VGL Clients Help Advance Our Understanding of MYHM in Quarter Horses!

A recent study sheds new light on the prevalence and penetrance of the missense mutation in the type 2X myosin heavy chain (MYH1) gene that is associated with myosin-heavy chain myopathy (MYHM) in Quarter Horses. The UC Davis Veterinary Genetics Laboratory offers a genetic test for MYHM and reports the presence of this mutation as My and the absence of this mutation as N.

MYHM is a muscle disease that affects American Quarter Horses and related breeds. Two distinct clinical presentations exist for the disease; both involve muscle loss or damage: immune-mediated myositis (IMM) and non-exertional rhabdomyolysis. While the two presentations are linked to the same genetic variant, not all horses with this variant will display signs of muscle disease. Thus, MYHM is said to be a dominant disease (only one copy of the variant is needed to develop disease) with variable penetrance (not every horse with the variant will be affected).  More information is needed to better understand what proportion of horses with the variant will develop the disease and to identify the specific environmental triggers that contribute to disease onset.

Led by Dr. Stephanie Valberg, Mary Anne McPhail Dressage Chair in Equine Sports Medicine from Michigan State University, and Dr. Carrie Finno, Gregory L. Ferraro Endowed Director of the UC Davis Center for Equine Health (CEH), in collaboration with Dr. Rebecca Bellone and Shayne Hughes from the UC Davis Veterinary Genetics Laboratory (VGL), the study consisted of a retrospective survey distributed to VGL clients, and the collected data allowed researchers to determine the prevalence of muscle atrophy and stiffness in American Quarter Horses tested for MYHM.

The prevalence of the MYH1 mutation in the study population was 29%, with homozygous horses being relatively rare in the population (3% of the study population were My/My). Muscle atrophy that was less likely to resolve occurred in 80% of homozygous horses whereas approximately 20% of the heterozygous horses experienced rapid muscle atrophy. Importantly, the study also showed that factors like vaccination or infectious diseases were not apparent in 75% of homozygous horses and 54% of heterozygous horses known to have developed atrophy or stiffness, indicating more work is needed to better understand additional environmental triggers. Dr. Valberg noted that 

“The tremendous participation of VGL clients allowed us to answer several of their most frequently asked questions: ‘What is the chance that my horse will develop atrophy if it tested My/My or My/N?’ We can now say that 80% of My/My horses develop muscle atrophy that is more recurrent and less likely to resolve than My/N horses. The good news we found was that only 20% of the My/N horses in the survey developed atrophy or severe muscle stiffness. From my perspective, this means I don’t recommend breeding heterozygous horses to each other to avoid the heartache of producing a homozygote. Another important question we have often been asked is ‘how do I manage a horse with My/My or My/N?’ Although we didn’t identify all the triggers, we found that in some horses, vaccination can be a trigger, especially influenza and equine herpes virus and strangles vaccines. We can now recommend that if you have a horse that develops atrophy after vaccination or easily gets muscle swelling after vaccination, spread out the vaccines in as small a grouping as possible within one syringe and try to identify which vaccine is causing the reaction.  In the future try a different brand of vaccine or, if it isn’t essential for your area, avoid that one. I really appreciate the fact that enough owners rapidly answered the survey that we could answer their important questions!”

Client participation and consent are essential for advancing research and understanding the connections between genetic findings and clinical manifestations. The VGL would like to thank our all of our clients who participated in this study.

Read the full publication for more information about the study.

Visit our website for more information about genetic testing for MYHM.