Effects of variations of water temperature and dietary lipids on the expression of experimental epizootic ulcerative syndrome (EUS) in sand whiting, Sillago ciliata

Outbreaks of epizootic ulcerative syndrome (EUS) in the Philippines occur during the cold season. A number of studies have shown that low temperatures delay the immune response in fish. To determine the effects of low temperature on the expression of experimentally-induced EUS, juvenile sand whiting were acclimated to 26°C and then subjected to either rapid or gradual drop in temperature and injected with zoospores of Aphanomyces sp. As some reports revealed an enhanced immune response in temperature-manipulated fish provided with dietary polyunsaturated fatty acids, the influence of dietary lipids on the experimentally-infected fish was also investigated. Qualitative histopathological examination showed significant delay in the inflammatory response in fish maintained at low temperature. Quantitative examination, using image analysis, showed a 6 to 8 days delay in the formation of mycotic granulomas. Peak inflammatory cell infiltration was at day 14 post injection (p.i.) for fish held at 26°C and at day 18 p.i. for fish at 17°C. In addition, reparative processes were likewise delayed in fish maintained at low temperature. The addition of fish oil in the diets did not improve the response of the fish to changes in temperature, and, if anything, further inhibited the cellular response. These findings indicate that low temperature prior to and during EUS outbreaks can reduce the host's response and that measures to ameliorate its effects among farmed species should be focused on improving the fish's immune response.