German Shepherd Dogs Among Breeds Affected by Masticatory Muscle Myositis

Swollen, painful masticatory (chewing) muscles and an inability to open the mouth are clinical signs of the rare disorder masticatory muscle myositis (MMM). Although the condition can occur in any breed, it occurs more commonly in large breeds, such as German Shepherd Dogs, Doberman Pinschers, Golden Retrievers, and Labrador Retrievers.

The average age of onset is 3 years, though puppies have been affected as well. Fortunately, if MMM is diagnosed early, dogs can be treated to increase the likelihood of a full recovery. Research of this disease at the University of California-San Diego led to the development of a blood test in 2004 that detects the presence of 2M antibodies and accurately identifies affected dogs.

Pathologist Diane Shelton, D.V.M., Ph.D., DACVIM, and her team found that type 2M muscle fibers are not present in any other muscle group and antibodies against this fiber type are not involved in any other muscle disease. “Masticatory muscle myositis is an inflammatory myopathy,” Shelton explains. “It is a unique myopathy, or muscle disease, in which dogs commonly have jaw pain and inability to open the jaw. The autoimmune process in this disease involves circulating antibodies that specifically target the masticatory muscles.”

Genes play a role in increasing susceptibility to autoimmune diseases, but MMM is likely impacted by hormonal, environmental and other unknown factors. The genes responsible for a dog’s susceptibility to autoimmune diseases are part of the major histocompatibility complex, which includes autoimmune thyroiditis, the underlying cause of canine hypothyroidism.

Signs of the disorder include an inability to open the jaw, jaw pain, swelling or atrophy of the jaw muscles, difficulty eating and drinking, reluctance to play with toys, and sunken or protruding eyes. Owners who recognize these signs should seek prompt veterinary care.

The most definitive confirmation of MMM is the 2M antibody test, which detects the antibodies that attach and destroy the 2M muscle fibers. The test is available to veterinarians through the Comparative Neuromuscular Laboratory at the University of California at San Diego. For information, visit http://medicine.ucsd.edu/vet_neuromuscular

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