IMPACT OF DIETARY LEVEL AND RATIO OF n6/n3 FATTY ACIDS ON DISEASE PROGRESSION, PHYSIOLOGICAL RESPONSE AND INFLAMMATION IN ATLANTIC SALMON CHALLENGED WITH AMOEBIC GILL DISEASE (AGD)

Gill infections are wide spread in commercial aquaculture operations and represent a source of progressive stress for the fish. Amoebic gill disease (AGD) is an ecto-parasitic infection in the gills causing a proliferative response in the gill epithelium, leading to impaired gaseous exchange. The significance of gill health in aquaculture is well recognized; nevertheless, dietary influence on gill infections is poorly understood. Inflammatory responses in AGD infected fish indicate a relation with the dietary n-6/n-3 fatty acid ratio. We therefore hypothesized that dietary n-6/n-3 ratio will influence the immune and inflammatory response in AGD infected gills.

Atlantic salmon (80 g) were fed with four different diets with different ratios of n-6/n-3 fatty acids; ranging from 1.3 to 6.0, (1.3 (diet 1), 2.4 (diet 2), 6.0 (diet 3) and 1.3 with high n-3, n-6 level (diet 4)).  The diet with the n-6/n-3 ratio of 6.0 was included to make sure potential n-6 effects, while the three other diets had more commercially relevant n-6/n-3 ratios and levels. After a pre-feeding period of 3 months, Atlantic salmon of ~300 g were used in a challenge trial with amoebic gill disease (AGD).

Fish from each diet regime were challenged by a standard and well-established laboratory challenge model using a clonal culture of Neoparamoeba perurans. The subsequent development of the disease was monitored (by gross gill score), and samplings conducted before infection and at weekly sampling points for 5 weeks (wk) after infection. AGD challenge did not have a significant impact on the growth of the fish during the 5 wk period, but fish given the feed with the highest n-6/n-3 ratio had reduced growth compared to the other groups. Total gill score for all surfaces and severity of gill lesions showed a significant increase with time, reaching a maximum at 3 wk post-challenge and declined thereafter irrespective of the dietary fatty acid ratios (Fig.1). Other physiological and inflammatory responses are being examined to better understand the impact of dietary n-6/n-3 fatty acid ratio on AGD in Atlantic salmon post-smolt. These include gill fatty acid composition and expression of immune and inflammatory markers in the gills, which will be presented.