Infectious pancreatic necrosis virus VP5 is dispensable for virulence and persistence.

Infectious pancreatic necrosis virus (IPNV) is the causative agent of infectious pancreatic necrosis (IPN) disease in salmonid fish. Recent studies have revealed variation in virulence between isolates of the Sp serotype, associated with certain residues of the structural protein VP2. The isolates are also highly heterogenic in the coding region of the nonstructural VP5 protein. To study the involvement of this protein in the pathogenesis of disease, we generated three recombinant VP5 mutant viruses using reverse genetics. The "wild-type" recombinant NVI15 (rNVI15) virus is virulent, having a premature stop codon at nucleotide position 427, putatively encoding a truncated 12-kDa VP5 protein, whereas rNVI15-15K virus encodes a 15-kDa protein. Recombinant rNVI15-deltaVP5 virus contains a mutation in the initiation codon of the VP5 gene that ablates the expression of VP5. Atlantic salmon postsmolts were challenged to study the virulence characteristics of the recovered viruses in vivo. The role of VP5 in persistent infection was investigated by challenging Atlantic salmon fry with the recovered viruses, as well as with the low-virulence field strain Sp103 and a naturally occurring VP5-deficient mutant of Sp103. The results show that VP5 is not required for viral replication in vivo, and its absence does not alter the virulence characteristics of the virus or the establishment of persistent IPNV infection.