Isolation and identification of infectious pancreatic necrosis virus from rainbow trout (Oncorhynchus mykiss)

This study investigated the etiology of Oncorhynchus mykiss that suffered fulminant death in Shimian of Sichuan. General observation, virus isolation, pathogenicity tests, multiple RT-PCR assay and histological examination were performed to explore its causes, and to indentify the pathogen WZ160509 in which the major structural protein VP2 was amplified and analyzed. Principal symptoms included darkened body, abdomen distension, yellow mucoid fluid fecal casts from anus. Internally, no food was found in digestive tract and enteritis, pale liver and kidney were observed. The filtrated homogenate was also inoculated into the O. mykiss gonad cell line cells, and the typical cytopathic effect was formed after three blind passages. The result of artificial infection test showed that IPNV could cause 90 % death rate, and the injected trouts displayed similar clinical symptoms as the fish that was naturally infected. Multiple RT-PCR assay was performed to detect the presence of the virus from the fish naturally infected, the fish artificially infected and the infected cells, and all the results showed IPNV-positive, and the homology analysis, in which the test sample mixed the main structural protein VP2 gene and genomic U.S. isolates 1 type, showed the highest homology between WZ160509-VP2 and IPNV-VP2 (AY026345) and the sequence identity was 95.8%. Histopathologic analysis, pancreas and liver showed degeneration and necrosis. A mild inflammation was observed in glomerular. Besides, increased capillary permeability and red floccule proteins were also observed in capsular space. Renal tubule showed vacuolar degeneration and proteinuria. All the results show the isolated virus is IPNV.