Virulence of infectious pancreatic necrosis virus is associated with the larger RNA segment (RNA segment A)

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Abstract
Infectious pancreatic necrosis virus (IPNV) contains two segments of double‐ stranded RNA (the larger RNA segment, A; and a smaller RNA segment, B). The IPNV‐ Buhl isolate belongs to the VR299 serotype, and eel virus European (EVE) is serologically the same as the Ab serotype. IPNV‐Buhl is virulent to rainbow trout fry, Oncorhynchus mykiss (Walbaum), whereas EVE is avirulent. To determine the genetic basis for the difference of the virulence, intertypic reassortants were made between IPNV‐Buhl and EVE. The reassortants with the genotypes of P/E (segment A of IPNV‐Buhl and segment B of EVE) and E/P (segment A of EVE and segment B of IPNV‐Buhl) were studied for their pathogenicity to rainbow trout fry. The P/E reassortant was highly virulent, similar to the IPNV‐Buhl parent (P/P), while the E/P reassortant was avirulent, as was the EVE parent (E/E). A high level of viral replication was observed in almost all organs of the fry infected with the P/E reassortant and the IPNV‐Buhl parent. In contrast, viral replication was restricted to the epidermal cells of the fry infected with the E/P reassortant and the EVE parent. The results demonstrate that the larger RNA segment (RNA segment A) of IPNV is related to cell tropism in vivo and is responsible for virulence in rainbow trout fry.