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Stress responses of juvenile salmonids to immunological challenge Ackerman, Paige Adrienne

Abstract

Physiological and cellular stress responses of fish to immunological challenge were examined using an experimental approach involving whole animal and tissue culture. The first study examined effects of sub-lethal ammonia exposure on stress responses and disease susceptibility of salmonids. In seawater at a pH of 7.8, salinity of 30 g/kg, and temperature of 10-12°C, the Environmental Protection Agency (EPA) defines acute ammonia exposure in terms of a Criteria Maximum Concentration (CMC) of 23 mg Total Ammonia Nitrogen (TAN)/L and chronic exposure as a Criteria Continuous Concentration (CCC) of 3.4 mg TAN/L. Juvenile chinook salmon (Oncorhynchus tshawytscha) were exposed to two sub-lethal concentrations (10.0 mg TAN/L and 2.5 mg TAN/L) for a period of 10 d. Samples taken during the exposure indicate that at levels well below EPA guidelines, metabolic changes occurred that were indicative of a physiological stress response. A subsequent disease challenge using the marine pathogen Vibrio anguillarum resulted in a significant increase in mortality of fish exposed to 10 mg TAN/L compared to controls. Mortality of fish exposed to 2.5 mg TAN/L was between that of controls and fish exposed to the higher concentration. The cellular stress response as measured by heat shock protein 70 (hsp70) changed in relation to exposure concentration. Hsp70 increased in liver tissue in fish exposed to 2.5 mg TAN/L but both concentrations resulted in hsp70 decrease in head kidney, although this was only significant in fish exposed to 10.0 mg TAN/L. Results suggest that current environmental standards do not protect fish from an immunological point of view. To further investigate the relationship between hsp70 levels and disease, juvenile rainbow trout (O. mykiss) were experimentally infected with the pathogen V. anguillarum. Changes in blood pathogen levels and corresponding indicators of physiological and cellular stress were examined over a period of 8 d. Chloride ion concentrations significantly decreased by 6 d post-challenge. At this point blood pathogen concentrations had reached peak levels. Haematocrit and haemoglobin levels also changed over time. The decrease in plasma protein concentration at 6 d post challenge, indicating a probable haemodilution, was a causative factor in these changes. Plasma Cortisol and liver and head kidney hsp70 levels increased from day 3, reaching peak values 24 h prior to clinical signs of disease. Hsp70 is involved in the glucocorticoid response and the observed increase of both hsp70 and Cortisol may have functional consequences in the immune response. A simple explanation for responses to live challenge may be that infectious bacteria damage cellular components, thus altering cellular homeostasis and inducing stress proteins.

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