Eosinophilic myositis (masticatory myositis) is a condition that causes the masticatory muscles in a dog's mouth to become inflamed and irritated. The inflamed muscles prevent a dog from opening its mouth to chew or pant. The disease is manageable with some quality of life modification. Dogs that respond favorably to drug therapy may regain full range motion of the jaw.
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About Masticatory Muscles
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The masticatory muscles are located on the side of the dog's face. Their primary function is to aid the dog in opening and closing his mouth for chewing. Muscle fibers known as 2M support the muscle's function. Maticatory myositis (MM) disease is caused when the dog's immune system begins attacking the 2M muscles fibers inhibiting their function. The exact reason why this occurs is unknown.
Patient Characteristics
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According to Mar Vista Animal Medical Center, a veterinary practice in Los Angeles, California, the average age for dogs with eosinophilic myositis is 3. The condition is more frequently found in German shepherds, Labrador retrievers, Doberman pinschers and certain spaniel breeds.
Symptoms
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Early stage symptoms of dogs with eosinophilic myositis include swollen masticatory muscles. Its eyes may bulge when the muscles behind the eye also swell. The dog may run a fever, and its lymph nodes may be enlarged. As the condition reaches more advanced stages, the dog's jaw tightens, making it nearly impossible for him to open its mouth. It's not usually until this point that the dog's owner notices the problem.
Diagnosis
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The veterinarian will examine the dog's mouth to ensure a broken tooth or dislocated jaw isn't the cause of symptoms. In more-severe cases, a general anesthesia may be required to sedate the dog so the veterinarian can open his mouth. A diagnosis of eosinophilic myositis is possible with a blood test to detect the presence of 2M antibodies that the veterinarian sends to a lab for evaluation. The veterinarian may conduct a biopsy of the muscle to check for possible scarring that may impact treatment.
Treatment
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The veterinarian will provide the dog high doses of anti-inflammatory medication to relieve swelling and block his immune system from attacking his 2M muscle fibers. Prednisone, dexamethasone and azathioprine are three prescription medications a veterinarian may prescribe. Once the dog's jaw opens normally, usually within two weeks, Michigan Veterinary Specialists recommends cutting back the dosage gradually over the next four to six months. Regular blood tests may be needed to monitor the effects of the drugs on the dog's body. Bladder infections caused from the medications are possible.
Eosinophilic myositis impacts only the masticatory muscles. No other muscles are affected by the disease. In addition, when the disease is diagnosed in its early stages, the prognosis is good. Full and partial range of motion of the jaw is possible.
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