Environmental influence on the occurrence of epizootic ulcerative syndrome (EUS) in freshwater fish in the Bellanwila–Attidiya wetlands, Sri Lanka

Water quality and rainfall conditions were analysed from November 1996 to September 1997 in selected sampling sites in the Bellanwila–Attidiya wetlands of Sri Lanka, an area where the epizootic ulcerative syndrome (EUS) has occurred, in order to examine the environmental factors that may be associated with EUS. Quantitative sampling of fish populations was carried out over the study period and the recurrence of EUS was monitored by gross clinical signs and histopathology. The occurrence of EUS from the sampling sites was recorded from January 1997 to March 1997. The outbreak was preceded by heavy rainfall. Diurnal variations in air temperature were relatively high during the outbreak period. Declining dissolved oxygen concentrations in water coincided with initiation of the disease outbreak. There were significantly low levels of dissolved oxygen in the water and significantly low rainfall in the area during the period of EUS in comparison with non-EUS periods. The biochemical oxygen demand of the water was significantly higher during the outbreak period than that in the period prior to the outbreak. Of the fish species sampled, the prevalence of EUS was highest in Trichogaster pectoralis (snake skin gourami). Histopathological studies on ulcerative lesions of fish confirmed that diffuse proliferative mycotic granulomatosis is a consistent feature of EUS. The results indicate that interaction between rainfall, deteriorating water quality and presence of pathogens could provide stressful conditions for fish, thereby inducing EUS lesions in susceptible fish populations.