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13 Signs of Masticatory Muscle Myositis in Dogs (in 2023)

Medically reviewed by Dr. Irish on April 28th, 2023

Do you have a puppy that can’t seem to open his or her mouth without severe pain? Is your puppy unable to pick up toys, eat, or chew?

If so, it could be something called masticatory muscle myositis (MMM), a painful immune system disorder that causes inflammation in the jaw muscles.

Without treatment, the jaw muscles will atrophy and weaken, leaving your dog unable to fully close his or her mouth.

If the dog can’t eat or drink properly, he or she will also suffer from weight loss, nutritional deficiencies, and long-term muscle damage.

Early detection and aggressive treatment increase the chances of a full recovery. The key is early diagnosis.

In this post, we’ll help you identify signs of masticatory muscle myositis in dogs. You’ll also learn more about the disease, how it’s diagnosed, and treatment options.

About Masticatory Muscle Myositis in Dogs

Masticatory muscle myositis might be easier to understand if it were called “jaw inflammation”. It’s an immune-mediated disease that only attacks the muscles of the head and jaw.

This extremely painful condition specifically attacks the Type 2M muscle fibers. These muscle fibers are in the temporalis muscle and masseter muscles and play an important role in chewing and drinking.

Illustration of temporalis and masseter muscles in dogs

The pain caused by inflammation makes it hard for dogs to eat and drink.

The average age of diagnosis is around 3 years of age. It can, however, occur in much younger puppies.

Acute Masticatory Myositis

In acute cases, the jaw muscles become swollen, and the dog will have a lot of difficulty opening the jaw. This phase can present vague symptoms that may not be noticed by the pet owner.

Dogs may present with symptoms like tonsil inflammation, enlarged lymph nodes under the jaw and neck, or protruding eyeballs. It’s easy to confuse these symptoms to other potential causes.

Dogs in the acute phase of masticatory myositis may have vague symptoms that include:

  • Fever
  • Lethargy or fatigue
  • Unwilling or unable to eat.
  • Excessive drooling

Chronic Masticatory Myositis

Over time, the symptoms will become more obvious. The dog won’t be able to eat and will suffer from weight loss. Signs of pain become much more obvious as he or she has difficulty opening the jaw.

Because the jaw muscles are not working properly, they’re not able to maintain tone or strength. This can ultimately lead to muscle atrophy.

Masticatory muscle myositis in dogs

At What Age do Dogs Develop MMM?

Dogs are typically diagnosed at around 3 years of age but can develop the condition much earlier than that.

What Dog Breeds Are More Susceptible to Masticatory Muscle Myositis?

Any dog breed can develop this disease and it occurs in both male and female dogs. Breeds that may be more susceptible include the following:

  • Golden retrievers
  • Doberman pinschers
  • German shepherds
  • Labrador retrievers
  • Cavalier King Charles spaniels
  • Rottweilers
  • Weimaraner
  • Large breed dogs in general

What Causes Masticatory Muscle Myositis in Dogs?

It’s not entirely known what triggers MMM in dogs. There are, however, suspected triggers including the following:

  • Bacterial infection
  • Viral infection
  • Vaccinations
  • Stress
  • Exposure to allergens
  • Reaction to medications
  • Environmental toxins
  • Abnormal immune reaction
  • Cancer

How is Masticatory Muscle Myositis Diagnosed in Dogs

There are a few tests that can help make an accurate diagnosis. These include the following:

Complete Physical Examination

The veterinarian will examine your dog for signs of other conditions that may affect the jaw. These include things like dental disease.

Blood Test

A complete blood panel may show signs of:

  • Anemia
  • An increase in white blood cells (neutrophils, leukocytes, and eosinophils)
  • An increase in globulins, an increase in creatine kinase (CK)
  • An increase in a liver and muscle enzyme known as aspartate animotransferase (AST)


The 2M antibody ELISA assay is a specific blood test used to detect circulating autoantibodies.

While the test can confirm a definitive diagnosis, it can’t determine how much muscle mass has been lost or prognosis of normal jaw function.

It’s important for the blood test to be performed before treatment because treatment can produce false negatives.

Muscle Biopsy

If circulating autoantibodies are not detected, but MMM is suspected, a muscle biopsy can determine the level of inflammation present. It can also determine how much the muscle has deteriorated from the scar tissue.

