IMM / MYHM in Reining and Cow Horse Bred Quarter Horses

I have been struggling with unexplained back soreness and lameness in El since I’ve had her (almost 4 years). It was driving me nuts! I had tried saddle fitting, fancy saddle pads, chiropractic, massage, lameness exams, stretches, everything. Although those things did help, nothing got rid of the soreness in her back and haunches, but exercise seemed to help it most.

I recently got a call from the ranch where El was raised and they informed me their stud had recently been tested for IMM / MYHM and they found out he was a carrier. So they suggested I test El for it too, which I did. Turns out, she’s homozygous (double positive) for the IMM muscle mutation. Suddenly, everything made sense.

What is IMM / MYHM?

IMM is an acronym for immune mediated myositis. Meaning when the horse’s immune system is activated in response to an infection (bacterial or viral), the immune system can become “over-active” and attack the horse’s muscles. This can cause severe muscle loss on the horse’s topline and haunches. Interestingly, there is a range of ages that IMM seems to be active, usually horses under the age of 8 and over the age of 17. Due to limited research, it is unclear if this is just the sample set of horses they tested or if there is truly a gap between ages 8 – 17 where the horses aren’t symptomatic.

The Gene

IMM is caused by both genetics and environment. MYH1 is a mutation present in the Myosin Heavy Chain gene which predisposes a horse to IMM. It is also called Myosin Heavy Chain Myopathy (MYHM). It is a co-dominant gene meaning that if a horse has 1 copy (is a carrier- N/My), they do have a chance of having an IMM episode. If a horse has 2 copies of the mutation (double positive- My/My), they have a much greater chance of having an IMM episode in their lifetime.

A paper from a group at Michigan State University estimated that 25% of reining horses, and 20% of cowhorses, and 18% of halter horses have at least one copy of the IMM mutation. However, due to AQHA not including IMM/MHY1 mutations on their standard breed panel test, the actual prevalence may be higher.

Most of the genetic mutations in horses follow the 2 allele rule. Meaning, they get one copy of a gene from the sire, and the other copy from the dam. So genetic panel tests for horses are shown as “copy 1/ copy 2”. If there is an “N” in one or both of those spots, that means that one or both copies/alleles are negative for the mutation. If one or both spots have something else (Hy, My, etc), that means one or both copies are positive for the mutation. So if you’re testing for IMM, “N/N” = both copies do NOT have the mutation (homozygous negative), “N/My” = one copy has the mutation (heterozygous positive), and “My/My” means both copies have the mutation (homozygous positive).

Environmental Factors

Strangles is a major cause of IMM episodes. Even the strangles vaccine, no matter the form, is known to trigger IMM episodes. Other infections and vaccines (especially IM vaccines) can cause IMM episodes. But, it’s also important to vaccinate your horse. Work closely with your vet and plan to give vaccines one at a time (give one, wait a month, give another, etc.). Closely monitor your horse and keep notes on which vaccines cause problems/inflammation. Although I recommend giving most vaccines, it is generally accepted to avoid strangles vaccines in this case. But remember, talk to your vet. If your vet is not familiar with IMM, consider consulting another vet who can give you and your vet advice.

I typically vaccinate El for rabies. To give her the best chance of limited reaction, I give her a 500 lbs dose of banamine the night before the vaccine, the morning of the vaccine, the night of the vaccine, and the morning after the vaccine. All of this is done under vet advisement. There was one year she was given the rhino/flu vaccine which caused her to severely tie up the day after despite banamine. Luckily she didn’t have any muscle wasting but with continued banamine and a watchful eye, her tie up episode resolved after a few days.

An IMM Episode

Having the mutation, either one copy or two, is not usually a death sentence. When managed correctly, horses can have a full life and performance career. However, it’s important to be able to recognize the symptoms of an IMM episode. Common signs of an attack can be one or more of the following symptoms.

  • Rapid muscle loss (40% loss in 48 hours) in the top line and gluteals. (Start to see the spine becoming prominent and the top of the butt looking flat and angled)
  • Stiff gait, not wanting to extend (very broad symptom, may not be IMM episode)
  • Fever of 103

For your vet diagnostics

  • Leukocytosis in blood and/or muscle (white blood cells indicating an infection)
  • Increased creatine kinase and aspartate transaminase (enzymes from muscle damage)

If you see these symptoms or if your horse is laying down and can’t get up, call your vet immediately. Usually corticosteroids and sometimes antibiotics, if the vet suspects an infection as well, can treat and stop the muscle damage. Of course, the quicker you can get it treated, the better. It usually takes 2-3 months for the muscles to recover.

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a Normal muscle mass in an MYH1 E321G homozygote prior to developing IMM. b The same horse 4 months after an episode of IMM. The spine is prominent due to loss of epaxial muscles (arrow). c Atrophy of middle and superficial gluteal muscles (arrow) is present. From: Finno, C. et al. 2018.

