Virulence and serological studies of recombinant infectious hematopoietic necrosis virus (IHNV) in rainbow trout

Infectious hematopoietic necrosis virus is a highly contagious disease of juvenile salmonid species. From the IHNV HLJ-09 isolated in China, two recombinant viruses were generated by reverse genetics using the RNA polymerase II transcription system. The recombinant viruses were confirmed by RT-PCR, indirect immunofluorescence assay and electron microscopy. They were referred to as rIHNV HLJ-09 and rIHNV-EGFP. rIHNV HLJ-09 and rIHNV-EGFP could stably replicate in EPC cell lines and had the same cellular tropism as wtIHNV HLJ-09. But the titer of rIHNV-EGFP was significantly lower than rIHNV HLJ-09 and wtIHNV HLJ-09. rIHNV-EGFP strain could express EGFP stably at least in 20 passages, and the fluorescence could be observed clearly. To assess the virulence and pathogenicity of the recombinant viruses in vivo, juvenile rainbow trout were challenged by intraperitoneal injection with 20 μl of rIHNV HLJ-09, rIHNV-EGFP or wtIHNV HLJ-09 (1 × 106 pfu ml−1). Fish challenged with rIHNV HLJ-09 and wtIHNV HLJ-09 exhibited clinical signs typical of IHN disease and both produced 90% cumulative percent mortality, whlie rIHNV-EGFP produced only 5%. Pathological sectioning results showed that the tissues (liver, kidney, heart muscle, back muscle) of the fish infected with rIHNV HLJ-09 exhibited pathological changes, with the exception of cerebral neurons and the cheek. However, no lesions of liver, kidney, heart, muscle, brain in rainbow trout of rIHNV-EGFP or the control group were observed. Indirect ELISA results showed that a high level of serum antibody was detected in the experimental fish challenged with rIHNV HLJ-09, just as the same as wtIHNV HLJ-09, while a lower titer was detecred in the fish infected with rIHNV-EGFP. This indicated that the recombinant viruses could induce humoral immune response in the experimental fish. The recombinant viruses had unique genetic tags and could be used for genetic engineering, laying new ground for further investigation of IHNV pathopoiesis molecular mechanism, host tropism and the development of novel vaccines against IHN.