The Effect of in Vitro Passage of Infectious Pancreatic Necrosis Virus (IPNV) on Virulence and Sensitivity of the Virus to Rainbow Trout Serum

Abstract

Three infectious pancreatic necrosis virus (IPNV) isolates (A1 serotype, Buhl subtype) were passaged five times in rainbow trout gonad (RTG)-2 cells with either minimum essential medium supplemented with 10% fetal bovine serum (MEM-10) or MEM-10 supplemented with 1% rainbow trout Oncorhynchus mykiss serum (MEM−RTS) to determine the effect of passage of the virus on its virulence and sensitivity to RTS. Mortalities in brook trout Salvelinus fontinalis fry were highly variable during viral passages; however, in general, most of the IPNV isolates were virulent under all conditions. The IPNV mortalities ranged between 30% and 89%. Maintenance of virulence during the five passages was dependent on the IPNV isolate and culture conditions. Rainbow trout serum did not always facilitate maintenance of viral virulence during the passages. Isolate mortalities after multiple passages were virulence dependent: the highly virulent isolate showed peak mortality from 4 to 11 d postexposure, while the low-virulence isolate showed delayed peak mortality from 8 to 14 d postexposure. The virus isolates passaged in MEM-10 or MEM−RTS were generally resistant to in vitro inactivation by RTS except for the isolate from noble crayfish Astacus astacus passaged in MEM-10. This isolate became highly sensitive to RTS, demonstrating a reduction in tissue culture infectious dose with 50% endpoint of 10−8.0/mL in virus titer. However, increased RTS sensitivity of the virus did not correlate with decreased virulence. Overall, no relationship was observed between virulence and the RTS sensitivity of the virus. All isolates passaged showed identical monoclonal reaction patterns with a panel of 11 monoclonal antibodies, irrespective of viral virulence and sensitivity to RTS.