Since the extensive studies of DUNKIN and LAIDLAW (1926), many virological and immunological studies using ferrets have been carried out on canine distemper. On the pathology of ferrets infected with canine distemper, however, few studies have been carried out, although a great deal has been studied pathologically on dogs infected with canine distemper. In addition, it seems that the viral strains used by some workers before 1948 were insufficient immunologically. The present studies were undertaken to clarify the characters of the lesions of ferrets infected with canine distemper by inoculation with such canine distemper virus as immunologically and internationally recognized. The four viral strains used in the present studies are shown in Table 1. The experimental animals used consisted of 36 ferrets which had died of canine distemper or had been sacrificed in the moribund period following the virus inoculation and 6 ferrets which had succumbed of any other illness. An outline of these experimental ferrets is given in Table 1. The severity and distribution of the main histological lesions in all ferrets are shown in Tables 4 to 9. The histological findings in ferrets infected with experimental canine distemper were summarized as follows. Acidophilic cytoplasmic inclusion bodies were found abundantly in epithelial cells of the urinary and the digestive tracts and the excretory ducts, reticulo-endothelial cells of the whole body, and the Malpighian layer of the epidermis of of the skin (Table 3). Fatty change of hepatic cells in the center of the lobule and swelling and proliferation of reticulo-endothelial cells in the liver, numerical reductive atrophy of the lymphatic cells and swelling and proliferation of reticulo-endothelial cells in the spleen and the lymphatic tissues, pneumonitis of alveolar septa of varying severity occurring in the lungs, degeneration of nerve cells in the central nervous system, hyperplasia of the Malpighian layer, and hyperkeratosis and parakeratosis of the horny layer of the skin were found characteristically. Moreover, the common histological changes found in all ferrets and related to no canine distemper infection were summarized as follows. Cellular hyperplasia complicated with myeloid metaplasia in the red pulp of the spleen and the appearance of fatty foam cells and, furthermore, cholesterol crystals in the subpleural alveoli accompanied by the following organization with giant cells were found remarkably. The autopsy findings were summarized as follows. In ferrets infected with canine distember, cloudy swelling and fatty change in the liver, obscurity of the follicles in the spleen, thickening and erythema or eczema of the skin (particularly at the lips, the nose, the eyelids, the hypogastric region and the pads) were remarkable. In all ferrets, including. those infected with canine distemper, enlargement of the spleen (Table 2) and the appearance of a number of white subpleural spots in the lungs were characteristic.

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