Pathogenicity analysis of infectious haematopoietic necrosis virus isolate

In order to analyze the pathogenicity of a new strain of infectious haematopoietic necrosis virus(IHNV-Sn1203)isolated from rainbow trout farm of Shandong Province,rainbow trout fry were subjected to artificial infection test.The artificial infection test showed an accumulated average mortality rate as high as 100%(8 days),and with typical symptoms.Diseased fish samples were collected,homogenized,filtered,then inoculated onto monolayer cultures of carp epithelial cell(EPC),and further analyzed with plaque assay,electron microscopy observation,virus titer detection and homology analysis.Cytopathic effect(CPE)of the EPC cells can be observed 24 h after inoculation,most of the cells disintegrated off 72 h after inoculation.In EPC cells the virus titer was 10^8.36TCID₅₀/mL,and formed about 2 to 4 mm plaques.Rhabdovirus particles in large numbers were clearly observed in the cytoplasm by electron microscope.Phylogenetic tree was constructed based on the amino acid sequences of nucleoprotein.The results showed that the amino acid sequence of IHNV-Sn1203 nucleoprotein had 97% and 93% identity with RB-1 and WRAC,respectively,and had homology as high as 99% with the domestic zyx isolate.These results indicate that the IHNV-Sn1203 isolate can stably propagate in cells cultured in vitro with high viral titer,and can infect rainbow trout fry with high mortality.