Additional Tests

Other tests including CT scan might be required to rule out a potential infection or cancer.

Treatment of Masticatory Myositis in Dogs

The severity of the disease often guides the treatment options. Most dogs, however, require six to eight months of treatment. In some cases, lifelong immunosuppressive therapy is required.

The idea is to treat long enough to stop the dog from relapsing.

Corticosteroid Therapy

Treatment involves a high dose corticosteroid like Prednisone. This is given over a period of 30 days until the symptoms are under control. High doses of corticosteroids will suppress the immune system.

Some dogs may need to be on the medication for the rest of their lives where other dogs can be weaned successfully without a return of symptoms.

Other Immunosuppressive Drugs

Other drugs that can be used in combination with prednisone include:

  • Azathioprine
  • Cyclosporine
  • Mycophenolate

These can be prescribed to fight inflammation with the goal of aggressively stopping the immune system’s attack on the muscle.

Pain Medication

Other pain medications could be prescribed until the steroids have had a chance to reduce inflammation.


In the past, dogs were put under general anesthesia to try and stretch the tissues of the jaw. This has recently been cast aside because it can lead to jaw injury and increase inflammation.

13 Clinical Signs of Masticatory Myositis in Dogs

In the early stages, MMM causes swelling of the muscles on the top of the head. As the weeks go by, the pain will be severe enough to stop a dog from wanting to eat or drink. Without treatment, the muscles can become fixed in place.

The faster a dog is diagnosed with this disease, the better the outlook for a full recovery.

1. Unable to Open Jaw

Dogs with MMM will have a lot of pain when trying to open their jaw. This is due to the inflammation caused by the disease.

2. Severe Pain in the Jaw

A dog with this condition will express signs of physical pain (howling, crying, etc.)

3. Difficulty Eating

Even if the dog can open his/her mouth partially, eating will be painful and difficult.

4. Difficulty Drinking

Getting adequate water intake will be hard for a dog with an inflamed jaw.

5. Unable to Pick Up a Toy

Any dog in severe pain won’t want to engage in activities like play. Regardless, the inability to manipulate the jaw comfortably will present the dog from even being able to pick up a toy.

6. Sunken Eyes

The appearance of sunken eyes is caused when the muscles behind the eyes begin to shrink. This might happen in chronic forms of MMM when the muscles around the head and jaw have begun to atrophy.

7. Muscle Loss

Muscle loss in dogs with MMM is a serious concern.

Inactivity can cause muscle loss throughout the body, but the disease itself can actually waste away the muscles of the jaw and around the head.

8. Swelling on the top of the head

Swelling on top of the head is in relation to the degree of inflammation in the jaw.

9. Protruding Eyes

Protruding eyes is caused by the inflammation of muscles around the eye.

10. Stiff Movements

Dogs in a lot of pain will express it in different ways including keeping their head down and walking with a stiff gait.

Read “13 Warning Signs Your Dog is in Pain” by Corey Whelan, Reader’s Digest

11. Weakness

Weakness in dogs with MMM can occur due to lack of food and water.

12. Third Eye Protrusion

Sometimes the swelling in and around the eyes can cause the third eye to protrude.

13. Red Eyes

Inflammation around the eyes can cause blood vessels to burst causing red eyes.

Masticatory myositis in dogs can be treated with steroids

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Pet owners should be alert for sudden pain in the jaw, unusual appetite changes, and signs of pain in dogs.

This autoimmune disease typically makes an appearance at around 3 to 4 years of age.

Although the exact cause is unknown there are a few theories as to what might trigger it. Some of these things include environmental toxins, possible vaccination side-effects, and genetics.

The sooner treatment is received, the better chance of a full recovery.

Keep in mind that jaw pain doesn’t necessarily point to a diagnosis of MMM.

Pain in the jaw can be caused by dental disease, injury, or any number of other things. Whatever the outcome, it’s vitally important to get your dog to a veterinarian for any of the above signs and symptoms.

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Works Cited

“Masticatory Myositis in Dogs | VCA Animal Hospitals.” Vca, Accessed 6 Apr. 2023.

“Myositis Fact Sheet | Davies Veterinary Specialists.” Davies Veterinary Specialists, Accessed 6 Apr. 2023.

@vetster. “Masticatory Myositis in Dogs – Causes, Treatment and Associated Conditions – Vetster.” Vetster Online Vets, 8 Mar. 2023,

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