IMM 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 2 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 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?

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 small 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.

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. Since AQHA does not require an IMM test, an AQHA 5 panel will not be helpful in determining the IMM status of your horse. You will need to find a 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. We are working hard to get it managed through exercise and diet.

I will be posting about my personal experiences with IMM and El in future posts. This will contain her symptoms, our management, my opinions on genetic lines, and more. Stay tuned!

Further Reading and Sources

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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