What Is Masticatory Myositis in Dogs?
Masticatory Muscle Myositis (MMM), or eosinophilic myositis, is an autoimmune disease where the body attacks the muscles of the head and jaw required for chewing (or mastication). Myositis means muscle inflammation (swelling).
This disease attacks the special fibers found only in the muscle of the head and jaw, and this extremely painful condition inflames muscles, preventing your dog from opening his or her mouth to eat, chew, or play with toys.
The disease affects the Type 2M muscle fibers, which are unique to the temporalis and masseter muscles of the head and jaw. There are acute and chronic forms of this disease which describe the progression of the disease. Dogs will progress from the acute to the chronic form of the disease over time, if untreated.
Both male and female adult dogs can be affected and are mostly young- to middle-aged dogs.
Most common breeds affected include:
Symptoms of Masticatory Myositis in Dogs
The acute phase may often go unnoticed or confused with other conditions of the head, eye, teeth, mouth, and jaw, as symptoms can be vague. Sometimes, the symptoms can be confused with other diseases that involve inflammation of the tonsils, enlarged lymph nodes under the jaw and neck, or protrusion of the eyeball (exophthalmos) which is caused by the swelling of inflamed muscles in the head.
Initially, the acute phase may include vague symptoms such as:
Unwilling to eat
As time goes on, the symptoms will progress, and the chronic form of Masticatory Myositis may include:
Muscle shrinkage as the bones of the head become more prominent
Difficulty for your dog to open its mouth to eat
Eyes may appear sunken due to loss of muscle mass
Causes of Masticatory Myositis in Dogs
Most autoimmune diseases start for unknown reasons, but they can be due to parasitic or viral infections, vaccines or other reactions to medications, exposure to environmental toxins or allergens, stress, or an early sign of cancer. The cause of this disease is unknown, but there is an increase in the immunoglobulin G to the fiber protein myosin, a component in muscle.
How Veterinarians Diagnose Masticatory Myositis in Dogs
Your veterinarian may suspect MMM based on a few test results. A panel of bloodwork may show any of the following:
Anemia, a decrease in red blood cells
An increase in certain white blood cells called neutrophils, leukocytes, and eosinophils
An increase in globulins, which is seen with many diseases that stimulate the immune system
An increase in creatine kinase (CK), a muscle enzyme
An increase in aspartate aminotransferase (AST), a liver and muscle enzyme
If a tooth or oral condition was initially suspected, your veterinarian may recommend a thorough oral examination under anesthesia. An awake dog may clench its jaw shut as a pain response, but a dog under anesthesia will no longer have muscle control. A dog suspected to have MMM will not be able to have its jaw opened even under anesthesia, and your veterinarian will recommend further testing.
There is a specific blood test that defines circulating autoantibodies to the 2M fibers. It is called a 2M antibody ELISA assay and it can help diagnose masticatory muscle myositis. A muscle biopsy (removal of a sample to examine) is needed, however, to help determine the progression of the disease and subsequent prognosis.
Ideally, this test should be done before treatment is initiated. Steroids are typically the treatment of choice, but it can interfere with the 2M antibody test as steroids decrease the quantity of circulating antibodies in the blood and may result in a false negative. End-stage destruction of muscle fibers may also result in a false negative test because of the severity of fibrous tissue replacing muscle. The muscle biopsy will examine the amount of inflammation and how much fibrous tissue has formed from deteriorating muscle.
Treatment of Masticatory Myositis in Dogs
In some cases, dogs can spontaneously improve with a regression of the autoantibodies and not require therapy. However, most dogs need a course of therapy that may last weeks to months (in some cases six to eight months) or even lifelong immunosuppressive therapy.
The length of treatment often depends on the severity of the disease and how quickly the disease was diagnosed. The most important aspect of treatment is to treat for a long enough period that the dog does not have a relapse. For this reason, it is important for owners to follow the prescription and administer all the medications prescribed by the veterinarian.
Corticosteroids—at immunosuppressive doses—are often prescribed for at least 30 days before tapering the dose. Some dogs need lifelong medication, and others can stop therapy after a tapering-down dose regimen. Usually, prednisone (or prednisolone) is the drug of choice.
Unfortunately, chronic steroid use will also lead to muscle wasting, like the effects of MMM, and the bones of the head may become prominent.
Other immunosuppressive drugs (such as azathioprine, cyclosporine, cytarabine, cyclophosphamide, or mycophenolate) may be prescribed to help combat the inflammation and can be used in combination with prednisone. The goal is to aggressively halt the immune system’s attack on the muscle and return the dog to normal as quickly as possible, which improves prognosis and prevents permanent damage to the muscles.
Your dog may need other pain medications until the steroids reduce inflammation and circulating antibodies.
It was previously recommended that dogs undergo anesthesia to have the jaw forcibly opened to stretch the fibrous tissue and muscles. However, this process has gone out of favor as it can lead to trauma such as mandibular luxation (dislocation) or fracture, and increases the inflammation in the torn muscles. It also does not help the outcome of the disease.
Recovery and Management of Masticatory Myositis in Dogs
You can expect any medications prescribed to take a few weeks to work before your dog feels better and shows some improvement. Depending on the degree of improvement, your veterinarian may start to taper medications slowly to prevent relapse. If you are worried about any side effects, please contact your veterinarian to discuss the symptoms.
Successful feeding may require softening dry food with warm water or canned food or blending it into a gruel to lap up or syringe feed, if necessary. There are also calorie-dense liquid formulations that may be prescribed by your vet.
Masticatory myositis can be a recurrent condition, and it can be more difficult to treat each time. The prognosis declines with subsequent episodes of the disease because of fibrous changes to the muscle.
The most common causes of a relapse, or worsening condition with MMM, are inappropriate doses of immunosuppressive medications, or an inappropriate length of time the medications are prescribed or given to the dog. It is very important to give all medications as prescribed and to work closely with your veterinarian.
Masticatory Myositis in Dogs FAQs
Is masticatory myositis fatal in dogs?
Masticatory myositis can be fatal as the dog cannot open its mouth to eat or drink properly. If the disease goes untreated or it has progressed to the point that most of the muscle fibers of the jaw have been replaced by fibrous tissue, this disease is not curable. Otherwise, dogs that respond to therapy can be expected to live a normal life with this disease.
What is the life expectancy of a dog with masticatory myositis?
Prognosis is usually favorable if dogs are treated aggressively. In a recent study conducted at the University of Pennsylvania School of Veterinary Medicine, cases of MMM were evaluated over a 17-year period. It showed many dogs improving within a couple of days of therapy, and within four weeks of treatment, dogs typically regained normal chewing function. Approximately 27 percent of affected dogs experienced a relapse and required continued steroid therapy for the condition to be resolved.
Couto, C. Guillermo and Richard W. Nelson. Small Animal Internal Medicine. 3rd ed. Elsevier Health Sciences Division;2003.
Harari, Joseph. Merck Veterinary Manual. Masticatory Myositis in Dogs and Cats (Eosinophilic Myositis). November 2020.
Mariotti, Evan. MSPCA Angell. Masticatory Muscle Myositis.
UC San Diego School of Medicine Comparative Neuromuscular Laboratory. Masticatory Muscle Myositis.
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