EXPERT T●PIC
4
needed to give a suite of approaches to disease control to farm managers. This article will focus on one such approach, involving the optimisation of the mineral component of the diet. Knowledge of the impact of mineral nutrition on immunological function and health status of fish, together with our greater understanding of the salmonid genome and a new suite of molecular tools, may offer a new perspective enabling better prophylactic control of stress and disease.
Fish immunology
Immunonutrition in fish farming:
A natural and sustainable solution by D. Pacitti, S. A. M. Martin, C.J. Secombes , Scottish Fish Immunology Research Centre, Institute of Biological and Environmental Sciences, University of Aberdeen, United Kingdom
T
he rise of aquaculture has been one of the most profound developments in global food production over the past 100 years, with production approximately doubling each decade. Aquaculture now delivers 39 percent of aquatic food products with the FAO recording 310 species under culture in 2010.
Among these, salmonid fish (primarily rainbow trout and Atlantic salmon) are the most intensively farmed fish in more than 30 countries representing 90 percent of global marine aquaculture production. Salmonid production, particularly Atlantic salmon, increased from 299,000 tonnes in 1990 to 1.9 million tonnes in 2010, at an average annual rate exceeding 10 percent. Salmon is one of the food categories that is growing at a significantly higher rate than the world’s human population (FAO, 2012). However, the salmon farming industry is vulnerable to the adverse impacts of disease. For example, in 2007 an outbreak of infectious salmon anaemia (ISA) in Chile caused more than $2 billion in losses and reduced by half the Chilean production of Atlantic salmon (Godoy et al, 2008). The common causative agents of infectious diseases in aquaculture include a range of bacteria, viruses, parasites and oomycetes. Whilst vaccines exist for some of these diseases, it is clear that additional measures are
The immune system protects an organism against disease and participates in the maintenance of stable conditions during development and growth, inflammatory reactions and tissue injury. As in the human immune system, the fish immune system is divided into innate and adaptive components. The innate system is an ‘ancient’ system that is based on a non-specific recognition of a pathogen, that gives an instant reaction but has a short duration. The innate immune system is of prime importance in the immune defence of fish and is commonly divided into three compartments: the epithelial/mucosal barrier, secreted soluble mediators (e.g. complement system, interferons, antimicrobial peptides) and the cellular components (e.g. phagocytic cells such as macrophages and granulocytes). The epithelial and mucosal barrier of the skin, gills and alimentary tract is an extremely important barrier in fish, being constantly immersed in
media containing potentially harmful agents. The humoral and cellular defences represent the first response of the organism once subject to pathogen attack. However, a second encounter with the same pathogen will not result in an enhanced response. In contrast, the adaptive arm is character-
ised by specific activity, which is not a heritable trait but reflects the immune experience of each individual. The response of the adaptive immune system is relatively slow initially but is long lasting and has a memory component, giving faster and larger responses on a second encounter. The main effector cells are a different white blood cell type called lymphocytes. During infection, the fast but generally shortlived innate immune response precedes the longer lasting more specific adaptive immune response. In fish this lag period can be as much as 10-12 weeks, which has to be kept in mind when considering prophylactic immunological control of fish disease (Magnadottir, 2010).
Immunonutrition Traditionally the use of antimicrobials and vaccination has been used to fight disease in fish farms. Today, farmed Atlantic salmon are routinely vaccinated against a
number of bacterial and viral diseases before seawater transfer. However, fish vaccinology is still a young and maturing science, and vaccines for many pathogens have not yet been developed. It is a well-accepted concept that appropriate feed and feeding regimes support optimum health. However the sustainability of fishmeal and fish oil stocks has brought about changes in aquafeed formulations that are demanding a greater understanding of the role that alternative ingredients, feed additives, macro- and micro-nutrients and their balance plays as they can directly or indirectly influence fish health and immune function (Figure 1).
34 | International AquaFeed | November-December 2012