Journal of Advances in Biology & Biotechnology

Epizootic ulcerative syndrome (EUS) is an ulcerative disease of freshwater fish associated in this study with seasonal changes in pond conditions and infection by Aphanomyces invadans. The study monitored EUS in Dalahi Pond, Darbhanga, Bihar, from August 2022 to March 2023, with Labeo rohita as the principal species examined. Pond physicochemical parameters, including temperature, pH, dissolved oxygen and total alkalinity, were monitored alongside disease occurrence and mortality. Blood samples were analysed for haematological indices, and ulcerated tissues were examined through fungal isolation and histopathology using haematoxylin–eosin and Periodic Acid–Schiff staining. Ulceration was observed during the post-monsoon and winter periods, when water temperature and dissolved oxygen were comparatively low. Among 428 fish, the highest recorded infection and mortality occurred in January, involving 170 infected fish and 151 deaths. EUS-infected fish showed reduced haemoglobin, packed cell volume and erythrocyte values, while several leucocyte-related parameters differed from those of control fish. Histopathological examination demonstrated fungal hyphae, myonecrosis, epithelial and muscular tissue loss, granulomatous and inflammatory changes, and alterations in the liver and kidney. The findings indicate that seasonal pond conditions, particularly declining temperature and dissolved oxygen, coincided with increased EUS occurrence. Monitoring environmental conditions together with haematological and histopathological indicators may support local surveillance and forecasting of EUS in cultured freshwater fish.