ABSTRACT
Fish may carry different pathogens (virus, bacteria, parasites, fungi) without any disease sign or any negative effect on their physiology or survival. This is a frequent scenario in natural fi sh populations, in which a balance between the host and the pathogen has been reached as a result of millions of years of co-evolution. Farming practices alter this equilibrium, favouring the emergence of diseases and posing a major problem for the aquaculture industry. These practices are likely to select for fast-growing, early-transmitted, and hence probably more virulent pathogens (Mennerat et al. 2010). Farmed fi sh are also constantly exposed to different types of stressors such as bad or poor culture conditions, inadequate diets and other anthropogenic factors that might compromise their performance and survival (Fig. 1).
