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Key takeaways

Polymyositis is an inflammatory disorder in dogs affecting two or more muscles in the body.

  • The immune system attacks healthy muscles, causing muscle tissue to be replaced with scar tissue, leading to weakness
  • Polymyositis can be caused by immune-mediated disease, a reaction to certain drugs, or cancer
  • Symptoms include muscle weakness, changes to the gait, muscle swelling or atrophy, muscle pain, intolerance to exercise, and regurgitation
  • Dogs with these symptoms require prompt medical care
  • Diagnostic tools include physical examination, blood work, urinalysis, X-rays, and muscle biopsy
  • Treatment includes corticosteroids and treatment of any underlying infectious conditions
  • Long-term management requires a rehabilitative exercise program to rebuild muscle, and changes to the diet if megaesophagus has developed
  • Prognosis with treatment is good in most cases If the underlying cause is cancer, the prognosis is poor
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A closer look: Polymyositis in Dogs

Polymyositis is a common cause of muscle weakness in dogs, but is rare overall. Dogs walking differently than usual require prompt veterinary attention.

The severity of polymyositis depends on the underlying cause and the extent of damage to the muscles. Muscles affected by polymyositis lose muscle fibers which are replaced by scar tissue. The more scar tissue, the weaker the muscle. Left untreated, the muscle loses its ability to function. Therefore, early detection and treatment are important to stop progression of the disease.

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Risk factors

In cases of chronic polymyositis, the muscles may atrophy, becoming noticeably smaller and weaker over time, due to long-term damage to muscle fibers.

In cases associated with megaesophagus (the enlargement of the esophagus), symptoms can vary.

Certain breeds are more susceptible to polymyositis including:

  • Newfoundlands
  • Boxers
  • Pembroke welsh corgis
  • Vizslas

Dogs with an underlying autoimmune disease or cancer are also at greater risk.

Possible causes

The causes of polymyositis are not fully understood. This disorder occurs when the immune response to a disease, drug, or infectious agent malfunctions and damages healthy muscle tissue instead of working on the intended target.

In cases where the damage to the immune system is permanent, chronic episodes of polymyositis occur.

Main symptoms

Testing and diagnosis

Diagnostic tools to identify polymyositis include:

  • Physical examination, including reflex checks
  • Blood tests
  • Serum creatine kinase (a test that checks for muscle damage)
  • Infectious disease testing
  • Urinalysis
  • Muscle biopsy

Steps to Recovery

Treatment for polymyositis is medication, specifically corticosteroids, to suppress the immune response. Treatment for underlying infectious diseases is sometimes necessary.

Recovery also requires the rebuilding of lost muscle tissue through daily exercise. In cases associated with megaesophagus, special feeding techniques including tube feeding or elevated feeding, and changes to the diet are necessary.

The prognosis for most cases of polymyositis is good with early treatment. In cases where treatment is delayed, recovery of lost muscle takes longer. In cases where the underlying cause is cancer, the prognosis is poor.


There are no proven preventative measures to avoid polymyositis. Avoiding infectious diseases by controlling contact with infected pets, environments, and other vectors, including fleas and ticks, reduces risk. Treating immune diseases early also reduces susceptibility in some cases.

Is Polymyositis in Dogs common?

Polymyositis is a common cause of muscle weakness in dogs, but is rare overall.

Typical Treatment

  • Corticosteroids

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