What is myositis?

Myositis, also called inflammatory myopathy, refers to a group of diseases that cause inflammation of the muscle. There are several different types of myositis in cats and dogs, and these can either affect a singular muscle or a group of muscles:

  • Masticatory Muscle Myositis (MMM), which affects the muscles used in chewing. This is the most common inflammatory myopathy in dogs.
  • Extraocular Myositis (EOM), which affects the muscles that move the eyeball.
  • Polymyositis, or Generalised Myositis, which affects more than one muscle group at a time and can affect any muscle throughout the body.
  • Dermatomyositis, which affects the skin and blood vessels.

What causes myositis in cats and dogs?

In both dogs and cats, muscle inflammation can be caused by a range of different issues such as:

  • A response to an infection, either parasitic or viral
  • Autoimmune disorders that attack the muscle
  • An allergic reaction to toxic substances or other medication
  • Cancer

In some pets, it is also an inherited condition that is more common in certain breeds or ages. For example, while Masticatory Muscle Myositis can occur in any breed or age, it is more common in younger dogs, and certain breeds, such as Golden Retrievers and German Shepherds are more likely to develop the condition.

How do I know if my pet is suffering from myositis?

Symptoms of myositis can vary greatly, and depend entirely on which muscle is affected by the condition. The most common symptoms are:
Masticatory Muscle Myositis (MMM):

  • Swollen and painful chewing muscles
  • Difficulty and pain when opening the mouth
  • Inability to eat or drink

Extraocular Myositis (EOM):

  • Swelling around the eye
  • Protruding eyeballs
  • Vision impairment


  • Lethargy and weakness
  • Depression
  • Muscle pain and swelling
  • Muscle atrophy
  • Limping
  • Stiff gait
  • Weight loss


  • Skin lesions or ulcers
  • Hair loss
  • Muscle atrophy


How is myositis diagnosed?

As with any type of muscle problem, without early and direct treatment all forms of myositis can cause long-lasting damage and muscle debilitation. If left untreated, conditions can cause blindness or an inability to walk or eat. However,

First, any potential infections or abnormalities need to be ruled out to get to the cause of the myositis and so a full physical examination will be conducted. Your veterinary professional will usually ask for complete blood count to determine whether your pet has any other underlying health conditions, while X-rays and ultrasounds of the chest and abdominal region may be used to rule out cancer or infections.

The best way to diagnose myositis in dogs and cats is by taking a muscle biopsy, which will be examined for inflammatory cells. Unlike other forms of myositis, masticatory muscle myositis can be diagnosed by a blood test called a 2M antibody test, which measures the level of antibodies within the blood. Similarly, electromyography (EMG) may also be used to show any abnormal electrical activity produced by muscles.



How is myositis in dogs and cats treated?

Early treatment of myositis is usually quite aggressive but offers the best chance of recovery. The best way to treat inflammatory myopathy is by prescribing a high dose of immunosuppressants or steroids in order to gain control over symptoms. This dose is then gradually reduced until these symptoms can be managed. It’s important to remember that immunosuppressants often cause side effects of increased urination and excessive thirst and appetite.

When it comes to looking after a dog suffering with any form of myositis, owners should make changes to keep their pet comfortable. For example, dogs suffering from MMM should be fed soft foods that don’t aggravate the chewing muscles. Similarly, as muscle inflammation decreases with treatment, owners should encourage pets to exercise and continue activities to build up strength.

What is the prognosis of myositis in dogs and cats?

As with many conditions, the outlook for a dog or cat suffering from myositis, in any form, depends on the severity of the individual condition and the degree of muscle inflammation, along with other factors such as general health, age and the specific treatment given. Throughout treatment, your vet will closely monitor your pet and make notes of any changes or side effects.

If you have any further questions about myositis in dogs or cats, you should speak to your veterinarian, who will be able to offer you more information. If you’re a veterinary professional, it’s also easy to make a referral with us, simply fill in your details online or download our PDF form now.