A generalized disease is induced experimentally in the rat by administration of Freund's adjuvant. The primary clinical and pathologic lesions are arthritis, periarthritis, peritendinitis, and periostitis in the joints of the extremities and tail. Accompanying the arthritis in some cases, and never observed in its absence, are other specific tissue lesions including iridocyclitis, nodular lesions in the glabrous skin (ear, genitalia, feet, tail), transient rashes, a chronic skin disease, genitourinary lesions, and diarrhea. These make up a striking and characteristic picture. The arthritis usually precedes the other lesions and, together with the skin disease may show a prolonged and fluctuating course. Visceral lesions do not occur.

Histologically, the basic lesion is a lymphocytic and histiocytic infiltration, initially perivascular and subsequently more disseminated. In addition, in the region of the joints and in the corpora cavernosa of the penis, there is extensive proliferation of mesenchymal cells, especially fibroblasts. Foci of fibrinoid necrosis are seen in the articular and nodular lesions, and destructive lesions of the joints are common.

Such a combination of tissue lesions has not previously been described in experimental pathology. The experimental disease is shown to have both similarities to and differences from Reiter's syndrome, rheumatoid arthritis, and certain other disorders which occur in man.

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