Aquaculture is providing increasing proportion of fish for human consumption, due to the decline of wild stocks. During the production cycle, fish are regularly subjected to various stressors due to farming practices and their effect in the context of a disease outbreak remains unknown . Our experiment evaluated the effects of unpredictable repeated stress in rainbow trout challenged with the ciliate Ichthyophthirius multifiliis, causing white spot disease in freshwater fish.
Prior to the pathogen exposure, fish were stressed once a day and post pathogen exposure twice a day with a rotation of three different stressors (chasing, air exposure and transfer). At 7 d ays p ost infection (dpi) , the parasite burden was evaluated in fish and in the tank water (e nvironmental DNA), and the local (gill) and systemic (spleen) immune response was investigated. The fish mortality was recorded from 0 to 12 dpi when all the fish from both infected groups died.
Mortality was partially delayed for the fish subjected to stress (Figure 1A) . There was no statistical difference in parasite burden between the stressed and unstressed infected fish groups. The immune gene expression analysis suggested an organ-dependent bimodal immune response. In spleen, a type I immune response was initiated whereas in gill , it was a type II immune response . The unpredictable repeated stress induced mainly upregulations of immune genes (e.g. cat-1 , hep , il-10) in gill and downregulations (e.g. il-2 , il-4/13a , il-8) in spleen (Figure 1B). Our results suggested that the stress protocol did not immunocompromise the fish against Ichthyophthirius multifiliis.