MYHM: From Generations Past to Present Concerns, Deciphering the Enigma of Equine Myosin-Heavy Chain Myopathy, an article published on StallionCompare.

In the world of equine health, understanding genetic factors and their implications is crucial for responsible breeding practices and proactive veterinary care. One such genetic risk factor gaining attention is Myosin-Heavy Chain Myopathy (MYHM), an autoimmune disease that can manifest as Immune-Mediated Myositis (IMM) or Nonexertional Rhabdomyolysis. This article delves into the intricate details of MYHM, exploring its clinical presentations, genetic underpinnings, symptoms, and the importance of knowing your horse's risk factor.

MYHM is an autoimmune disease linked to a specific genetic variant known as MYH1. This variant predisposes horses to two distinct clinical presentations – IMM and Nonexertional Rhabdomyolysis. IMM is characterized by the presence of inflammatory cells, specifically lymphocytes, and muscle atrophy. On the other hand, Nonexertional Rhabdomyolysis shares similar symptoms but lacks the pronounced inflammatory component.

Genetic Variant (MYH1)

The MYH1 gene serves as the common denominator for both IMM and Nonexertional Rhabdomyolysis. This genetic variant plays a pivotal role in the susceptibility of horses to MYHM. It is crucial for horse owners and breeders to comprehend the genetic risk associated with MYH1 to make informed decisions about breeding and horse care.

Clinical Presentation and Symptoms:

When triggered by factors such as infections or certain vaccinations (e.g., influenza), MYHM can lead to rapid and severe muscle loss, particularly in the gluteal and vertebral regions. The inflammation targets muscle fibers and surrounding blood vessels, resulting in atrophy within a remarkably short period – up to 40% in just 72 hours. Horses affected by MYHM may exhibit symptoms like general malaise, muscle stiffness, and weakness unrelated to exercise.

Age-Related Susceptibility:

MYHM does not discriminate solely based on age; however, there is a noted trend of its prevalence in horses older than 17 or younger than 8 years of age. Understanding this age-related susceptibility can aid in early detection and proactive management of the disease.

Genetic Risk Factors and Proactive Measures:

It is imperative to emphasize that the presence of MYHM as a genetic risk factor does not guarantee the manifestation of the disease in the horse. Horses can be classified into three categories based on their MYHM genetic status: homozygous for MYHM (MY/MY) with increased risk, heterozygous (n/MY) with moderate risk, and those with no variant detected (n/n).

Knowing your horse's risk factor becomes instrumental in proactive equine health management. By collaborating with veterinarians, horse owners can develop tailored strategies to mitigate the risk of MYHM and other potential health concerns. This proactive approach is not only applicable to MYHM but also extends to other conditions, such as West Nile Virus Symptom Susceptibility and Chronic Idiopathic Anhidrosis.

While genetic testing has become an invaluable tool in understanding and managing equine health, questions linger about the potential for horses to develop Myosin-Heavy Chain Myopathy (MYHM) without it being evident on their genetic testing reports. Current research suggests that, in some cases, environmental factors and triggers such as infections or specific vaccinations may play a role in the manifestation of MYHM, even in horses with no detected genetic variant. This intriguing aspect challenges our conventional understanding, urging further exploration into the intricate interplay between genetics and environmental influences. As the equine community continues to unravel the complexities of MYHM, it is imperative to remain vigilant and open to evolving insights that could reshape our approach to diagnosis, prevention, and proactive management of this enigmatic condition.

21%

of the 37 reining stallions tested heterozygous (n/MY) for the mutation, one of which was homozygous (MY/MY)¹

17%

of the 41 reined cow horse stallions tested heterozygous (n/MY), none of those tested were homozygous (MY/MY)¹

16%

of the 50 halter stallions were heterozygous (n/MY), none were homozygous¹

Want to learn more about MYHM/IMM? Reach out to industry experts, Etalon Equine Genetics.

References

Finno, C.J., Gianino, G., Perumbakkam, S. et al. A missense mutation in MYH1 is associated with susceptibility to immune-mediated myositis in Quarter Horses. Skeletal Muscle 8, 7 (2018). https://doi.org/10.1186/s13395-018-0155-0

Natashya V.

Founder, StallionCompare.com | Creative Director

Natashya, the founder of StallionCompare.com, is a dynamic force in both the equine and creative industries. As a lifelong equestrian, Natashya’s journey with horses began with Quarter Horses in her hometown in British Columbia, Canada. During her college years she took a brief (and eye opening) hiatus from Quarter Horses to work in the Thoroughbred industry – returning to the AQHA circuit as a young adult. Natashya was also able to reconnect with the quarter horse industry through design. She worked with former owners of Pleasurehorse.com and Show Horse Today on their design needs prior to Robyn Duplisea purchasing the business. Natashya also worked at Horse Canada (HPG) as the Creative Director for many monthly, quarterly, and annual equine magazines. Her passion (read: obsession) for breeding, genetics, and equine pedigrees ran ever-increasingly deep, which lead her to create StallionCompare.com.

Beyond her equine pursuits, Natashya is the Creative Director & Co-Founder of a leading design agency known for its boutique, customer-focused approach. With her expertise, she blends creativity and technology to make a lasting impact in both horse breeding and design.