Mucosal responses in the skin and gill filaments of Atlantic salmon ( Salmo salar ) infected by Neoparamoeba perurans

Gill health is crucial for maintaining respiratory and immunological functions in fish, yet it is often compromised by parasitic infections and treatments. This study investigates the effects of amoebic gill disease (AGD) and peracetic acid (PAA) treatment on mucous cell (MC) parameters in the gill filaments of Atlantic salmon ( Salmo salar ). Although exposure to AGD, with or without PAA treatment, did not cause changes in the mucosal parameters of the skin in Atlantic salmon, there was an increase in the mucosal parameters in the gill filaments 24 h post-treatment and a significant increase four weeks post-treatment. These findings suggest that while AGD and PAA treatment do not affect skin mucosa, they disrupt mucosal homeostasis in the gill filaments as well as in the gill lamellae. In AGD-affected fish, hyperplasia and hypertrophy of MCs in both gill lamellae and filaments increase the diffusion distance across the water-blood barrier, leading to respiratory disturbances.