The pathogenesis of epizootic ulcerative syndrome (EUS) with particular reference to factors potentially influencing outbreaks of the disease in the Philippines

Epizootic ulcerative syndrome (EUS) outbreaks in the Philippines had been consistently associated with periods of decreasing and fluctuating water temperature usually after the rainy season. Therefore, one of the main objectives of this study was to examine the influence of low and highly fluctuating temperature on the pathogenesis of the disease based on the histopathological and epidermal changes. Also, the influence of water temperature variations on the nonspecific immune responses of a fish exposed to \({Aphanomyces}\) \({invadans}\) was determined. Moreover, the efficacy of four therapeutic substances to enhance the defense mechanisms of susceptible fish against EUS was assessed. A model of the sequential pathology of EUS was developed and described in three spot gourami \({Trichogaster}\) \({trichopterus}\) and sand whiting \({Sillago}\) \({ciliata}\). Both species of fish exhibited histopathological changes typical of EUS, such as chronic granulomatous response and inflammatory cell infiltration in the muscle and skin tissues, at day 8 post-inoculation of 55 to 70 spores/fish. From the results of comparative granuloma counts and percentage of cellular infiltration in a sampled lesion area using image analysis, it was shown that the three-spot gouramis developed a more vigorous response than the sand whiting. Based on the EUS model, the effects of either rapid or gradual decrease in temperature on the histopathology of EUS in sand whiting was documented. Qualitative and quantitative histopathological examinations revealed a marked delay in the inflammatory response of fish maintained at low temperature. EUS lesions were also induced in skin-abraded three-spot gouramis exposed to motile A. invadans zoospores, with higher mortality in fish subjected to daily temperature variations (26 °C to 19°C) than in fish held at 26°C. Histopathological examination also revealed minimal inflammatory cell infiltration and delayed mycotic granuloma formation with extensive tissue damage in temperature-stressed fish. Fish oil, L-cysteine ethyl ester, yeast glucans and levamisole were trialled as prophylactics against EUS. Only yeast glucans and levamisole gave encouraging results. Results from this study showed that low and fluctuating temperature could affect the host's defense mechanisms against EUS and that preventive measures should be aimed at the maintenance of the epidermal integrity and improvement of the nonspecific immune response of fish