Injection of Forty -Five Day Old Fingerlings of Common Carp with Zoospores of Aphanomyces invadans Leads to Histopathological Lesions Suggestive of Epizootic Ulcerative Syndrome (EUS)

Using artificial infection tests with Aphanomyces invadans, the etiological agent of Epizootic Ulcerative Syndrome (EUS), the present investigation examined the disease susceptibility of fingerlings (forty five days old and averaging 6.5±0.18 cm) of common carp (Cyprinus carpio, Cyprinidae), considered to be one of the resistant fish species to EUS. The study was carried out in two different experiments. The first experiment was carried out for a period of 12 days and the number of mortalities was recorded daily. The second experiment was carried out for a period of 10 days. Regular sampling was done to know the sequence of progression of infection. Results of the first experiment indicated that over an experimental period of 12 days, there was 50% mortality in the injected fish and the histopathological features of moribund and / or dead fish was typical of a disease condition of EUS. Results of second experiment indicted that the injected zoospores of A nvadans were able to germinate in the muscle of all the experimentally infected common carp fingerlings. In due time, the germinated hyphae were able to proliferate massively, invade the neighbouring tissues and induce extensive necrotic lesions in large areas of myotome in about 60% of the fish. Among the remaining 40% of the fish, the lesion area was restricted to the injected side but within the injected side, large proliferative lesions were observed and within the lesion area a large number of fungal hyphae was observed. This is the first report where such extensive necrotic pathology and /or mortalities have been recorded in the common carp due to A. invadans infection.