Spawning Rash – Gross Pathology

Rainbow trout, multi-focal granulomatous dermatitis

The bacterium Renibacterium salmoninarum is a small (~ 1.0 μm), intracellular, non-motile diplobacillus, gram +, that is slow growing and a fastidious pathogen. The external gross findings on the skin could be a good example of chronic-active dermal o subdermal lesion associated with R. salmoninarum. “Spawning rash” can be observed in adults, particularly at or around spawning time. Lesions may be largely dermal, often centering round scale-pockets to produce multifocal chronic-active dermatitis. Alternatively the response, mainly in the superficial muscle, may be so severe as...