Masseter Myositis in Dogs

If your canine companion avoids his food, has difficulty chewing or can’t open his mouth, masseter myositis could be the cause. Commonly known as masticatory or eosinophilic myositis, it's an immune-mediated disease that causes inflammation in the masseter muscles, or those responsible for chewing. This condition comes in both acute and chronic forms, with certain breeds showing hereditary connections or predispositions. While masticatory myositis is treatable and often responds well, the chances of a positive prognosis depend on when treatment starts.

With an immune-mediated condition, the immune system essentially goes into overdrive, attacking cells in the body. In the case of masticatory myositis, the immune system targets 2M muscle fibers. Unlike other immune-mediated conditions that can affect the entire body, the masseter muscles are the only ones in the body with these 2M fibers. This keeps the condition isolated to the chewing muscles.

In an acute case of masticatory myositis, you may notice swelling in the masseter muscles. The swelling can create pressure behind the eye, leading to eye protrusion. You may notice your dog does not want to eat, drools excessively or has difficulty opening his mouth. Other symptoms can include fever, enlarged lymph nodes and tonsil inflammation. If you see any of these symptoms, consult your veterinarian. Early diagnosis with a 2M antibody blood test allows for early treatment and reduces the risk of progression to chronic masticatory myositis.

Cases of chronic masticatory myositis can occur after treated acute bouts or come about without any previous history. Symptoms of chronic cases are more severe, leading to a complete inability to open the mouth. With chronic masticatory myositis, the masseter muscles atrophy and are replaced with scar tissue. You may notice your dog has a sunken facial appearance.

While any breed can suffer from masticatory myositis, young large-breed dogs have the greatest risk. Specific breeds that show a hereditary or predisposed risk include German shepherds, Labrador retrievers, Doberman pinschers, golden retrievers and cavalier King Charles spaniels.

Treatment for masticatory myositis begins with drugs that suppress the immune system. The corticosteroid drugs commonly used include prednisone and dexamethasone. These medications are administered at high doses for at least six months, or until the muscles return to normal. Your veterinarian will then gradually decrease the dose of corticosteroids. Never stop treatment abruptly.

If this medication is not effective, chemotherapy medications may be necessary to suppress the immune system. The prognosis depends on how soon treatment begins. If scar tissue is already present, medications may have little effect. Liquid diets are often necessary during recovery to provide nutrition. Never try to pry open your dog’s mouth, but rather encourage chewing with toys and treats as the muscles start to heal.