Fish skin acts as the first line of defense against pathogen invasion. Evidence suggests that skin defenses can be improved by preventive supplementation of health additives. In this study, the mechanisms of a phytobiotic-based additive (APEX®, Adisseo) in supporting fish skin health were investigated using the Guppy-Gyrodactylus infection model along with a shotgun proteomics approach. Guppies were fed a diet supplemented with the additive for 14 days, after which each fish was infected with Gyrodactylus turnbulli and continued to receive the additive for 17 days. The number of parasites in each fish was counted every 48 hours, and fish were classified based on their susceptibility to infection. Skin samples were collected on days 13 and 17, corresponding to the peak and the end of the infection, respectively. Protein identification and quantification of skin were conducted using nano LC-MS/MS. The differential expression of proteins regulated by additive supplementation in both responsive and resistant fish was assessed to determine the phytobiotic’s mode of action in response to infection.
Two primary mechanisms for combating parasite infection were revealed. In responsive fish, the additive induces skin cornification at the peak of infection. However, in resistant fish, the additive activates the complement system. Overall, the results suggest that phytobiotic-based additives can enhance defense mechanisms against ectoparasite infection by strengthening physical barriers and immunocompetence. These findings provide insights into the dual protection effect of the additive, confirming its effectiveness as a preventive strategy to help fish deal with skin infections.