Histopathological Study on Vibriosis of the Salmonids

Vibrio sp. RT group infection had broken out among the salmonids in autumn 1974. Histopathological observation was made on 15 diseased rainbow trout yearlings and 5 diseased amago salmon yearlings. Diseased fish showed a small boil or erosive lesions in the body surface or/and in the head in early stages but advanced cases showed large ulcerative lesions in the body surface or/and in the head, exophthalmos and enlarged spleen.Histopathologically an early lesion was formed in the skin in which considerable bacterial multiplication, edematous dissociation and infiltration of inflammatory cells were observed in the dermal loose connective tissue. In the extended ulcerative lesions bacteria spread extensively through the lateral musculatures and made many large colonies in the interstitial tissues. In these lesions dermal necrosis associated with epithelial slough and considerable edema, slight hemorrhages, infiltration of inflammatory cells, degeneration or necrosis of muscle fibers in the affected musculature were observed. In the lesion formed in the head bacteria diffusely spread through subcutaneous connective tissue and adipose tissue and edema, slight hemorrhage, infiltration of inflammatory cells and tissues necrosis were caused. In an eye ball showing exophthalmos, which was usually associated with lesions formed in the head, marked dilatation of coroid capillaries, detachment of the retina accompanied by necrosis of the pigmented layer, iris hemorrhage and degeneration of the conea were observed. In cases of systemic infection metastatic lesions appeared in the heart and spleen, and bacterial emboli were caused in sinusoids of the liver and renal hematopoietic tissue and in lamellae capillaries.