IMM in Horses: What It Is and How to Manage in Your Horse

Immune-mediated myositis (IMM) in horses is a genetic disorder that causes severe muscle atrophy (wasting). It’s important to understand how this condition affects horses and to work with a veterinarian if you suspect your horse may have the condition.

What Is IMM in Horses?

Immune-mediated myositis (IMM) is an autoimmune muscle disorder in horses that causes rapid onset of muscle atrophy along the topline and hindquarters of the horse.

IMM is one of two disorders classified under Myosin-Heavy Chain Myopathy (MHCM), which is caused by a mutation in the MYH1 gene in Quarter Horses or Quarter Horse-related breeds such as the Thoroughbred.

The prevalence of the MYH1 gene mutation in Quarter Horses is currently thought to impact about 7% of the horse population, but it is higher in bloodlines created for certain Western disciplines, such as:

• Reining (24%)

• Working cow (17%)

• Halter (16%)

Currently, IMM is not considered a common disease, because not every horse with the mutation will show any clinical signs during their lifetime.

While muscle atrophy is not a medical emergency, if you suspect muscle atrophy in your horse, it is recommended to seek veterinary care for an evaluation as soon as possible.

Symptoms of IMM in Horses

Horses with IMM will experience rapid muscle atrophy along the topline and hindquarters. Within a week, the atrophy of the muscle may progress to 50% of the horse's muscle mass. This can lead to generalized weakness.

A horse with atrophy may appear to be bony or almost like they have carved-out areas in their neck, spine, or buttocks. 

Causes of IMM in Horses

Horses that may develop IMM must have at least one copy of the MYH1 mutation genetically passed on to them by one of their parents.

The MHY1 mutation can cause the development of two different diseases in horses—IMM or non-exertional rhabdomyolysis. Research about IMM in horses is still being collected today to better understand this disease, as many horses with the mutation may never show any symptoms or develop either disease.

Specific triggers that may lead to IMM are still being researched today, as they are poorly understood. However, some triggers may include:

• Intramuscular injections

• Strangles vaccination

• Intramuscular vaccines given together

How Veterinarians Diagnose IMM in Horses

Your veterinarian will perform a physical exam, blood work, and likely genetic testing if they suspect an MYH1 mutation in your horse.

MYH1 mutations can be diagnosed by examining a simple genetic test from a hair sample.

A muscle biopsy may be used in some cases, but this is less common since the genetic test became available. A horse is sedated for the biopsy and a sample of muscle is taken from the horse.

Biopsies may be performed with a large needle or biopsy tool to be examined for defects or abnormalities at a lab. The area may need to be sutured up and have time to heal afterward.

How Do You Treat IMM in Horses?

There is no direct treatment for IMM in horses.

Muscle atrophy in horses with IMM generally develops gradually and the muscles recover within two to three months (with or without treatment).

Some horses may receive steroid (corticosteroid) treatment. It may be recommended that horses with severe atrophy who compete receive steroid treatment in hopes of improving recovery time. 

Recovery and Management of IMM in Horses

Horses with IMM can completely recover with time and generally have no lifelong issues.

Horses may be more susceptible to recurrence of atrophy once they have had one episode, or if they are homozygous (have both copies of the gene mutation) from their parents.

There are no dietary changes that can help with IMM, but your veterinarian may recommend a muscle health supplement.

Horses with IMM should be given intramuscular injections with caution, since the injection can cause an IMM flare-up in the muscles where it is given. If your horse is given seasonal intramuscular vaccines, your veterinarian may opt to administer them four to six weeks apart to reduce the risk of an IMM episode.