Immune-mediated myositis


Good news!   

Your horse is clear of immune-mediated myositis and does not have this gene mutation.

Your horse cannot pass on this gene mutation. There are no breeding or performance implications. 

What is immune-mediated myositis?

A horse with immune-mediated myositis has an immune system unable to sustain a specific protein in the body, which causes them to experience rapid, diffused muscle atrophy, stiffness, and lethargy.  

Immune-mediated myositis episodes are usually triggered by environmental stimuli such as certain infections or vaccinations. Sadly, the syndrome has no cure. When an affected horse survives an episode there is a chance of relapsing in the future.  

Genomic Details

Immune-mediated myositis is associated with the MYH1 gene. This gene can cause the loss of muscle mass. The fast twitch muscle fibers that are used for sprinting are particularly vulnerable to the influence of this gene.

Immune-mediated myositis is a dominant genetic disease. This means only one gene mutation from one parent is needed for the disease to occur. However, it is an unusual form of dominant genetic disease:

Not all horses carrying the dominant disease gene will get immune-mediated myositis.

Just having the disease gene is not enough of a cause in some horses: there also needs to be a second trigger, such as a virus, infection, or vaccine, which interacts with the gene to cause the disease.

Having two MYH1 disease genes often makes immune-mediated myositis more severe.

The mutation that causes immune-mediated myositis is a switch in one of the four base pairs that make up a DNA molecule.  Adenine (A) is replaced by Guanine (G) changing the MYH1 gene. Therefore, a horse’s genetic result for immune-mediated myositis is one of three possibilities:

  • A/A: Normal. Does not have the disease. Does not have the mutation.

  • A/G: Affected. Has one disease mutation from one parent. This is enough to cause the disease. It might be passed to their foal.

  • G/G: Affected. Has two disease gene mutations, one from each parent. One mutation will be passed to their foal.