Oral administration of lipopolysaccharide to Atlantic salmon ( Salmo salar L.) fry. Uptake, distribution, influence on growth and immune stimulation

Abstract

Atlantic salmon fry were fed Aeromonas salmonicida lipopolysaccharide (LPS)-coated feed for 62 days and then challenged with virulent A. salmonicida bacteria. The fry were fed LPS-coated feed also after challenge. Fry that were fed LPS feed (0.1% LPS) showed a higher mortality throughout the challenge period (accumulated mortality of 57% after 42 days) compared with fry fed control feed (accumulated mortality of 36.5% after 42 days). Fry receiving LPS-coated feed showed an increase in mean weight of 10.7% at day 62 compared with fry receiving control feed. The increase was, however, not statistically significant. Sixty days of feeding with LPS-coated feed did not result in measurable amounts of specific antibodies against A. salmonicida LPS in the homogenised fry. The total amount of immunoglobulins (Igs) was, however, slightly increased. Studies of immunohistochemical localisation and radioactive LPS distribution revealed high levels of LPS in the intestinal epithelial cells. Head kidney, liver and heart showed low levels of radioactivity and no immunohistochemical staining. In another set of experiments, Atlantic salmon fry were fed LPS-coated feed (0.03% and 0.01%, respectively) for 64 days and then challenged with A. salmonicida and Vibrio anguillarum. The accumulated mortality in fry challenged with A. salmonicida was 40.9%, 34.3% and 30.8% after feeding 0.03% and 0.01% LPS and control feed, respectively. The accumulated mortality in fry challenged with V. anguillarum was 47.0%, 55.0% and 55.1% after feeding 0.03% and 0.01% LPS and control feed, respectively. Fry receiving 0.03% and 0.01% LPS-coated feed showed a statistically significant increase in mean weight at day 64 compared with fry receiving control feed.