The autoimmune masticatory myositis is, indeed, the most likely cause of the inability to fully open the mouth. Don't hesitate to have the serum type 2M fiber antibody titer blood test performed. Antibodies against type 2M fibers are detected in serum of 81% of dogs with masticatory myositis. It's important that blood be drawn prior to instituting prednisone. Supportive evidence of the acute form of this disease includes decreased activity, lethargy, fever, regional lyphadenopathy (swollen lymph nodes), dysphagia, reluctance to eat, weight loss, drooling, change in bark (more high pitched), and pain on yawning or when grabbing toys.
There are quite a few differential diagnoses, however. They include maxillofacial trauma, temperomandibular joint disease (TMJ); bone and soft-tissue neoplasia (rare in a 6 month old), foreign body (a plant awn, e.g.) penetration caudal to the last maxillary molar, tonsillar region, pharyngeal wall, and sublingual tissues; ocular disease and space-occupying orbital/retrobulbar lesions; ear diseases, other inflammatory muscle disorders, and tetanus.
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