EFFECTS OF UNPREDICTABLE REPEATED STRESS ON RAINBOW TROUT (Oncorhynchus mykiss) IMMUNE RESPONSE AGAINST THE FISH PARASITE Ichthyophthirius multifiliis

Aquaculture is providing increasing proportion of fish for human consumption, due to wild stock limits. 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 is remains unknown. Our experiment evaluated the effects of unpredictable repeated stress in rainbow trout challenged with the ciliate Ichthyophthirius multifiliis, known to cause white spot disease in freshwater fish.
Prior and post the pathogen exposure, fish were stressed with a rotation of three different stressors (chasing, air exposure and transfer). At 7 days post infection (dpi), the parasite burden was evaluated in fish and the tank (water environmental DNA), and the local and systemic immune response was investigated in respectively gill and spleen. The fish mortality was recorded from 0 to 12 dpi when all the fish from the infected groups died.
Mortality was partially delayed for the fish subjected to stress. There was no statistical difference in parasite burden between the two 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. The unpredictable repeated stress has 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 immunocompromised the fish. In addition, the study also demonstrated that more work on Ichthyophthirius multifiliis is needed to fully understand virulence differences between isolates.