Masticatory Myositis in Dogs: Causes and Symptoms

Masticatory myositis in dogs is an autoimmune disease in which the immune system attacks the masticatory or chewing muscles. The attacked muscles become inflamed and lose mobility and function.

Masticatory myositis (MMM) in dogs has a genetic component and is more common in large breeds. The condition is believed to be triggered by environmental factors like allergen, toxin, and infection exposure.

Muscle atrophy, pain, malnutrition, difficulty drinking, aspiration pneumonia, fibrosis, trismus, and stress are common signs and complications of dog masticatory muscle myositis.

The dog muscle for mouth movement and chewing wastes over potentially fatal time. Myositis in dogs requires aggressive treatment with steroids and immunosuppressants.

Dogs respond well to treatment in most cases. An end-stage masticatory myositis dog requires surgical removal of a jaw section or the placing of a gastric feeding tube.

What is Masticatory Myositis in Dogs?

Masticatory myositis in dogs is an autoimmune condition that manifests as inflammation, atrophy, and necrosis of the masticatory (chewing) muscles.

The masticatory muscles in dogs (temporalis, masseter, pterygoid, and rostral digastricus) have unique 2M fibers not found in other muscles.

Masticatory myositis develops when the immune system mistakenly identifies the 2M fibers as a threat. The immune system produces fiber-type-specific autoantibodies against type 2M fibers, triggering an attack.

The attacked masticatory muscles lose their mobility and function, rendering the dog unable to open and close its jaw, eat, and drink. Masticatory myositis causes the muscle tissue to be replaced by fibrous or connective tissue.

Masticatory myositis in dogs is manageable when treated aggressively. MMM is potentially life-threatening when it is untreated.

What are the other terms for Masticatory Myositis in Dogs?

The other terms for masticatory myositis in dogs are atrophic myositis, eosinophilic myositis, and masticatory muscle myositis (MMM). The acronym MMM is for ease of pronunciation. The term atrophic describes muscle wasting. Eosinophilic describes the type of cells accumulating in the inflamed muscles.

Is Masticatory Myositis a common autoimmune disease in dogs?

Yes, masticatory myositis is a common autoimmune disease in dogs. A flawed immune system causes autoimmune conditions. The immune system mistakenly identifies its body structures as foreign threats.

The exact incidence of MMM as a dog autoimmune disease is unknown. Masticatory myositis is common but poorly documented and lacks research studies.

How Masticatory Myositis in Dogs developed?

Masticatory myositis in dogs develops acutely or chronically. Untreated acute cases of MMM tend to progress into chronic conditions that last over a year.

The condition starts when the immune system mistakenly recognizes the 2M fibers in the dog’s masticatory muscles as foreign and makes antibodies. The attacks trigger muscle inflammation followed by atrophy and necrosis.

The median age of masticatory myositis onset in dogs is three years. MMM cases, however, have been reported in puppies as young as four months.

How common are Masticatory Myositis in Dogs?

Masticatory myositis in dogs is common. MMM is a frequently reported inflammatory myopathy, especially among certain large dog breeds.

“Masticatory muscle myositis (MMM) is the second most common inflammatory myopathy,” says a “Case report: Atypical and chronic masticatory muscle myositis in a 5-month old Cavalier King Charles Spaniel. Clinical and diagnostic findings, treatment and successful outcome” published in Frontiers in Veterinary Science in 2022. The study notes that MMM is not common in puppies.

MMM is rare in cats, according to “Trismus, masticatory myositis and antibodies against type 2M fibers in a mixed breed cat,” in JFMS Open Reports in 2018.

What breeds are more prone to developing Masticatory Myositis?

The breeds more prone to developing masticatory myositis are listed below.

German Shepherds: German Shepherds are predisposed to masticatory myositis and autoimmune conditions in general.
Retriever Breeds: Canine masticatory muscle myositis is widespread in Retrievers, including Labrador and Golden Retrievers.
Cavalier King Charles Spaniel: MMM is commonly reported in Cavalier King Charles Spaniels despite the condition’s predisposition for large breed dogs.
Doberman Pinschers: Autoimmune conditions, including masticatory myositis, are often diagnosed in Doberman Pinscher breed members.

What does Masticatory Myositis in Dogs look like?

