The emergence of a severe infection in albino rats during cortisone administration is reported. Evidence is present that the causative agent of the disease was Corynebacterium pseudotuberculosis murium, a microbial species not demonstrably a part of the usual bacterial flora of the host. It has been possible to reproduce the disease in rats by a relatively large infecting inoculum of this strain of Corynebacterium, but the susceptibility of normal rats to infection has been found to be low.

The disease occurred in 41 of 50 rats given cortisone and in 28 of 30 instances in which isolation of the etiologic agent was attempted, this strain of Corynebacterium was recovered. The disease was characterized by widespread necrotizing lesions, with multiple coalescent lesions occurring in the lungs, and similar, though smaller lesions of the pericardium, pleurae, liver, and kidneys. In its gross appearance the pulmonary disease was similar to that of a disseminated, far advanced tuberculous process. The histologic appearance of the lesions, however, was sufficiently distinctive so that they could be readily differentiated from the lesions characteristically produced by Mycobacterium tuberculosis. Moreover, the host-parasite relationship established by the experimental infection with avian tubercle bacilli, was not markedly altered by the factors which led to the emergence of the pseudotuberculosis.

In contrast to the high frequency of pseudotuberculosis in rats given cortisone, no instance of this disease has been encountered in a similar group of animals given large quantities of corticotropin.

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