Ectoparasite infections represent a significant challenge in fish aquaculture, with the fish skin immune system serving as the frontline defense and comprising multiple mechanisms to protect against microbial invasion. Incorporating health-promoting additives as a preventive approach has enhanced skin mucus defense mechanisms and reduced parasite prevalence. This study aimed to examine the modulatory effects of a phytobiotic-based additive, APEX® BRANCHIA (Adisseo), on fish skin defense against ectoparasitic diseases- The Guppy-Gyrodactylus infection model and a shotgun proteomics approach were applied to evaluate the additive’s impact on fish skin.
Material and methods
Forty individually housed guppies (Poecilia reticulata ) were initially provided with a diet containing the functional additive for 14 days. Subsequently, each guppy was inoculated with two Gyrodactylus turnbulli parasites, and the additive-supplemented feed was continued for an additional 17 days. Parasite numbers on each fish were assessed every 48 hours, and based on infection susceptibility, fish were classified into responsive, resistant, and susceptible groups. Skin samples were collected on days 13 and 17, corresponding with peak immune activities and the end of infections, respectively. Differentially expressed proteins in the skin tissue among the various susceptibility groups were identified using nano LC-MS/MS.
This analysis elucidated the mode of action of APEX® BRANCHIA in relation to fish response to infection, revealing two primary strategies for combating parasite infection. The first strategy involved skin cornification in susceptible fish, wherein parasite burdens peaked on day 13 and subsequently declined sharply by day 17. Cornification is a programmed cell death process generating a dense barrier of dead cells, effectively impeding parasite entry. The second strategy was employed by resistant fish, which consistently displayed low parasite levels throughout the study. In these fish, the additive was found to activate the complement system, initiating a series of proteolytic events that facilitated the elimination of invading pathogens. In conclusion, the dual protection mechanism delivered by APEX® BRANCHIA is likely a key mechanism to reduce the severity of infection observed in field testing.