MYH1-associated myopathy
Rapid muscle loss or a painful muscle episode without exercise may lead a veterinarian to investigate MYHM. The name covers two clinical presentations associated with a variant in MYH1. The MYHM test looks for a MYH1 variant that increases the risk of these muscle disorders. That increased risk is called genetic susceptibility. Having one or two copies of this variant does not mean the horse will necessarily develop disease. Keeping genetic risk separate from current health is essential when reading a report or making plans for the horse.
Codominant inheritance
The MYHM-associated variant changes a protein used in muscle contraction
Myosin contributes to the contraction of muscle fibres. MYH1 provides instructions for one form of this protein, found in particular muscle fibres. The MYHM-associated variant changes that protein. It is different from the glycogen-storage change in PSSM1 or the calcium-channel change in malignant hyperthermia: these conditions involve different mechanisms and different targeted tests.
The initial study identified a strong association between the variant and susceptibility to immune-mediated myositis. In that presentation, immune cells contribute to muscle damage. The disease nevertheless involves a more complex interaction than a gene that inevitably makes a horse ill. Triggering factors and the reasons some positive horses remain without signs are still being investigated.
MYHM can cause rapid muscle loss or muscle injury without exercise
Immune-mediated myositis, usually shortened to IMM, is muscle inflammation involving cells of the immune system. It causes rapid loss of muscle, particularly along the topline and hindquarters. Weakness, stiffness or reduced appetite can precede the visible wasting. Inflammation found in appropriate muscle samples helps characterise this presentation. A horse losing muscle needs an assessment of the cause, rather than an assumption based on breed or appearance.
Nonexertional rhabdomyolysis involves major muscle-fibre injury without the usual exercise trigger. A horse may be stiff, painful, take short steps, lie down and struggle to rise. Urine can become dark. Visible muscle wasting is not essential in this second form, so absence of atrophy does not mean a muscle episode is mild or harmless.
A horse with one copy of the MYHM-associated variant can remain free of muscle disease
One copy can increase susceptibility. Laboratories describe an autosomal codominant pattern with variable penetrance: one or two copies of the MYH1 variant can have effects, but not every horse with the variant develops disease. Copy number can change the consequences. The practical point is that a positive horse may remain without signs, while two copies can be associated with more severe expression if disease develops. Neither result is a precise personal forecast.
Infections or other immune-related events have preceded some episodes. Many affected horses, however, have no clearly recognised trigger. A positive result cannot automatically assign responsibility to a recent event. Whether disease develops and how severe its signs become are separate questions; a targeted genetic test alone cannot answer both for an individual horse.
The MYHM-associated variant is documented mainly in Quarter Horses and related breeds
The variant is documented mainly in Quarter Horses and related breeds, including Paint Horses and crosses with Quarter Horse ancestry. Studies have examined different groups and performance disciplines. A breed name or sporting discipline does not impose an individual genotype, and the presence of affected relatives does not reveal exactly which copy a particular horse inherited.
Recruitment affects study figures. Owners testing because their horses were ill do not form a random sample of the entire breed. The proportion showing signs in such a study cannot simply become a prediction for every positive horse. Pedigree, individual test results and clinical history remain different kinds of evidence, and population findings need their original sampling context.
How MYHM DNA testing helps investigate the cause of muscle problems
A veterinarian considers the speed of muscle loss, pain, recent activity and other health changes. Blood tests can show muscle injury. Depending on the presentation, biopsy can provide information about inflammation and muscle structure. Genetics helps connect findings to a known susceptibility, but does not describe the horse’s current lesions or their full explanation on its own.
Other myopathies, infections and other causes of muscle loss or weakness also need consideration. The discovery study included horses with myositis that lacked the identified MYH1 variant. A negative result therefore does not exclude all inflammatory muscle disease. Severe pain, dark urine or inability to rise needs urgent veterinary assessment regardless of the genetic result.
Care for a horse showing signs of MYHM
The veterinarian chooses care according to the horse’s signs and complications, such as inflammation, muscle injury or pain. They plan the support needed during recovery and adjust feeding and return to activity to the horse’s condition. A horse whose DNA test identifies the MYH1 variant but which has no signs should not automatically receive treatment intended for a horse in an acute episode.
A horse can make a substantial, sometimes complete, recovery. However, illness can recur, and some horses retain muscle loss. Illness can be severe, and affected horses do not all have the same outcome. Infection prevention and decisions that stimulate the immune system, including vaccination, need to be discussed with the vet for each horse. A MYH1 test result alone is not a reason for an owner to withdraw vaccination or other preventive care.
How a horse can pass the MYHM-associated variant to its foal
The MYHM test counts copies of the tested MYH1 variant. A parent without this variant cannot pass it on. A parent with one copy has a 50% chance of transmitting that copy at each conception. A parent with two copies passes the variant to every offspring. A parent can therefore transmit the variant even without ever having visible muscle problems.
If the mare and stallion each have one copy of the MYH1 variant linked to MYHM, every conception has a 25% chance of a foal without this variant, a 50% chance of a foal with one copy and a 25% chance of a foal with two copies of the same variant. These are not percentages of inevitable disease. Decisions should consider increased susceptibility, potentially greater severity with two copies, both parents’ results and future foal welfare. Testing does not summarise every aspect of the offspring’s future health.
Understanding MYHM DNA test results
Tested MYHM-associated variant not detected
Susceptibility associated with this tested variant is not identified. Other causes of myositis and muscle injury remain possible.
In a MYHM report, N means a copy without the tested variant and My means a copy with it. N/My therefore describes one variant copy and My/My describes two. A result without the variant does not mean the MYH1 gene is missing or exclude every muscle disease.
Questions about the condition: MYH1-associated myopathy
Why was the MYHM test also called the IMM test?
The variant was first linked to immune-mediated myositis. The name MYHM was adopted to include nonexertional rhabdomyolysis too. Check which variant was actually tested instead of comparing panel names alone.
Can a horse with the MYHM-associated variant remain free of muscle disease?
Yes. The result identifies susceptibility, and not every positive horse develops disease. It does not change the copies the horse can pass on or remove the need to assess new signs if they appear.
Can a horse with MYHM regain lost muscle?
Recovery is possible and can be substantial, but depends on the presentation, severity and course. Episodes may recur or leave persistent effects. DNA testing alone cannot establish an individual prognosis.
Scientific sources about the condition: MYH1-associated myopathy
This article brings together the research and the university and laboratory resources listed below. Numbers after each section link to the references used. Sources consulted in November 2026.
- Myosin-Heavy Chain Myopathy (MYHM)
UC Davis Veterinary Genetics Laboratory · Test documentation
- MYH1 myopathy: immune-mediated myositis and nonexertional rhabdomyolysis
UC Davis Center for Equine Health · University resource
- Finno et al. (2018) — MYH1 and susceptibility to immune-mediated myositis
Skeletal Muscle · Research
- Valberg et al. (2022) — Clinical expression of MYH1-associated myopathy
Journal of Veterinary Internal Medicine · Research
- MYHM (IMM): genetic diagnostic testing
Racing Australia — Equine Genetics Research Centre · Test documentation