An acute arthritis is followed in 6 to 8 months by the development of chronic polyarthritis when young Yorkshire swine are given a single intraperitoneal inoculation of Mycoplasma hyorhinis. Sequential antibody studies, using the metabolic inhibition test, revealed a brisk response in infected animals followed by sustained elevations of titers. Synovial fluid titers were found to exceed those of serum, suggesting local antibody production. The organism, readily recovered in the acute phase, became progressively more difficult to isolate.

A Mycoplasma pulmonis strain, recovered from the arthritic joints of mice employed in the serial passage of a chemically induced tumor, was found to be arthritogenic for mice under experimental conditions. Some joint involvement occurred in all mice challenged intravenously with this strain, and M. pulmonis was recovered frequently from the enlarged joints. The arthritis was migratory, appearing first in the radiocarpal joints and later in the tibiotarsal joints. There was little evidence of a generalized mycoplasmal infection as a consequence of the experimental challenge. Histopathologically, the early stages of the infection in the joints was characterized by an inflammatory response in the synovium and periarticular tissues. Exudate in the joint space contained about equal numbers of polymorphonuclear and mononuclear cells. The polyarthritis resolved slowly, but some residual joint enlargement was noted for as long as 4 months. Two other M. pulmonis strains were also observed to be arthritogenic for mice. Rats were not susceptible to M. pulmonis challenge. Characteristics of the nonsuppurative M. pulmonis arthritis in mice were compared to M. arthritidis joint infections in rats.

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Mycoplasrna hyorhinis arthritis, induced in young Yorkshire swine, can be considered as a three-stage process. The first 3 weeks of the disease were characterized by a severe polyserositis and mild synovitis. In the second stage, mononuclear cell infiltration of the synovium and other tissues was apparent. In the final stage, beginning about 3 months after inoculation, lymphocytic aggregates had become nodular, synovial hypertrophy was pronounced, and bone and cartilage destruction by pannus was common. Continued activity was apparent to Day 224.The polyarthritis which develops after the intraperitoneal inoculation of Mycoplasma hyorhinis into young Yorkshire swine is described in the accompanying report (1). That pilot study was undertaken in order to evaluate more chronic disease than has been previously reported (2, 3) and to compare the observed findings to rheumatoid arthritis.The key features of such comparisons are the histopathologic changes, especially in the articular tissue, and the duration of active disease contrasting a potentially selflimiting mycoplasmal infection to the life- long nature of rheumatoid arthritis. This report details the evolution of the gross and histologic findings up to Day 224 of the experimental illness and points up certain features pertinent to its chronicity. MATERIALS AND METHODSThe animals studied, the experimental inocula used, and the circumstances of the study are described in the accompanying paper (1).At preselected intervals, or at times required by life threatening illness, control or infected animals were anesthetized with intravenous sodium pentobarbital and were sacrificed by carotid artery bleeding. In human anatomic terms, the shoulders, elbows, wrists, hips, knees and ankles were opened and examined grossly. Specimens for histology were taken from each of the 12 joints. Tissue was also obtained from a number of viscera and from the central nervous system. Specimens for histologic examinations were fixed in 10% buffered formalin and stained with hematoxylin and eosin. Selected tissues were also stained with toluidine blue, safranin 0-fast green, phosphotungstic acid-hematoxylin, Masson and Gram stain.In order to compare synovial specimens, a histologic grading scheme was devised. Seven major categories were evaluated: (a) presence of a fibrin and inflammatory cell exudate, primarily polymorpho- 202