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FEATURE

Immunostimulation in aquatic animals by Philippe Tacon, global aquaculture manager, Phileo

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survey made at the end of an aqua industry forum meeting in Vietnam last year has shown that for 63 percent of the participants, the most limiting challenge for developing aquaculture was health and disease management. Indeed, in recent years, we have seen numerous diseases appearing and impacting aquaculture production, such as WSSV and EMS in shrimp, or Infectious Salmon Anemia (ISA) in salmonids. Working around the classic Host-PathogenEnvironment triad, new technologies and management techniques have been developed to better control diseases in aquatic animals: vaccination, which has led to the decrease of antibiotic use in salmonids; biosecurity procedures in hatcheries and in farms; biofloc technology. All of these technologies have proven successful. Their further development and expanded use will certainly improve the way aquatic animals are farmed. Another strategy is to increase the health of the animal through feeding, and this magazine might be a good place to discuss it. Well balanced diets can certainly improve the health status of a fish or a shrimp, but in some challenging conditions, like a pathogen infection, the use of immune stimulants can be required to enhance the response of the immune system. When studying immune stimulation, it is important to understand that the immune system of aquatic animals differs not only between theirs and the mammalian one but also between teleost and crustacean. Fish are the first group in which a specific immune system appears in the evolutionary tree. The fish immune system therefore has a greatly inferior performance to that of mammals (see Tort et al 2003). It is less specific, less sensitive and has only oneclass of antibodies (IgM). Fish being poikilothermic animals, it is highly dependent on temperature, low temperature slowing down the immune response up to 10 to 12 weeks. Fish rely by then more on their non-specific immune system (also called innate immunity) to fight against pathogens. The innate immune system recognises nonself molecules that could be of foreign origin - also called pathogen associated molecular patterns (PAMP) - and molecular patterns exposed though damage to the host. These patterns are recognised by germline-encoded pattern recognition receptors (PRR) or pattern

recognition proteins (PRP). These molecular patterns can be for example peptidoglycans and lipopolysaccharides from bacteria cell walls, fungal b1, 3-glucan, viral double-stranded RNA and bacterial DNA (see Magnadottir 2006 for an overview of fish innate immunity). Fish innate immunity starts with first barrier defences such as mucus; it traps pathogens and includes lysozymes, antibacterial peptides which can eliminate pathogens. Neutrophils and macrophages are key cells of the innate immune complex as they can phagocytose pathogens (a mechanism which is not temperature dependent) and release Reactive Oxygen species, which are toxic to pathogens. Completing this cellular response, the humoral response implicates the synthesis and release of antimicrobial components. In shrimp, where the picture is even simpler as they rely only on innate immunity, we find the same type of mechanisms in place as in fish with phagocytosis performed by granulocytes (a specific form of the blood hemocyte cells) and humoral response. However the most effective mechanism of invertebrates (as arthropods) is cellular melanotic encapsulation. This requires the combination of circulating hemocytes and several associated proteins of the prophenoloxidase (proPO) activating system. Recognition of PAMPs such as LPS and β-1, 3 glucans by PRPs is an essential step for the activation of the proPO cascade (Amparyup et al 2013). Stimulation of the innate immune system, which would enhance the speed and the effect of the immune response, is therefore possible by mimicking the effect of PAMP on PRR and PRP. In that regard, beta glucans have been studied for a long time in aquaculture and seem ‘the ideal’ immune stimulant in aquaculture (see Meena et al 2013 and Ringo et al 2012) as they can specifically activate macrophages in fish and the proPO cascade in shrimp. Parietal fractions, such as Safmannan® are extracted from a selected Saccharomyces cerevisiae strain respecting strict EU manufacturing control standards. They contain beta glucans, mannan oligosaccharides that are all activators of the immune system (Song et al 2014). Earlier internal trials have shown that yeast cell walls and parietal fractions have different effects in mycotoxin binding and immunity in aquatic animals. Indeed several trials done at the Hellenic Center for Marine Research in Greece have shown that yeast fraction

18 | INTERNATIONAL AQUAFEED | March-April 2015

Mar | Apr 2015 - International Aquafeed magazine  

The March - April 2015 edition of International Aquafeed magazine