Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut elit tellus, luctus nec ullamcorper mattis, pulvinar dapibus leo.

Masticatory Myositis in Dogs

What Is Masticatory Myositis in Dogs?

Masticatory Muscle Myositis (MMM), also known as eosinophilic myositis, is an autoimmune disorder wherein the body’s immune system targets the muscles essential for chewing, located in the head and jaw. Myositis denotes inflammation within the muscles, leading to swelling and discomfort.

This condition specifically targets the specialized fibers present exclusively in the muscles of the head and jaw, resulting in severe pain that impedes your dog’s ability to open its mouth for eating, chewing, or engaging with toys.

MMM primarily affects the Type 2M muscle fibers unique to the temporalis and masseter muscles in the head and jaw region. It manifests in both acute and chronic forms, reflecting the progression of the disease. Without treatment, dogs may transition from the acute phase to the chronic phase over time.

Both male and female adult dogs, particularly those in the young to middle-aged range, can be affected by MMM. The breeds most commonly impacted include German Shepherds, Retrieving breeds like Labradors and Golden Retrievers, and Doberman Pinschers.


During the acute phase, symptoms of Masticatory Muscle Myositis (MMM) may be subtle and mistaken for other head-related conditions, such as issues with the eyes, teeth, mouth, or jaw. This phase can sometimes be confused with ailments involving inflamed tonsils, swollen lymph nodes under the jaw and neck, or protruding eyeballs (exophthalmos) resulting from muscle inflammation in the head.

Initially, the acute phase may present with nonspecific symptoms like lethargy, fever, reluctance to eat, and drooling. Over time, these symptoms progress, and the chronic form of Masticatory Myositis may manifest with:

  • Muscle atrophy, leading to more prominent head bones
  • Difficulty for the dog to open its mouth for eating
  • Anorexia and weight loss
  • Sunken appearance of the eyes due to muscle mass loss


The origins of most autoimmune diseases often remain elusive, although they may stem from various factors such as parasitic or viral infections, reactions to vaccines or medications, exposure to environmental toxins or allergens, stress, or potentially early indications of cancer. While the precise cause of Masticatory Muscle Myositis (MMM) is uncertain, there is a notable elevation in immunoglobulin G targeting the fiber protein myosin, a key constituent in muscle tissue.


Veterinarians diagnose Masticatory Muscle Myositis (MMM) in dogs by considering various test results. A comprehensive blood panel may reveal the following indicators:

  • Anemia, indicating a decrease in red blood cells.
  • Elevated levels of certain white blood cells like neutrophils, leukocytes, and eosinophils.
  • Increased globulin levels, often associated with immune system stimulation.
  • Elevated levels of creatine kinase (CK), an enzyme found in muscles.
  • Increased levels of aspartate aminotransferase (AST), an enzyme present in the liver and muscles.

If initial suspicions point towards dental or oral issues, a thorough examination under anesthesia may be recommended. While an awake dog might clench its jaw due to pain, anesthesia eliminates muscle control. If MMM is suspected, the dog will still exhibit difficulty opening its jaw even under anesthesia, prompting further investigation.

A specific blood test, known as a 2M antibody ELISA assay, detects circulating autoantibodies targeting 2M fibers, aiding in MMM diagnosis. However, a muscle biopsy is essential to assess disease progression and prognosis accurately.

Ideally, the 2M antibody test should precede treatment initiation. Steroids, commonly used for treatment, can interfere with the test by reducing circulating antibody levels, potentially yielding a false negative result. Additionally, end-stage muscle fiber destruction may also lead to a false negative due to extensive fibrous tissue replacing muscle. Muscle biopsy examines inflammation levels and the extent of fibrous tissue formation resulting from muscle deterioration.


In certain instances, dogs may experience spontaneous improvement with a reduction in autoantibodies, obviating the need for therapy. However, the majority of dogs require a course of treatment, which can extend from weeks to months, and in some cases, necessitate lifelong immunosuppressive therapy.

The duration of treatment hinges largely on the severity of the disease and the promptness of diagnosis. Ensuring an adequate duration of treatment is crucial to prevent relapse, underscoring the importance of owners adhering to the prescribed medication regimen.

Corticosteroids, administered at immunosuppressive doses, are commonly prescribed for a minimum of 30 days before gradually tapering the dosage. While some dogs may require lifelong medication, others may discontinue therapy following a tapering regimen. Prednisone (or prednisolone) typically serves as the preferred drug.

Regrettably, prolonged steroid use may result in muscle wasting akin to the effects of MMM, potentially accentuating the prominence of head bones.

Additional immunosuppressive medications (such as azathioprine, cyclosporine, cytarabine, cyclophosphamide, or mycophenolate) may be prescribed in combination with prednisone to mitigate inflammation. The primary objective is to aggressively halt the immune system’s assault on the muscles, thereby facilitating a return to normalcy as swiftly as possible, improving prognosis and averting permanent muscle damage. Supplementary pain medications may be necessary until steroids effectively reduce inflammation and circulating antibodies.

Previously, it was recommended to perform anesthesia to forcibly open the jaw and stretch fibrous tissue and muscles. However, this approach has fallen out of favor due to associated risks such as mandibular luxation or fracture, exacerbation of muscle inflammation, and negligible impact on disease outcome.

Living and Management

It typically takes a few weeks for prescribed medications to take effect and for your dog to begin feeling better and exhibiting signs of improvement. Depending on the level of improvement observed, your veterinarian may gradually taper the medications to prevent relapse. Should you have concerns regarding any potential side effects, it’s important to promptly consult your veterinarian to discuss symptoms.

Facilitating successful feeding may entail softening dry food with warm water, offering canned food, or blending food into a gruel for easy consumption or syringe feeding if necessary. Your veterinarian may also prescribe calorie-dense liquid formulations to aid in recovery.

Masticatory myositis can recur, and each subsequent episode may pose increased difficulty in treatment. The prognosis tends to worsen with recurring episodes due to fibrous changes within the muscles.

Common factors contributing to relapse or worsening of MMM include inappropriate dosages of immunosuppressive medications or inadequate duration of medication administration. It’s crucial to administer all medications precisely as prescribed and maintain close communication with your veterinarian throughout the treatment process.

Masticatory Myositis in Dogs FAQs

Is masticatory myositis fatal in dogs?

Masticatory myositis has the potential to be fatal since affected dogs may struggle to open their mouths to eat or drink adequately. If left untreated or if the disease has advanced to the stage where most jaw muscle fibers have been replaced by fibrous tissue, it becomes incurable. However, dogs that respond positively to therapy can typically lead normal lives despite having this condition.

What is the life expectancy of a dog with masticatory myositis?

A study conducted at the University of Pennsylvania School of Veterinary Medicine over a span of 17 years analyzed cases of masticatory myositis (MMM). The prognosis for dogs with MMM is generally favorable when treated aggressively. According to the study, many dogs showed improvement within a few days of therapy, and by four weeks of treatment, they typically regained normal chewing function. Around 27 percent of affected dogs experienced relapses and needed ongoing steroid therapy for resolution of the condition.

Scroll to Top