Synovial pathologic changes in spontaneous canine rheumatoid‐like arthritis

The synovial fluid and membrane were studied in 10 dogs meeting the American Rheumatism Association criteria for classic human rheumatoid arthritis (RA). Light microscopic pathologic features were consistent with those found in the human disease. Neutrophilic infiltration of synovium was somewhat more prominent than in chronic human RA, and activated lymphocytes in fluid or membrane were less frequent. The proliferative and plasma cell reaction seemed identical. Electron microscopy (EM) suggested microvascular injury with findings which included electron dense deposits in the vessel walls of 2 dogs. Seven dogs had meshworks of 20--25 nm tubules in tubuloreticular structures (TRS) similar to those seen in human systemic lupus erythematosus and only occasionally in human RA. There were also crystalline arrays of tubules, a configuration previously reported in tumors and virus infections and possibly suggestive of a cellular reaction to virus infection. To date no initiating agent has been identified, but this spontaneous canine disease which is very similar to human RA can provide a valuable model in which to examine pathogenesis of chronic arthritis.