Polymyositis in Dogs

Causes, Treatment, and Prevention

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Polymyositis is a muscle disease that can affect all the muscles in a dog's body, making it difficult to perform everyday activities. Weakness, lethargy, atrophy, and lameness are symptoms that indicate the development of the disease, which may be acute or become chronic. Although polymyositis is incurable, there are treatment options to help dog owners manage the condition and maintain their pets' quality of life.

What Is Polymyositis?

Polymyositis is a type of myopathy (skeletal muscle disorder) in which the muscles become inflamed and irritated, and the fibers degrade. It is often associated with autoimmune disorders.

Symptoms of Polymyositis in Dogs

Dogs with polymyositis exhibit several symptoms that demonstrate that their muscles are not working normally.


  • Weakness
  • Lethargy
  • Muscle pain
  • Lameness or limping
  • Depression
  • Weight loss
  • Muscle wasting

Weakness, lethargy, and generalized discomfort are common signs of polymyositis. A dog with weak, sore muscles may cry when rising off the floor or jumping on or off furniture. It may even wince or whine when being touched if the pain is severe. Limping or holding up sore legs can also be indicative. Pain often makes dogs feel depressed and less inclined to play or participate in normal activities.

As muscle fibers degrade, a dog's muscles will waste away, appearing as weight loss with visible atrophy of muscles. This is usually most obvious in the hind legs, along the back, and on the head where the skin may appear to sink into the skull.

What Causes Polymyositis in Dogs?

Polymyositis is thought to be an immune-mediated condition, and it is often seen alongside other autoimmune diseases like megaesophagus, myasthenia gravis, lupus erythematosus, and polyarthritis.

Some breeds of dogs seem more prone to developing the disease, so a genetic component may exist, but no one fully understands what causes this disease in dogs. At-risk breeds include:

How Vets Diagnose Polymyositis in Dogs

If polymyositis is suspected, your veterinarian will perform a full physical examination on your dog and discuss the symptoms you have observed. X-rays may be recommended to check for injuries, and a complete blood count (CBC) will check for changes in a muscle enzyme called creatine kinase (CK).

Occasionally, electromyography (EMG) tests will be performed to look for abnormalities in the electrical activity of the muscle tissues. If polymyositis is suspected, then a muscle biopsy may aid in forming a definitive diagnosis because it will reveal muscle tissue necrosis and inflammation on a microscopic level.

How to Treat Polymyositis in Dogs

There is no cure for polymyositis, but treatments are usually effective in managing the symptoms of the disease. Steroids and immunosuppressants are usually prescribed, and activity restriction may also be recommended.

Concurrent diseases may make it more difficult to manage polymyositis, and relapses of the disease can occur after symptoms appear to resolve. Steroids and immunosuppressants may need to be administered long-term depending on the severity of the disease.

Prognosis for Dogs with Polymyositis

Polymyositis can be an acute or chronic condition. Some dogs may experience ongoing, long-term symptoms, while others experience an acute form of the disease that occurs and resolves quickly.

In rare cases, symptoms are severe and treatments do not effectively manage the pain and weakness a dog experiences. In these cases, dogs with polymyositis must be euthanized.

How to Prevent Polymyositis

Since polymyositis is most likely an immune-mediated disease, it is still unknown whether or not there are any ways to prevent it from occurring.

If a dog has polymyositis, it is not a good candidate for breeding in case the disease is heritable.

Article Sources
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  1. Polymyositis in Dogs. Merck Veterinary Manual.

  2. Polymyositis. The MSPCA–Angell.