Masticatory myositis in dogs looks like a clenched jaw. The jaw is locked in (trismus), the eyes appear bulging, and saliva hangs from the dog’s mouth. The inflammation of the chewing muscles gives the dog’s face an asymmetrical appearance. Muscle loss in the jaw region is visible in the advanced stages.

What are the Causes of Masticatory Myositis in Dogs?

The causes of masticatory myositis in dogs are listed below.

Flawed Immune System: A faulty immune system is the primary cause of MMM in dogs. The condition starts when the immune system attacks its masticatory muscles.
Genetic Predispositions: MMM is common in German Shepherds, Retrievers, Cavalier King Charles Spaniels, and Doberman Pinschers. The breed predisposition of masticatory myositis suggests a genetic etiology.
Environmental Factors: Environmental factors like exposure to allergens, toxins, and pathogens contribute to MMM. Adverse reactions to medications and vaccinations are other risk factors.

What are the Symptoms of Masticatory Myositis in Dogs?

The symptoms of masticatory myositis in dogs are listed below.

Reluctance to Eat: The inflammation of the chewing muscles locks them, and the dog is unable or in pain when it tries to open its mouth.
Drooling: Dogs with MMM have normal appetites but are unable to eat due to their jaws being locked, resulting in excessive drooling or hypersalivation.
Fever: Increased body temperature is typical for dogs with underlying inflammation.
Muscle Wasting: The inflammation of the chewing muscles progresses to visible atrophy.
Weight Loss: Weight loss in the advanced and chronic stages of MMM is caused by prolonged inability to eat.
Lethargy: The ongoing pain and reduced appetite affect the dog’s energy levels, causing low stamina, increased sleepiness, and disinterest in daily activities.
Exophthalmos: The dog's eyes appear more bulging than usual due to the inflammation and swelling of the muscles beneath the eye socket.
Enlarged Lymph Nodes: Enlarged lymph nodes in the dog’s head and neck region are common masticatory myositis symptoms, especially in advanced cases.

Can Masticatory Myositis in Dogs Come on Suddenly?

Yes, masticatory myositis in dogs can come on suddenly. MMM occurs without warning and develops quickly or starts slowly and progresses gradually.

The condition starts when the dog experiences pain when opening its mouth. The early phases of MMM are non-specific. Pet owners confuse the early signs with other conditions.

When do Masticatory Myositis in Dogs Symptoms usually occur?

Masticatory myositis in dogs symptoms usually occurs at the age of three. MMM has been reported in young puppies and senior dogs.

The initial symptoms of MMM are fever, lethargy, drooling, and reluctance to eat. The early signs are easy to confuse. Muscle shrinkage, inability to open the mouth, and weight loss are striking and develop as the condition progresses.

What are the Risk Factors of Masticatory Myositis in Dogs?

The risk factors for masticatory myositis in dogs are listed below.

Allergens and Toxins: Environmental exposure to airborne allergens and various toxin types is associated with a high risk of autoimmune conditions in dogs.
Meds and Vaccines: Medication and ongoing vaccinations in some dogs increase the risk of MMM, especially if predisposed to autoimmune issues.
Infectious Diseases: Bacterial and viral infectious diseases in dogs trigger the immune system and are linked to autoimmune problems.
Stress: Long-term or frequent stress affects the dog’s quality of life and is a significant risk factor for autoimmune conditions.

Is Masticatory Myositis in Dogs Hereditary?

Yes, masticatory myositis in dogs is hereditary. Masticatory myositis is an autoimmune disease with a pronounced genetic or hereditary component.

Masticatory myositis is widespread in German Shepherds, Labrador and Golden Retrievers, Cavalier King Charles Spaniels, and Doberman Pinschers. Weimaraners, Rottweilers, and other large breeds are at a high risk of MMM.

What are the complications of Masticatory Myositis in dogs?

The complications of masticatory myositis in dogs are listed below.

Muscle Atrophy: Muscle atrophy occurs when muscle tissue wastes away.
Pain and Discomfort: Pain and discomfort are caused by muscle clenching, which occurs when the dog tries to open its mouth.
Malnutrition: Malnutrition is the effect of prolonged reduced food intake and is a complication of MMM in dogs.
Difficulty Drinking: The locked jaw prevents the dog from drinking and increases the risk of dehydration.
Aspiration Pneumonia: Aspiration pneumonia is a potentially life-threatening inhalation of food or liquid and is a severe MMM complication.
Fibrosis: Fibrosis is the excess production of fibrous or connective tissue in response to tissue damage.
Trismus: Trismus is when the jaw muscle gets so tight it prevents normal jaw movement and function.
Psychological Stress: Stress stems from the dog's inability to move its mouth and harms its quality of life.

