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Polymyositis in dogs

The essentials

  • Polymyositis is a muscle disease — It can affect any or all of the muscles in a dog’s body, making it hard to perform everyday activities.
  • It’s not curable — But it can be managed with treatment.
  • Large-breed dogs are predisposed — Any dog can get polymyositis, but it tends to be more common in larger breeds like Newfoundlands and boxers.
  • What to look out for — Muscle weakness, tenderness, weight loss, or depression are all common symptoms.

What is polymyositis in dogs? 

Polymyositis is a type of inflammatory myopathy. This group of diseases includes several types of myopathies, like inclusion body myositis and masticatory muscle myositis (MMM), which only affects the masticatory muscles (the ones you use for chewing). What makes polymyositis different, though, is that it can affect multiple muscles at the same time instead of one specific muscle group or area.

These inflammatory myopathies are often considered autoimmune diseases, which means the body creates autoantibodies, and the immune system attacks itself. Over time, your pup’s muscles become inflamed and irritated, and those autoantibodies break down the muscle tissue as the disease progresses.

Weakness, lethargy, and atrophy are all symptoms of inflammatory myopathies, but those symptoms don’t always stick around. Sometimes inflammatory diseases like polymyositis are acute, meaning that symptoms may show up but go away quickly, while other times, your pup may experience chronic, long-term symptoms. Although polymyositis isn’t curable, treatments are available to help pet owners manage the condition and maintain their pet’s quality of life.

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What causes polymyositis?

There are several types of polymyositis, and each has different causes. Three main contributing factors may trigger the disease process:

Infectious agents. This type of polymyositis is triggered by bacteria, viruses, protozoa, or parasites that can infect the muscle tissue.

Immune-mediated polymyositis. This is when the body’s immune system creates antibodies that attack healthy muscle tissue. This type of polymyositis has also been linked to other immune-mediated diseases like systemic lupus erythematosus and dermatomyositis.

Paraneoplastic polymyositis. This type of polymyositis is caused by cancer. Certain types, like lymphoma, can release substances into the body that damage muscle tissue.

Symptoms of polymyositis

Dogs with polymyositis often show symptoms that their muscles aren’t working normally. A dog with weak, sore muscles may cry or have difficulty when standing up or jumping off of furniture, or they may even whine when you are petting them if the pain is severe. These symptoms can make dogs feel depressed since it’s hard to do the things they normally love, like playing with toys or running around outside.

  • Depression. Pups with polymyositis may develop depression when their favorite activities cause them pain or discomfort.
  • Fatigue/lack of energy. Your dog may sleep more, seem weak, or try to limit their movement.
  • Weight loss. Your pup may lose weight and look skinnier than they used to.
  • Lameness. If your dog is in pain, they may limp or avoid certain movements.
  • Muscle tenderness or pain. As the muscle fibers degrade, it can cause tenderness or pain when moving or being touched.
  • Muscle atrophy. You may notice your pet losing muscle mass, which can make them look skinnier and less defined.
  • Stilted gait. Your dog may walk differently, using stiff movements or limiting their range of motion with each step.

🚨If you notice any of these signs in your dog, you should take them to a vet immediately.

How is polymyositis diagnosed? 

If your veterinarian believes your pup may have polymyositis, they’ll perform a full physical examination and see what symptoms your pup is showing. Your vet may also recommend diagnostic tests like thoracic X-rays or a blood test with a complete blood count to check for changes in an enzyme called creatine kinase (CK).

This enzyme is found in your skeletal muscles, heart muscle, and brain. When any of those tissues are damaged, they leak CK, which can be used to identify muscle injuries or disease. To make a definitive diagnosis, your vet may take a muscle biopsy to identify any muscle atrophy and inflammation on a microscopic level.

Treating polymyositis in dogs 

Unfortunately, the outlook for this disease isn’t as good as we’d like it to be. Polymyositis and other canine inflammatory myopathies often lead to euthanasia in affected dogs. But don’t give up hope just yet. In some cases, clinical signs rapidly improve with aggressive treatment.

Pups like this corgi were given immunosuppressants or high doses of steroids called corticosteroids, and went on to live a long and happy life despite the disease. Options like immunosuppressive therapy may also help to lessen your dog’s immune system response. These treatment options are especially effective in acute conditions. However, only your vet or vet specialist can help you decide the best path for you and your pup.

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While there is no known cure for polymyositis, a more healthful diet rich in DHA could help. You may notice improvements in your dog’s health within days of adding ZipZyme™ Omega to their daily meal regimen.

Breeds more prone to polymyositis

Although any dog can develop the disease, certain breeds of dogs have an increased risk of polymyositis. These may include:

Frequently asked questions

What are the first symptoms of polymyositis? 

The first symptoms of myositis may include weakness, lethargy, lameness, or weight loss.

How do you treat polymyositis in dogs? 

Polymyositis may be treated with high doses of corticosteroids or immunosuppressive therapy.

What is the life expectancy of dogs with polymyositis? 

The life expectancy for your pup will vary depending on the cause of the disease and how well it can be treated. The more severe the disease is when it’s found, the worse the prognosis is likely to be. If you notice your dog displaying any unusual symptoms, take them to a vet immediately.