Polymyositis in Dogs

Close Up of Newfoundland Dog Outdoors
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Polymyositis is a muscle disease in dogs and it can affect all the muscles in a dog's body making it difficult for it do everyday activities. This disease thankfully has treatment options but it is important for an owner to recognize the potential signs of polymyositis so the best possible outcome can be obtained.

What Is Polymyositis in Dogs?

Polymyositis is a type of myopathy or muscle disease. In polymyositis, the muscles in a dog become inflamed and the fibers necrose or die off. Even though it affects all the muscles in a dog but is not an infectious disease meaning it isn't caused by a bacteria, virus, or fungus spreading through the body. Polymyositis can be an acute or chronic condition so this means that for some dogs it may be an ongoing, long term condition but for others it may occur quickly and seem to go away with treatment. Another type of polymyositis is called dermatomyositis and includes pus-filled skin lesions in addition to the muscle issues but regular polymyositis only affects the muscles.

Symptoms of Polymyositis in Dogs

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

Overall weakness and lethargy are commonly seen signs of polymyositis along with muscle pain. Due to these symptoms, a dog may cry when rising off the floor or jumping onto furniture. They may even cry in pain when being pet if they are experiencing extreme muscle pain but others will remain stoic or simply limp or hold up sore legs. Being sore or in pain may make a dog depressed and seem less inclined to want to play and participate in normal activities.

As muscle fibers die off, muscles will waste away so weight loss and visible atrophy of muscles may be seen. 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.

Signs of Polymyositis in Dogs

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

In dogs with the dermatomyositis form of polymositis, skin lesions or wounds will be seen. These lesions are filled with pus, a thick material filled with bacteria and white blood cells.

Causes of Polymyositis in Dogs

Polymyositis is thought to be an immune mediated condition and it is often seen alongside some other immune mediated diseases such as megaesophagus. Due to the prevalence in certain breeds, a genetic component may also exist but no one fully understands what causes this disease in dogs.

Diagnosing Polymyositis in Dogs

If polymyositis is suspected, your veterinarian will perform a full physical examination on your dog and discuss the symptoms you have been seeing at home. Blood and urine tests along with X-rays may be recommended to rule out other diseases and injuries and to 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 still suspected a muscle biopsy may be needed to make a diagnosis and check for muscle tissue necrosis and inflammation on a microscopic level.

Treatment of Polymyositis in Dogs

There is no cure for polymyositis but treatments are usually effective in managing the symptoms of the disease. Occasionally, though, symptoms are so severe and the treatment cannot effectively manage the pain and weakness so dogs with polymyositis need to be euthanized.

Steroids alongside immunosuppressants are usually prescribed to treat polymyositis but activity restriction and pain medications may also be needed. If it is the dermatomyositis form of polymyositis then antibiotics will most likely also be prescribed. Concurrent diseases such as megaesophagus may make it more difficult to manage polymyositis but relapses of the disease can occur regardless. Steroids and immunosuppressants may need to be administered long term depending on the severity of the disease.

How to Prevent Polymyositis in Dogs

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. Some breeds of dogs have shown to be more likely to develop it than others though so there is potential that a genetic component is present. At-risk breeds include:

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