MUCOSAL MAPPING OF SALMON'S SKIN, GILLS AND ESOPHAGUS REACTION TO HYDROGEN PEROXIDE TREATMENT

Sea lice infestation has caused major economic losses for the salmon industry imparting a negative review from the public because of its adverse effect on the welfare and environment (Boxaspen et al., 2006). Hydrogen peroxide (H₂O₂) is one of several treatments used to control sea lice infection in Atlantic salmon, Salmo salar, production although acts on the slimy barrier tissues of all fish  . The barrier tissues of fish (skin, gills and guts) have mucosal epithelia (slime producing), which have been shown to play an important role in the defence against pathogens in Atlantic salmon (Svendsen and Bogwald, 1997). This study explores the effect of hydrogen peroxide treatment on the mucosal barriers of skin, gills and esophagus in commercial salmon farming using mucosal mapping and determines the recovery time from treatment. Mucosal mapping (Pittman et al., 2013) has been shown to allow direct comparison of volumetric size data and ratios of epithelia to mucous cell to indicate enhanced immunity dysregulation in the gut due to diet (Torrecillas et al., 2015), exposure to infectious agents in the gills and responses to parasites in the skin. Samples of commercially farmed Atlantic salmon (mean length 44 cm)  in seacages were taken prior to large scale H2O2 treatment (Day -1), Day 4, Day 11 and Day 18.

Samples were taken from standardized body sites of dorsolateral skin and caudal ventral, second gill arch and esophagus, fixed in formalin, decalcified, embedded in Technovit, sectioned, stained with Periodic Acid Schiff and Alcian Blue for unequivocal marking of the mucous cells, and analyzed for mucous cell area and density, mucous cell area:density ratio and filament:lamella ratio for gills.  The results show that different fish body regions exhibit different cellular architecture where mucous cell size and density varied significantly across the body sites with significant differences between groups. It takes at least two weeks for Atlantic salmon mucosal epithelia to recover after delousing. Mucosal Mapping may detect weaknesses, which are undetectable by traditional histopathology.   It provides an unbiased reference to understand the effect of hydrogen peroxide on mucosal barriers of salmonids and the corresponding recovery time. Patterns in mucosal epithelia are immunologically important, objectively reproducible and comparable across time, species and treatment may detect weaknesses, which are undetectable using traditional histopathology.