Effects of sea lice infestation on macrophage functions in Atlantic salmon

The effects of Lepeophtheirus salmonis, on non-specific defence mechanisms in Salmo salar, were determined by experimentally infesting 180 hatchery-reared post-smolts with laboratory-grown infective copepodids at moderate to high infection intensities ranging from 15 to 285 lice per fish. The effects of sea lice-induced stress were investigated by measuring the blood levels of cortisol and glucose as indicators of primary and secondary stress responses, and by changes in macrophage respiratory burst activity and phagocytosis as indicators of tertiary stress responses as well as non-specific defence mechanisms. Fish were sampled prior to sea lice infestation and at days 3, 7, 14, and 21 post-infestation. Blood levels of cortisol and glucose were found to be significantly increased in sea lice-infested fish throughout the experiment while respiratory burst and phagocytic activities were found to be significantly decreased at day 21. The reductions in both respiratory burst and phagocytic activities are presumably the results of chronic stress induced by sea lice. The results from this study also indicate that sea lice have effects on the development of chronic stress and on the suppression of host defence mechanisms when infested with moderate to high parasite intensities..