ISOLATION AND PRELIMINARY CHARACTERIZATION OF A PATHOGENIC ICOSAHEDRAL DEOXYRIBOVIRUS FROM THE CATFISH ICTALURUS-MELAS

A virus was isolated from dead specimens of catfish Ictalurus melas originating from a pond in which the whole catfish population had succumbed to an acute infection within a week. Diseased catfish showed clinical signs of oedema and haemorrhages while other fish species present in that pond remained healthy. The virus isolate grew well in Epithelioma papulosum cyprini (EPC), bluegill fry (BF2) and channel catfish ovary (CCO) cells at temperatures ranging from 15 to 25-degrees-C. The virus formed tiny round plaques (diameter 400 mum) under agarose overlays and the BF2 cells were the most sensitive for this assay. The infectivity of the catfish virus isolate was inhibited by chloroform treatment and its replication was suppressed by 5-iodo-2-deoxyuridine indicating the presence of lipid components and a DNA genome. Hexagonal virions with a diameter of 150 to 160 nm, some of them surrounded by an external envelope, were seen by electron microscopy in the cytoplasm of infected EPC cells. These virions were morphologically similar to those of the Iridoviridae. Infection trials conducted in subadult and adult catfish demonstrated the strong pathogenicity of the new virus isolate and high levels of virus were detected in the kidney and spleen, inducing a dramatic necrotising spleno-nephritis and subsequently death of most of the fish. The infection trial substantiates the causative role of this virus isolate in the massive catfish mortality observed and constitutes the first record of an overt natural viral infection of L melas.