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 to dissect the whole dermis away from the subcutis to produce large fluid-filled cavitations or blisters.

Dermal lesions may occur in the absence of detectable lesions elsewhere in the body, including the kidney, and they resolve after spawning. This time-limited susceptibility illustrates the immunological changes seen in the skin at spawning.

The cellular response involves mainly neutrophils and macrophages and is a good example of chronic-active dermatitis.

At gross pathology, in the initial phases of this disease, small elevations of the scales are observed with subsequent inversion in their position and detachment. Subsequently, ulceration and haemorrhage of the integument occur.

REFERENCES

• Benediktsdóttir, E., Helgason, S., & Gudmundsdóttir, S. (1991). Incubation time for the cultivation of Renibacterium salmoninarum from Atlantic salmon, Salmo salar L., broodfish. Journal of fish diseases, 14(1), 97-102.
• Ferguson, H. W., 2006, Systemic Pathology of Fish, London, UK, Scottian Press.