NEUROMUSCULAR CASE OF THE MONTH - JULY 2005

Masticatory muscle myositis in a young male Cavalier King Charles Spaniel
Contributed by Dr. Dana Graham
Caring Hands Pet Hospital
Indianapolis, IN 46236

Clinical History
The pup was born 3/15/05. An initial DHPP vaccination was given by the breeder, and booster DHPP and intranasal Bordatella vaccines were given at 9 weeks of age. At that time, physical examination was with normal limits with the exception of a small, reducible umbilical hernia. The dog was also dewormed and placed on preventative treatment for fleas. Ten days following the second vaccination, the dog presented for lethargy and anorexia.

Physical and Neurological Examination

A change in facial features was noted with swelling of the temporomandibular muscles and jaw pain. The dog was unable to open the jaws more than 1-1.5 cm. Mild serous exudate was present from both nostrils, and there was mild ocular discharge and conjunctivitis. The submandibular lymph nodes were also enlarged. The differential diagnosis included acute masticatory muscle myositis or craniomandibular osteopathy. Treatment was initiated with a non-steroidal anti-inflammatory (4 mg/kg SID) and clavamox (22 mg/kg BID). Since there was no improvement 24 hours later, prednisolone (1.5 mg/kg BID) was added.  

On re-evaluation two days later, the dog was brighter with decreased swelling of the masticatory muscles. The owner described that he was starting to play with chew toys and eating gruel. The jaw was easier to open and lymph nodes were decreased in size. At one week following initiation of prednisolone, the dog developed diarrhea. Coccidia sp was identified and the dog treated. Ten days later masticatory muscle atrophy was noted and the owner, in spite of recommendations to continue long term prednisone therapy, decreased the prednisolone dosage to 2 mg/kg SID, and weaned the dog off the prednisolone following only a three week course of treatment. Three weeks later, there was reoccurrence of masticatory muscle swelling, decreased jaw mobility, and enlargement of the submandibular lymph nodes.

Serum creatine kinase concentration 668 IU/L (reference 59-895)

Serum 2M antibody titer positive 1:4000 by ELISA (reference <1:100)

Diagnosis
The positive 2M antibody titer confirmed the diagnosis of masticatory muscle myositis and prednisolone therapy was again prescribed. The dog again responded to treatment and is now on an extended course with slow taper over a 4 to 6 month period of time. Relapse of clinical signs is a common occurrence in this myopathy if treatment is not continued for an adequate period of time. A female, full litter-mate was also noted to have had a “vaccine reaction”.

Conclusion by Dr. Diane Shelton
In a search of the database of the Comparative Neuromuscular Laboratory over the years 2001-2005, masticatory muscle myositis was confirmed in 11 young Cavalier King Charles Spaniels with onset at less than 6 months of age. In several cases, onset followed within 10 days of vaccinations. Although long-term information is not yet complete, there was resolution of clinical signs of masticatory muscle myositis in most cases. Of interest, other complications such as early onset hypothyroidism and allergies were reported. Additional information will be provided as more is learned about this early-onset masticatory muscle myositis in this breed.

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