Polymyositis in Dogs
Causes, Treatment, and Prevention
Reviewed by Amy Fox
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 chronic. There are many possible causes of polymyositis, some of which are curable, while others are incurable but can be managed with medications to help dog owners 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. There are many different causes for this condition.
Symptoms of Polymyositis in Dogs
Dogs with polymyositis exhibit several signs that reflect that their muscles are not working normally.
Symptoms
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 a sign. Pain often makes dogs feel depressed and less inclined to play or participate in normal activities including eating and seeking out affection.
As muscle fibers degrade, a dog's muscles will waste away, known as atrophy, and this may look like weight loss as bones become more visible due to the loss of muscle. 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. Additionally, dogs may also lose weight due to pain, generally not feeling well, or due to difficulty eating from muscle atrophy in their jaw and esophagus.
What Causes Polymyositis in Dogs?
Polymyositis can be caused by many different conditions. This includes certain infections from protozoa, tick-borne disease, or Leishmaniasis. It can also be caused by certain tumors, immune-mediated disease, or due to a reaction to an antibiotic called trimethoprim-sulfadiazine.Some breeds of dogs also have an inherited form of the condition. At-risk breeds include:
Pembroke Welsh Corgis
Vizslas
How Vets Diagnose Polymyositis in Dogs
Your veterinarian will perform a full physical examination on your dog and discuss the signs you have observed. X-rays may be recommended to check for other underlying injuries and/or abnormalities, and thorough blood work will be done to look for additional clues. Electromyography (EMG) tests will be helpful to look for abnormalities in the electrical activity of the muscle tissues. This is usually performed by a veterinary specialist so your vet may refer you for these tests if polymyositis is suspected. A muscle biopsy may also aid in making a definitive diagnosis because it will reveal muscle tissue necrosis and inflammation on a microscopic level.
How to Treat Polymyositis in Dogs
Treatment depends on identifying the underlying cause of polymyositis. Infectious causes will require treatment with the appropriate antibiotic or anti-parasitic medications. If this is a reaction to the antibiotic trimethoprim-sulfadiazine, discontinuing the medication will usually be the only treatment needed. In cases of immune-medicated polymyositis, steroids and other immunosuppressive drugs are usually prescribed. In all cases, activity restriction may also be recommended until signs start to improve. Some of these causes can be cured with the appropriate treatment, while others will be chronic and can be managed with medication but cannot be completely cured.
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 can be cured. Depending on the underlying cause, some of these cases can be cured with the right treatment and dogs can make a full recovery. This includes certain infectious causes and reactions to trimethoprim-sulfadiazine. Others, such as immune-mediated polymyositis will not be cured completely but can be managed long-term with medications. These medications can control the signs, but can also have some unwanted side effects. Finally, some causes of polymyositis such as cancerous tumors and some hereditary conditions do not have effective treatments and cannot be cured. In these cases, if symptoms are severe, euthanasia may be the most humane option.
How to Prevent Polymyositis
Polymyositis has many possible causes and it is hard to prevent them all. Using effective tick preventatives and thoroughly checking your dog for ticks on a regular basis can help to prevent tick-borne illness. Other infections can be prevented by practicing good hygiene, providing a clean living space for your dog, and not feeding raw or undercooked meats. Immune-mediated disease cannot be prevented, but it is important to see your vet as soon as you notice anything amiss with your dog. Early treatment can slow the progress of the disease and help your dog have a good quality of life. While the hereditary form cannot be prevented in an individual dog, any dog that has polymyositis should not be bred and this will prevent the condition from being passed on to another generation of puppies.
