Field and laboratory studies on Rhabdoviruses associated with Epizootic Ulcerative Syndrome (EUS) of fishes.

Abstract

Epizootic Ulcerative Syndrome (EUS) is a seasonal and widely spread ulcerative disease condition of fresh and brackishwater fishes in Asia caused by a complex of etiological agents. Viral agents have been found to be associated with EUS but the role of viruses in the complex etiology has still to be identified. Further virological examinations were, therefore, conducted in this study. Two warm-water fish cell lines were established from hybrid catfish, male Clarias gariepinus x female C. macrocephalus. The HCK line was derived from head kidney and the HCT line was derived from tail tissue. Both lines were susceptible to 3 birnaviruses, sand goby virus (SGV) and infectious pancreatic necrosis virus (IPNV) serotypes Ab and Sp, 2 reoviruses, golden shiner virus (GSV) and catfish reovirus (CRV), but refractory to all 6 strains of ulcerative disease rhabdovirus (UDRV), channel catfish herpesvirus (CCV) and 1 EUS-associated reovirus (T9231). Only the HCK line was susceptible to recent EUS-associated rhabdovirus strain T9204 and tench and chub reoviruses. Using HCK, BF-2 and SSN-1 fish cell lines, 9 rhabdoviruses were successfully recovered from EUS-diseased fishes during the first 2 weeks of a 1993- 1994 epizootic. Rhabdovirus strain T9412 caused death in fry and skin damage in juvenile striped snakehead fish. A combination of this rhabdovirus and pathogenic Aphanomyces fungus appeared to induce more severe EUS disease in snakehead fish than a single infection with the fungus. EUS transmission was also experimentally achieved by co-habitation of healthy and diseased fish. Three characterised virus strains T9415, T9416 and T9429 possessed a typical bullet- or bacillus-shaped morphology and also exhibited a lyssavirus-like electrophoreotype of structural proteins similar to snakehead rhabdovirus (SHRV) and rhabdovirus strain T9204, while UDRV strains SL11, BP and 20E possessed vesiculovirus-like eletrophoreotypes. The lyssavirus-like EUS-associated rhabdoviruses, except strain T9416, were structurally and serologically similar for which the ‘serotype Sh’ is proposed while the UDRV strains are grouped as a proposed ‘serotype Ud\ Strain T9416 could not be grouped in either serotype as the homologous antiserum was capable of neutralising viruses of both serotypes. The results of this study suggest that the rhabdovirus is one of a complex of etiological agents for EUS and that at least 2 serotypes of EUS-associated rhabdoviruses are identified.