Immunological responses in Atlantic salmon (Salmo salar) to co-infections with sea lice (Lepeophtheirus salmonis)

Intensive fish farming at aquaculture sites is reaching maximum exploitation of the environment, providing suitable conditions for simultaneous infections to occur in wild and farm systems. A major parasitic contributor that is negatively impacting the health of Atlantic salmon (Salmo salar) is the salmon louse, Lepeophtheirus salmonis. This thesis aimed to determine the pathogenic effects of several co-infection models with a concurrent L. salmonis infection by evaluating responses of host immune gene transcription identified by RT-qPCR. The first study investigated winter ulcer disease; a cold-water disease of salmonids caused by Moritella viscosa. When co-infected with L. salmonis at high lice loads, mortalities were significantly higher and skin lesions were more severe than in a single infection with M. viscosa or co-infections with lower lice loads. Evidence of osmoregulatory stress from increased potassium levels and significant up-regulation in genes associated with wound healing and humoral responses in skin was also prevalent in co-infected fish with high lice loads, suggesting that co-infection may have a significant impact on the ability of lesions to resolve, leading to increased mortality. The second study investigated functional feeds, which are an alternative anti-louse management strategy. Atlantic salmon were co-infected with L. salmonis and a high virulent isolate of infectious salmon anemia virus (ISAv). Fish were administered one of four functional diets with varying EPA/DHA levels; a control feed with a low EPA/DHA mix of 0.3% (Ctrl), an EPA/DHA enriched diet (1.0%) with pro-inflammatory supplementation (FA+I), an EPA/DHA enriched diet (1.0%) with anti-inflammatory supplementation (FA-I), and a low EPA/DHA feed (0.3%) with an immunostimulant added (Ctrl + IS). Fish fed fatty acid enriched (FA+I, FA-I) feeds had significantly lower lice abundance during single infection and lower survival during co-infections, suggesting the improved anti-parasitic responses may not be effective in a co-infection model. Reverse transcription qPCR was used to assess immune gene transcription modulation in the head kidney between single and co-infected fish fed functional diets. Pro-inflammatory and antiviral genes associated with the interferon system were modulated following co-infection with ISAv in all functional diets but the immunostimulated diet, while several genes accounted for high lice and high viral loads in diets FA+I and Ctrl + IS. The results generated from this thesis contribute to the growing body of knowledge of understanding fish immune responses during co-infections and developing alternative strategies, such as functional diets, for managing pathogenic outbreaks at farm sites.