Masseter Myositis in Dogs

When a dog chews, he uses several groups of muscles in his temples and jaw.

Masseter myositis may also be called masticatory muscle myositis (MMM) or masticatory myositis. Historically, the disease has also been called eosinophilic myositis or atrophic myositis. Masticatory myositis is an autoimmune disorder that affects the muscles that allow a dog to chew. Without early and aggressive treatment, this disease can cause irreversible jaw dysfunction.

Masticatory Muscles

Mastication is another word for chewing. When a dog chews, he uses several groups of muscles in his temples and jaw. The largest muscle in his jaw is called the masseter muscle. Other muscles involved in chewing include the temporalis muscles, the medial and lateral pterygoid muscles and the rostral digastricus muscle. All of these muscles may become inflamed and painful when a dog has masticatory myositis.

Autoimmune Disorder

White blood cells normally attack harmful foreign substances such as bacteria.

"Myositis" means "muscle inflammation." Normal inflammation occurs when white blood cells attack harmful foreign substances such as bacteria and viruses. But sometimes the immune system mistakenly treats healthy body tissue as if it were a threat, causing inflammation in the healthy tissue. This is called an autoimmune disorder.

The Comparative Neuromuscular Laboratory's report on masticatory muscle myositis explains that a dog's chewing muscles contain special molecular structures called type 2M muscle fibers. If a dog has masticatory myositis, the immune system attacks healthy 2M muscle fibers. These attacks destroy the fibers and cause swelling, pain, atrophy and scarring of the masticatory muscles

Symptoms of Masticatory Myositis

A dog with masticatory myositis will have trouble chewing. The jaw and sides of the head may be swollen and painful to the touch. In advanced cases the dog may unable to open his jaw (trismus). The Mar Vista Animal Medical Center recommends using a blood test that checks for 2M antibodies to confirm a diagnoses of masticatory myositis.

Dogs at Risk

Masticatory myositis primarily affects large breeds of dogs.

According to Mar Vista Animal Medical Center, masticatory myositis can occur in any breed of dog, but seems to be more prevalent in large breeds such as German shepherds, Labrador retrievers, golden retrievers, Cavalier King Charles spaniels and Doberman pinschers. The disease also seems to target younger dogs, with three years being the average age of onset.

Treatment for Masticatory Myositis

According to an owner information sheet published by Michigan Veterinary Specialists, dogs with masticatory myositis are treated with large doses of corticosteroids like prednisone or dexamethasone. Very high doses of these drugs will suppress the immune system and stop the damaging attacks upon the masticatory muscles. Once the jaw function has returned to normal, the drug dose is tapered off over a period of four to six months. In some cases, a low dose of the drug is continued indefinitely to prevent reoccurrence of the disease.

References

• Comparative Neuromuscular Laboratory: Masticatory Muscle Myositis
• Mar Vista Animal Medical Center: Masticatory Myositis
• Michigan Veterinary Specialists: Masticatory Myositis

Resources

• VetMed: Peripheral Nerve Diseases
• Medline Plus: Autoimmune Disorders