Background

The prevalence of clinical signs and factors triggering muscle atrophy and rhabdomyolysis associated with an MYH1^E321G mutation in Quarter Horses and related breeds (QH) remain poorly understood.

Hypothesis/objectives

Determine the prevalence and potential triggers of atrophy and stiffness in horses homozygous reference (N/N), heterozygous (My/N), and homozygous (My/My) for the MYH1^E321G mutation.

Animals

Two-hundred seventy-five N/N, 100 My/N, and 10 My/My QH.

Methods

A retrospective case-control study using a closed-ended questionnaire completed by clients of the Veterinary Genetics Laboratory at the University of California, Davis. History of clinical signs, disease, vaccination and performance were analyzed by genotype using contingency testing.

Results

Atrophy occurred in proportionately more horses with MYH1^E321G (My) than N/N QH and more frequently in My/My than My/N QH (P < .001; My/My 8/10 [80%], My/N 17/100 [17%], N/N 29/275 [11%]). More My/My horses had rapid atrophy (P < .001), with recurrence in 50%. Fewer My/My horses recovered versus My/N QH (P < .001). Stiffness was common across genotypes (P = .100; My/My 4/10 [40%], My/N 18/100 [18%], N/N 48/275 [17%]). Three months before the observed atrophy and stiffness, 47% of MYH1^E321G QH were vaccinated or had respiratory or gastrointestinal disease. Horses achieving 100% expected performance did not differ across genotypes (50% My/My, 71% My/N, 55% N/N), but, only 4/10 My/My QH were competing. My/N horses achieved national or world championships or both.

Conclusion and clinical importance

Approximately 20% of My/N QH develop rapid atrophy. Atrophy is more common (80%) in homozygous My/My QH and less likely to resolve. Inciting causes such as vaccination and infection are inapparent in over half of cases.