1. Muscle Atrophy

Muscle atrophy is when the muscle tissue shrinks in size. The muscle tissue is made of fibers. The fibers lose volume and become smaller. The complication develops due to a lack of proper use of the muscles.

Muscle atrophy is known as wasting away and is widespread in MMM dogs. The atrophy of the masticatory muscles is visible when looking at the dog and makes the facial bones appear more prominent.

2. Pain and Discomfort

Pain and discomfort develop when the dog attempts to move its jaws. The muscles clench tight, and simple mouth movements are hard. Pain and discomfort are devastating complications that develop due to the jaw muscles losing their elasticity.

Acute forms of MMM are more painful but short-lasting. Dogs with chronic masticatory myositis are able to move the mouth slightly, but it is uncomfortable.

3. Malnutrition

Malnutrition is defined as the lack of proper nutrition. Dogs are malnourished when they do not eat enough or are unable to absorb nutrients from their food.

MMM locks the dog’s mouth and inhibits food intake. Some dogs with masticatory myositis are able to open their mouths, but it is too painful. Malnutrition contributes to weight loss.

4. Difficulty Drinking

Difficulty drinking is uncomfortable or painful drinking. Dogs with MMM have trouble opening their mouths. MMA causes difficulty swallowing (dysphagia), too.

The inability to drink and swallow causes dehydration and worsens malnutrition. A dehydrated dog is in life-threatening danger.

5. Aspiration Pneumonia

Aspiration pneumonia is the inhalation of food or liquid. The inhaled material causes lung inflammation and infection. Aspiration pneumonia in dogs is potentially fatal.

Dogs with MMM inhale food or liquid accidentally due to loss of swallowing function. The use of semi-liquid diets in dogs with masticatory myositis increases the risk of aspiration pneumonia.

6. Fibrosis

Fibrosis is the formation of fibrous or connective tissue, similar to scarring. It occurs as a response to damage or injury. Dogs with MMM develop fibrosis of the masticatory muscles.

The new fibrous tissue is not as elastic as the original muscle tissue, and the scar tissue impairs normal jaw function.

7. Trismus (Lockjaw)

Trismus refers to a mucle spasm in the temporomandibular joint. The complication is defined as jaw muscle clenching. The muscle clenches tightly and prevents the mouth from opening.

Trismus is a hallmark MMM complication in dogs and is called lockjaw. Lockjaw is stressful and potentially fatal when it impairs the dog’s eating and swallowing ability.

9. Psychological Stress

Psychological stress is the anticipation of danger. Stress in dogs with masticatory myositis arises from the inability to open the jaw.

The locked jaw is confusing to the dog. The dog becomes stressed when the locking persists. Stress is not life-threatening but harms the dog’s life quality and causes mood swings.

Is Masticatory Myositis in dogs contagious to other animals or humans?

No, masticatory myositis in dogs is not contagious to other animals and humans. MMM is an autoimmune condition. Autoimmune diseases in dogs are not transmissible.

Masticatory myositis, however, is described in other species. Humans frequently develop MMM, which is seen in cats but very rarely.

How do Veterinarians Diagnose Masticatory Myositis in Dogs?

Veterinarians diagnose masticatory myositis in dogs based on a specific serum 2M antibody test and muscle biopsy.

The vet takes the dog’s history, performs a physical exam, and considers the clinical signs and symptoms to make a presumptive diagnosis. A definitive diagnosis requires serum testing and muscle biopsy.

The serum test is highly sensitive and detects circulating 2M antibodies. Dogs that are already treated with steroids give false negative results. Muscle biopsy, in such cases, is recommended to confirm the MMM diagnosis.

Where can you seek a diagnosis for Masticatory Myositis in dogs?

You can seek a diagnosis for masticatory myositis in dogs at the veterinarian’s office. General veterinarians refer potential MMM dogs to specialists in severe cases.

Masticatory myositis must be differentiated from other conditions causing similar signs. Potential differential diagnoses include generalized inflammatory myopathies.