When to Suspect IMM (without an IMM episode)

IMM positive horses are prone to non-exertional rhabdomyolysis which causes muscle pain in the topline/loin and in the haunches/gluteals/hamstrings, stiffness, and reluctance to extend. This form of rhabdomyolysis is not considered an IMM episode and muscle wasting is not usually seen. Non-exertional rhabdomyolysis is observed as “tying up” or stiffness when the horse has not been active, like when they’re kept in a stall overnight. Exercise every day is critical for horses with non-exertional rhabdomyolysis, with plenty of extended trotting before any other exercise.


Management of IMM horses has not been researched extensively. However, after talking with some veterinarians, I have found that IMM is often managed similarly to PSSM. Low starch and high protein feeds, limited to no alfalfa (conflicting reports on the safety of alfalfa), and no access to grass (especially fresh spring grass). In severe cases, grass hay is soaked in water to get rid of excess starch.

Where and How to Test

Testing is easy. Just collect 20-30 strands of mane or tail hair, with the roots, in a ziplock bag then send it off. As of December 2022, AQHA has added MYHM1 to their panel. Please double check this if you go through AQHA for testing. You can also find non-AQHA direct IMM test or go through a 6-7 panel. I personally did a 6 panel through . They had a quick turn around and easy sample submission process. You can also go through UC Davis, links are below.

What About El?

We believe El’s soreness is caused by the non-exertional rhabdomyolysis which she’s genetically at risk for due to her IMM status.

Two years post her diagnosis, I have been relatively successful managing El’s NER through diet and exercise.

Her daily feeding includes Triple Crown Low Starch as her main feed supplemented with grass hay and Standlee Teff Pellets (a low sugar hay pellet). When we were in Colorado, the grass hay was good enough quality that she didn’t need hay pellets but the hay here in Connecticut is not great, so we supplement. Make sure to watch your horse carefully during this time and work with local vets to determine the need for hay pellets.

She gets plenty of supplements to help out. Through SmartPak, she gets the “E-Se-Mag” pellets, the “Smart & Simple Vitamin E” pellets, and the “SmartMuscle Recovery” pellets. The research suggests that vitamin E supplementation can help with IMM and NER, especially with horses that are not turned out on fresh grass. These supplements together supply 3,500 IUs of vitamin E. Selenium is needed for proper vitamin E absorption and magnesium has also been noted to help with muscle soreness.

Anecdotally, I have found that the Vitalize Hyaluronex liquid supplement has really helped loosen El’s muscles. I originally tried it for joint protection but after about 60 days on it, I saw a big difference in El’s stretching and collection abilities as well as her muscle soreness. For full disclosure, I am a content creator for Vitalize and I believe in their products, but this product is the product that has helped El the most and honestly, I probably would not have tried this product had they not asked me to. But, I am so glad they did. I don’t want you to feel like I’m pushing product on you, but if you’re curious, their link is below under “Further Reading”.

An important note: some owners with IMM/MYHM positive horses have had good success with alfalfa hay because it provides amino acids to help in muscle regeneration. This has not been my experience. I would love to have El on alfalfa, but it it makes things worse for her for some reason. My advice, try switching on/off alfalfa for 60 days (without any other diet or exercise changes) to determine if it helps or hurts your horse. This seems to be a vary on a case by case basis.

IMM/MYHM Genetics

I have really thought about adding genetic lines on this blog because it would be helpful to everyone. But, I have decided against it due to legal reasons. I know it is a pain, but I would suggest breeding to studs/mares that have their IMM/MYHM1 status disclosed (preferably negative). When in doubt, ask. If the breeder isn’t willing to disclose, maybe look for someone else. If you are curious if your horse is positive and you don’t know if their parents are positive, I would suggest you run a whole panel anyway.

There is a facebook page dedicated to IMM/MYHM owners, they give great advice and may tell you the pedigree of their positive horse. It is called “IMM/MYH1 Forum”. Should you have any more questions, it is a great place to start.

Further Reading and Sources

Vitalize Supplements : Use code HANAH10 (double check spelling) for a discount.

Valberg, Stephanie. 2006. Immune-Mediated Myopathis. AAEP Proceedings. 52:354-358.

Lewis, S. et al. 2008. Suspected Immune-Mediate Myositis in Horses. Journal of Veterinary Internal Medicine. 21: 495-503.

Finno, C. et al. 2018. A missense mutation in MYH1 is associated with susceptibility to immune-mediated myositis in Quarter Horses. Skeletal Muscel 8:7.

Gianino, G. et al. 2019. Prevalence of the E321 MHY1 variant for immune-mediated myositis and nonexertional rhabdomyolysis in performance subgroups of American Quarter Horses. Journal of Veterinary Internal Medicine. 32:897-901.

If you would like any of the scientific articles and cannot access them, please email me or DM me on instagram. I will be happy to send you a pdf.


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