An integrated approach to evaluate the immunomodulatory effect of functional feed on rainbow trout (Oncorhynchus mykiss) during an A. salmonicida outbreak.

Different strategies have been proposed to boost the resistance of fish against Aeromonas salmonicida, one of the major pathogens of salmonids. This study conceived an integrated approach to evaluate the immunomodulatory effects of two commercial feeds (Protec and Optiline, Skretting) on rainbow trout during an Aeromonas salmonicida outbreak that caused high mortality in a fish farm. Welfare indicators, biometric, autoptic, and haematology indices, and innate immunity parameters (leukocyte viability, reactive oxygen species (ROS) production, and phagocytosis) were evaluated in fish fed with either Protec (containing β-glucans, vitamin C, E, and zinc) or a control diet (Optiline). Trout fed with Protec showed a higher proportion of blood lymphocytes (4.58 vs. 3.35; p < 0.05) and thrombocytes (2.21 vs. 1.54; p < 0.05), and a lower number of neutrophils (1.83 vs. 2.54; p < 0.05). Leukocytes isolated from individuals fed with Protec showed significantly higher viability (53.73 vs. 42.59; p < 0.01), lower apoptosis (29.61 vs. 36.49; p < 0.05), and lower death (10.99 vs. 20.38; p < 0.05), parallelled by lower ROS production. Furthermore, they displayed a higher proportion of phagocytic-ROS-producing live myeloid cells (20.91 vs. 14.34; p < 0.05). These data suggest that the immunostimulant-enriched diet strengthens rainbow trout's innate immune system, partially mitigating A. salmonicida infection.