The epizootic ulcerative syndrome (EUS) pathogen was transmitted to catla (Catla catla) using two experimental infection models: intramuscular injection and cohabitation. Oomycetes, recovered from naturally infected ulcerated bata (Labeo bata), were identified as Aphanomyces invadans based on morphology and histopathology. Lesions typical of EUS were reproduced in the catla using an intramuscular injection of 1×105 zoospores/ml autoclaved water from a EUS-affected pond. Lesions were first visible six days after injection; all lesions were swollen and ulcerative 10 days after injection. In the cohabitation experiment with EUS-affected bata, apparently healthy catla exhibited lesions eight days after infection. Histopathology of the muscle and liver from experimentally-infected catla showed the presence of hyphae and granuloma. Twelve days after infection, immunological parameters (superoxide anion and nitric oxide production, leukocyte proliferation, lysozyme activity) of experimentally-infected catla were significantly higher (p0.05). Biochemical parameters (total serum protein, serum glutamate pyruvate transaminase, serum glutamate oxaloacetate transaminase, and serum alkaline phosphatase were significantly higher (p<0.05) in intramuscularly-injected catla than in apparently healthy fish, however, in catla infected by cohabitation, only total serum protein significantly differed from the control (p<0.05)
