Observations of pathology in Atlantic salmon (Salmo salar L.) in association with three subsequent mechanical delousing treatments

Mechanical delousing methods are widely used in Atlantic salmon aquaculture, however, the effects of these treatments on fish welfare remain underexplored. This study followed a commercial production of Atlantic salmon that underwent three subsequent mechanical delousing treatments. The study focused on pathological changes in the gills, thymus, nasal cavity, skin, and hearts of the fish. The sampling was done every sixth week over a period of 8 months. Additionally, the study investigated changes in growth and mortality and the expression of specific immune and stress genes in the gills and thymus using qPCR analysis. Comparison of data from samplings before vs. after delousing showed reduced specific growth rate, increased mortality, more external injuries, cellular stress in the gills, and pathological changes in the exposed organs after all treatments. The immunohistochemistry showed that the number of PCNA-positive lymphocyte-like cells increased after each treatment, and especially after the third. At this sampling, we also observed degeneration and necrosis of pillar cells, and an increase in the number of mucus cells in the gills. However, the data does not allow us to determine whether these additional changes after the third treatment were due to a gradual weakening of the fish from repeated treatments or other co-correlating factors. An interesting observation near the end of the study was a significant increase in PCNA levels in the thymus and that the study of T cell markers showed a simultaneous increase in TCRδ and CD28. • Repeated mechanical delousing reduced growth and increased post-treatment mortality. • Mild to moderate pathological changes were observed in gill, thymus, and nasal cavity post-treatment. • Repeated mechanical delousing affected the cellular composition and increased thymus cell proliferation.