Infectious agents, autoimmune issues, and neoplasms cause general inflammatory myopathies in dogs. Making a definitive MMM dog diagnosis is challenging.

How long can a dog survive with Masticatory myositis?

A dog with masticatory myositis can survive for a long time. Properly managed MMM ensures good quality of life and longevity.

“MMM has a good prognosis with relatively quick return to masticatory function,” reports a study, “Treatment Outcome of 22 Dogs With Masticatory Muscle Myositis (1999-2015),” published in the Journal of Veterinary Dentistry in 2018.

The promptness of diagnosis and treatment is the answer for pet owners asking, “How long can a dog live with masticatory myositis?”

The MMM in dogs life expectancy is reduced when most or all muscle tissue is replaced by connective tissue, which happens if the treatment is delayed.

What are the treatments for Masticatory Myositis in dogs?

The treatments for masticatory myositis in dogs are listed below.

Corticosteroids: High corticosteroids (prednisone or dexamethasone) are the standard for MMM treatment. Steroids have serious adverse effects, and long-term use must be carefully monitored.
Immunosuppressants: Azathioprine and cyclosporine are combined with steroids to allow lower steroid doses. Immunosuppressants are given to dogs experiencing side effects from steroids.
Surgery: Surgery to remove the locked portion of the jaw is suggested for dogs with end-stage MMM. Dogs with removed jaw parts lap food with the help of their tongues.
Physical Therapy: Dogs that respond to treatment benefit from physical therapy to keep the muscles active. Encouraging the dog to play with chew toys is an excellent MMM treatment.
Diet Modifications: A semi-liquid diet is recommended for dogs with locked jaws. Severe cases require a gastric feeding tube to administer food directly into the stomach.

Is CBD oil effective for treating pain in dogs with Masticatory Myositis?

Yes, CBD oil is effective for treating pain in dogs with masticatory myositis. CBD (cannabidiol) is not a cure for MMM but is helpful when used correctly and as a part of a multimodal approach.

CBD modulates the immune system and reduces inflammation, helping with the cause and symptoms of masticatory myositis in dogs.

Cannabidiol relaxes the muscles (myorelaxant) and eases pain. A human study, “Myorelaxant Effect of Transdermal Cannabidiol Application in Patients with TMD: A Randomized, Double-Blind Trial,” published in the Journal of Clinical Medicine, shows CBD's myorelaxant effect.

CBD works through the endocannabinoid system in humans and dogs. CBD oil is believed to help relax the masticatory mussels in dogs with MMM.

What is the typical cost range for diagnosing Masticatory Myositis in dogs?

The typical cost range for diagnosing masticatory myositis in dogs is from $150 to over $500. The 2M antibody ELISA test is $146. The price does not include the lab referral fee and the procedures performed before sending the dog’s sample for ELISA testing.

The exact cost of diagnosing masticatory myositis in dogs depends on the region and the vet clinic performing the diagnostic process. Expect to pay several hundreds of dollars for MMM diagnostics.

Can Masticatory Myositis in dogs be cured?

No, masticatory myositis in dogs cannot be cured. Dog MMM is manageable with aggressive treatment. Dogs regain jaw mobility and function within four weeks of starting treatment, but the condition is not entirely curable.

Relapses are common, and each recurrence manifests more severely. Dogs that relapse require continued therapy.

How can Managing the Immune System Help Treat Masticatory Myositis in Dogs?

Managing the immune system can help treat masticatory myositis in dogs in the following ways.

Immunosuppressive Therapy: Immunosuppressive therapy is the mainstream when treating autoimmune conditions. Suppressing the immune system prevents unnecessary overreactions and helps control the MMM symptoms.
Natural Immunomodulation: Immunomodulation is achieved with CBD products made for pets. CBD or cannabidiol has the ability to modify the immune system, preventing overreactions while boosting a dog’s immune system against pathogens.
Inflammation Reduction: Keeping the immune system in check reduces inflammation. Inflammation is a critical component of MMM and autoimmune diseases. Inflammation fuels the MMM symptoms.
Lifestyle Changes: Simple lifestyle changes, like a healthy diet and regular exercise, promote healthy weight in MMM dogs. Excess fat in dogs has a pro-inflammatory effect and avoiding obesity helps prevent unnecessary inflammatory processes.