test result and consider your long-term goals. Are you purchasing this horse as your forever heart horse, or are you investing in one with future resale or breeding value?
Your intended market matters. Experienced owners, trainers, and breeders who routinely manage elite-level performance horses are often well versed in the maintenance, monitoring, and veterinary coordination required for horses carrying genetic mutations like IMM. In those programs, carrier status is frequently managed proactively and may not present a significant obstacle.
On the other hand, smaller owner-breeders or participants operating at a more regional or recreational level may have different comfort levels, budget considerations, or resale markets that are more sensitive to genetic status. Understanding your resources, your support team, and your intended market will help ensure that your purchase aligns with both your expectations and your long-term goals.
𝐈𝐧 𝐜𝐥𝐨𝐬𝐢𝐧𝐠....
MYHM/IMM is: A genetic susceptibility linked to the MYH1 mutation
Associated with immune-mediated muscle damage
Variable in expression
Influenced by environmental and immune triggers
Manageable with thoughtful veterinary and nutritional planning
With education, proactive management, and responsible breeding strategies, a majority of MYHM-positive horses live productive, successful lives.
Knowledge creates confidence.
And confidence allows the industry to move forward responsibly.
𝐀 𝐅𝐢𝐧𝐚𝐥 𝐍𝐨𝐭𝐞
While education empowers owners and breeders to make informed decisions, it is important to remember that we are not veterinarians. This article is intended for informational purposes only and should not replace professional veterinary advice, diagnosis, or treatment. Every horse is an individual, and management decisions, including vaccination strategy, nutritional planning, and breeding considerations... should always be discussed directly with your licensed veterinarian.
If you have concerns about IMM/MYHM, genetic testing results, or muscle health in your horse, consult your veterinarian to develop a plan tailored specifically to your horse’s needs.
𝑺𝒄𝒉𝒐𝒍𝒂𝒓𝒍𝒚 𝑹𝒆𝒇𝒆𝒓𝒆𝒏𝒄𝒆𝒔
Finno CJ, Gianino G, Perumbakkam S, et al. A missense mutation in MYH1 is associated with susceptibility to immune-mediated myositis in Quarter Horses. Skeletal Muscle. 2018.
Valberg SJ, Henry ML, Perumbakkam S, Gardner KL, Finno CJ. An E321G MYH1 mutation is associated with nonexertional rhabdomyolysis and immune-mediated myositis in Quarter Horses. Journal of Veterinary Internal Medicine. 2018.
Gianino GM, Valberg SJ, Perumbakkam S, et al. Prevalence of the E321G MYH1 variant in performance subgroups of American Quarter Horses. Journal of Veterinary Internal Medicine. 2019.
Durward-Akhurst SA, Finno CJ, et al. Myosin Heavy Chain Myopathy and immune-mediated muscle disease in horses. Veterinary Clinics of North America: Equine Practice. 2025.
Ochala J, Finno CJ, Valberg SJ. Functional effects of the E321G MYH1 mutation in equine muscle. Cells. 2021.
