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Fish Immunostimulation through dietary manipulation 9The aquaculture sector has shown a rapid growth over the last 30 years with an associated increase in 10 disease problems as result of rapid expansion and amongst other factors high stocking densities. In order
11to maintain fish health and to improve performance immunostimulants have been used as dietary 12 additives to improve weight gain, feed efficiency, and/or disease resistance in cultured fish.
13An immunostimulant is a natural or chemical substance that stimulates the immune system by specific
14(vaccines or antigens) or non-specific (irrespective of antigenic specificity) routes. In Aquaculture, non-
21The immunostimulant effect of dietary supplements in fish has been focused mainly on the evaluation of 22 non-specific immune parameters and, therefore, on the consequences of these treatments on the innate 23 immune system. The innate immune system has both cellular and humoral components by which it carries 24 out its protective function. The major components of the innate immune system at cellular level are 25 leukocytes, mainly monocytes, macrophages and granulocytes [1,2]. Among the granulocytes, neutrophils
26are the most abundant cell-type and their presence has been described in Salmoniformes, Cypriniformes 27 and Perciformes [3]. Neutrophils and macrophages are responsible for the production of bioactive 28 molecules for pathogen recognition and destruction, cellular communication and activation, initiation of 29 an adaptive immune response and later, resolution of an inflammatory response and tissue repair.
30Furthermore, these cell types are responsible in the majority for phagocytosis [4], one of the main 31 mediators of innate immunity to remove pathogens such as bacteria, viruses, and parasites. For this 32 reason, these immune cell types are also called phagocytes. This microbe/killing mechanism triggers 33 diverse antimicrobial processes that use a wide variety of mechanisms including cellular activation, 34 production of oxidative radicals, and the production of cytokines driving the inflammatory response 35 amongst others.
36Two of the most important antimicrobial systems of phagocytic cells are the NADPH phagocyte oxidase 37 and inducible nitric oxide synthase (iNOS) pathways, which are responsible for the generation of 38 superoxide (O2 − ) and nitric oxide (NO) radicals, respectively. NADPH oxidase, a multi-subunit complex, 39 catalyses a one-electron reduction of molecular oxygen into superoxide anion (O2 − ), also referred to as 40 reactive oxygen species (ROS), which is either spontaneously converted to H2O2 or enzymatically by 41 superoxide dismutase (SOD). In comparison to neutrophils, the size of the respiratory burst is much 42 reduced in macrophages [5]. Since O2 − is the first product to be released from the respiratory burst, the 43 measurement of O2 − has been accepted as a direct and accurate way of measuring respiratory burst 44 activity [6]: the reduction of ferricytochrome c to determine extracellular O2 − , and th...
