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13 Signs of Masticatory Muscle Myositis in Dogs (in 2021)

In laymen’s terms, masticatory muscle myositis (MMM) in dogs is a painful inflammatory disorder that only affects the muscles responsible for chewing.

This inflammation causes a painful and serious condition in which the dog cannot open his mouth wide enough to eat properly. Progression of the disease will lead to severe muscle atrophy.

The breeds with the highest risk factor includes German Shepherds, retrievers, Doberman pinschers and Cavalier King Charles Spaniels, although it can occur in any breed.

The age of onset is around 3 years of age; however, it has been seen in much younger puppies.

Early detection and aggressive treatment increases the chances of a full recovery. The key is to identify the disease early enough to halt or slow further progress.

This post will help you identify early warning signs of the disease.

What Causes Masticatory Muscle Myositis in Dogs?

Auto-immune disorders occur when the body’s natural immune defenses begin to attack muscles, joints, organs, and/or tissue that it suddenly sees as the enemy.

It’s often difficult to pinpoint the exact reason for this type of response.

Ultimately, MMM in dogs is caused by the presence of 2M fibers in the jaw muscles. These fibers are not present in any other part of the body.

Antibodies see these proteins as the enemy and attack. This attack results in inflammation of the chewing muscles.

There are a few theories about what might cause the body to attack its own immune system including:


No single vaccination has been proven to directly cause MMM in dogs. However, some dog owners have reported trismus (inability to open the jaw) within about 10 days of their dog receiving a rabies vaccine.

Source: Masticatory Muscle Myositis in a Maltese Dog. Journal of Veterinary Clinics, Volume 31, Issue 3, pages 223-225/214, The Korean Society of Veterinarian Clinics.

Reaction/Side-Effects of Medication

No specific medications have been identified; however, prescriptions and over-the-counter medications have the potential to cause side-effects ranging from mild to severe.

Environmental Toxins

Pesticides, lawn chemicals, certain food preservatives, medications, and water contaminants may play some role in autoimmune disorders.

Masticatory Muscle Myositis in dogs is a painful autoimmune disorder.
Thank you to Phil Botha for this image – available on

How is MMM Diagnosed in Dogs

A biopsy involving the removal of a small section of the affected muscle can confirm whether there are inflammatory antibodies or not.

A pathologist can examine the tissue under a microscope and determine the level of inflammation as well as how much the disease has progressed.

2M Antibody Assay (Blood Test)

In 2004, a blood test was developed by researchers at the University of California-San Diego used to detect the antibodies that attack 2M fibers.

The test can confirm a diagnosis but cannot determine how much muscle mass has been lost or the prognosis for a return of normal jaw function.

If the disease is considered to be in the acute stage with little loss of muscle mass, the 2M antibody assay may be enough to make a diagnosis.

In chronic cases of MMM in dogs, it might be necessary to conduct a biopsy to determine the stage of the disease.

Treatment of Masticatory Myositis in Dogs

Treatment involves a high dose of an immune-suppressive dose of corticosteroids like Prednisone.

This is given over a period of 4 months until the symptoms are under control. Gradually, the dose of Prednisone is reduced to the lowest point possible. The idea is to get the dose as low as possible while keeping symptoms from returning.

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.

13 Signs of Masticatory Myositis in Dogs

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

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.

Severe Pain in the Jaw

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

Difficulty Eating

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

Difficulty Drinking

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

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.

Sunken Eyes

The appearance of sunken eyes is caused when the muscles behind the eyes begin to shrink.

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.

Swelling on the top of the head

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

Protruding Eyes

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

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.


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

Third Eye Protrusion

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

Red Eyes

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


If you have one of the more susceptible breeds to masticatory muscle myositis, 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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