Clinical History
A 2 year old FS Labrador Retriever presented for a one week history of depression, lethargy, decreased appetite and abnormal gait.  Physical examination by the referring veterinarian was normal with the exception of pelvic limb weakness. A chemistry profile revealed elevated ALT (2326 U/L, reference 12-118), increased AST (4292 IU/L, reference 15-66) and elevated creatine kinase (CK; 39,605 IU/L, reference 59-895).

Physical Examination
On presentation, physical examination, temperature, pulse and respiratory rate were normal. The neurological examination including mentation, postural reactions, spinal reflexes and cranial nerve evaluation was also normal.  Muscle pain was not evident. Mild pelvic limb weakness was noted with fine muscle tremors in the musculature.  Neck movement was restricted. When walking, a decrease in flexion of the stifles was noted giving a paddling appearance with the pelvic limbs (Go to video clip). 

Diagnostic Testing
CBC - Normal
Serum Chemistry Panel – Elevated ALT (2272 U/L, reference 12-108), elevated CK (25,284 U/L, reference 64-350)
Urinalysis – SG 1.006, pH 6.5, +1  protein, large amount of blood, <10 RBC/hpf
Antibody titer for Toxoplasma gondii – IgM 1:512  and  IgG – negative
Antibody titer for Neospora caninum – IgG – negative
Tick titer panel – E. canis – negative,  RMSF – negative,  Lymes disease – negative
PCR for Hepatozoon  americanun – negative

Electrodiagnostics – No significant abnormalities were found on electromyography and the motor nerve conduction studies (ulnar and sciatic nerves) were within the reference range.

Muscle biopsy – Moderate to severe inflammatory myopathy (myositis)  with myonecrosis and  fibrosis in the  triceps brachii and biceps femoris muscles.  No organisms were identified  in any of the  biopsy sections. Immunohistochemical staining  for Toxoplasma spp.was unremarkable.

Treatment
Treatment was initiated with trimethoprim sulfa at 15 mg/kg PO BID for 3 weeks. The owner reported that the gait and range of motion of the neck were markedly improved. The lethargy and decreased appetite resolved.

Conclusion
Recheck examination was performed by the referring veterinarian at 3 weeks following completion of trimethoprim sulfa therapy. At that time the convalescent antibody titer for Toxoplasma gondii  IgM was 1:16 and IgG was  negative.  Serum chemistry profile revealed a reduced ALT and serum CK activity was normal.  The assumption is that this was a case of Toxoplasma myositis.  An antibody titer of 1:512 may be dismissed since it can be found in dogs with little suspicion for Toxoplasma infection.  However, the favorable response to antibiotic  therapy without corticosteroid treatment suggests that an infectious cause of myositis was the  most likely diagnosis.