A migratory polyarthritis occurred in over half of the affected animals, and cardiac involvement of varying intensity was present in all. Histologically, the arthritis was characterized by a mononuclear synovial inflammatory reaction. In the heart, the valves and myocardium demonstrated a similar type of response. Cultures of involved joints and hearts for pyogens and pleuropneumonia-like organisms were negative.

The lymphoid tissues showed progressive changes in three stages: (a) disappearance of follicles; (b) increase in plasma-like cells and histiocytes; and (c) fibrosis. The second stage tended to coincide with the development of the polyarthritis. The kidney showed mild interstitial mononuclear infiltration.

The pathologic changes described appear to be a consequence of reaction between foreign immunologically competent cells and host antigens, but the possibility that these lesions represent a response to antigens derived from unknown infectious agents is, however, not ruled out.