EQUUS ‘Farm Calls’ episode 25: Equine muscle disorders

There are many different muscle disorders in horses, including various forms of tying-up.

In this episode of EQUUS Farm Calls, we are talking to Dr. Erica McKenzie about equine muscle disorders.

Dr. McKenzie is a specialist in large animal medicine as well as sports medicine and rehabilitation. She is a professor of Large Animal Internal Medicine at Oregon State University. Her research interests are largely related to exercise physiology and muscle function.


McKenzie talked about various forms of “tying-up” in horses. She said the onset is usually when the horse is exercising. He becomes slow, sweaty and stiff, she said. Affected horses can refuse to exercise or have “lameness” that gets worse as the horse is warming up. Horses that are tying-up will have high heart and respiratory rates.

McKenzie talked about seeing horses that are experiencing their “first” tying-up episodes at 17 or 18 years old but that have actually had underlying disease all their life.

“Tying-up is a challenging situation because some owners think horses have bad behavior when actually they have tying-up,” said McKenzie.

The disorders that contribute to tying-up include polysaccharide storage myopathy (PSSM) type 1. https://cvm.msu.edu/research/faculty-research/comparative-medical-genetics/valberg-laboratory/type-1-polysaccharide-storage-myopathy This genetic issue is usually seen in Quarter Horses, Paints and Appaloosas, but it can occur in other breeds.

Thoroughbreds and Standardbreds that experience tying-up usually have a genetic disorder called recurrent exertional rhabdomyolysis (RER). See: https://cvm.msu.edu/research/faculty-research/comparative-medical-genetics/valberg-laboratory/recurrent-exertional-rhabdomyolysis.

Other genetic diseases that cause muscle disorders include PSSM type 2 ( https://cvm.msu.edu/research/faculty-research/comparative-medical-genetics/valberg-laboratory/type-2-polysaccharide-storage-myopathy), myofibrillar myopathy (MFM) (https://cvm.msu.edu/research/faculty-research/comparative-medical-genetics/valberg-laboratory/myofibrillar-myopathy), malignant hyperthermia (https://ceh.vetmed.ucdavis.edu/health-topics/malignant-hyperthermia-mh#:~:text=What%20is%20malignant%20hyperthermia%3F,occasionally%20by%20stress%20or%20excitement) and myosin heavy chain myopathy (MYHM) (https://cvm.msu.edu/research/faculty-research/comparative-medical-genetics/valberg-laboratory/inflammatory-myopathies).

McKenzie said it is important to use genetic testing that has been proven by research. She also warned owners that just because a horse carried a defect, it doesn’t mean the horse will show clinical signs of disease.

Other Myopathies

Myopathies are neuromuscular disorders that can be inherited or acquired.

Two types of acquired disorders that can cause muscle disorders in horses are selenium deficiencies (or toxicities) and vitamin E deficiency. McKenzie noted that horses can have both issues at the same time.

Selenium issues are regional, and supplementation can help. However, said McKenzie, offering a salt block with selenium to horses living in deficient regions of the United States is not a good solution. “It is best given orally” as a supplement, she said.

Vitamin E deficiencies early in a horse’s life can cause ataxia. As the horse ages, it can have muscle wasting, severe tremors and will stand with its feet close together.

McKenzie said horses across the nation are at risk if they have only dry forage and no supplemental vitamin E. She said synthetic vitamin E can be very effective and economic, but that natural vitamin E is better absorbed by the horse.

(Editor’s note: Check out the Michigan State University College of Veterinary Medicine’s Equine Neuromuscular Diagnostic Laboratory page for more information about muscle disorders: https://cvm.msu.edu/research/faculty-research/comparative-medical-genetics/valberg-laboratory).

About Dr. Erica McKenzie

Dr. Erica McKenzie graduated from Murdoch University in Western Australia in 1996, then completed her residency and PhD at the University of Minnesota in 2003. At that college, she was involved in developing nutritional and pharmacologic methods of controlling exercise associated muscle disease in Thoroughbred horses while concurrently achieving specialist certification in large animal internal medicine.

Following her residency, McKenzie completed a two-year post-doctoral fellowship at Oklahoma State University performing research investigations in exercising horses and long-distance sled dogs. She then accepted a faculty position in large animal internal medicine at Oregon State University in 2005.

McKenzie is a charter diplomate of the American College of Veterinary Sports Medicine and Rehabilitation. She is the author of more than 30 research manuscripts relevant to exercising horses and dogs, and she iis the current chair of the International Committee for the International Conference on Equine Exercise Physiology.

About the landing page photo: Jewell is a Quarter Horse who belonged to the author and who tested positive for PSSM type 1.

This episode of EQUUS “Farm Calls” podcast is brought to you by Farnam—Your Partner in Horse Care. Visit farnam.com to learn more.




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