Type I immune-mediated polyarthritis with azathioprine therapy in a Shih-tzu dog

A 2-month-old female Shih-tzu dog was referred because of lameness, exercise intolerance, depression, elbow and stifle joint swelling. Physical examination, complete blood counts, serum-chemistry, radiography, synovial fluid analysis, antinuclear antibody test, and rheumatoid factor measurement were initiated. On radiography, soft tissue swelling of elbow and stifle joints without erosiveness were founded. The results of synovial fluid analysis revealed severe neutrophilic pleocytosis (nondegenerative), decreased viscosity, increased turbidity, positive on mucin-clot test, and negative on bacterial culture. The results of rheumatoid factor measurement and antinuclear antibody test were negative and below 1 : 40, respectively. Based on all tests, we diagnosed this case as juvenile onset type I immune-mediated polyarthritis. Azathioprine (1 mg/kg body weight, per os q 24 h, for 4 weeks) was then administered and clinical signs improved gradually. Four weeks after azathioprine administration, clinical signs were disappeared. This report describes the clinical findings, imaging characteristics, synovial fluid findings, and other laboratory results of type I immune-mediated polyarthritis and successful management with azathioprine therapy.