Susceptibility of Atlantic cod Gadus morhua juveniles to different routes of experimental challenge with infectious pancreatic necrosis virus (IPNV).

Atlantic cod Gadus morhua L. juveniles weighing 40 g were challenged with infectious pancreatic necrosis virus (IPNV) by intraperitoneal (i.p.) or intramuscular (i.m.) injection or by bath. The amount of infectious virus was determined over 6 wk in head kidney, heart and pylorus tissues. No mortality or clinical signs were observed in either of the challenged groups. However, 6 wk after challenge virus was still present in the fish, which shows that IPNV can persist asymptomatically in cod. I.p. and i.m. injections were the most efficient routes of challenge giving the highest virus recovery. The prevalence of individuals with a viral titre > or = 500 infectious units g(-1) tissue was lower in the group of fish challenged by bath; thus bath was a less efficient route of challenge than injection. Our data also show that pylorus and head kidney are target organs for IPNV in cod, and levels of virus recovery were not considerably different between these 2 organs. Challenged by injection, the cod heart is also a target organ for IPNV. Compared to head kidney and pylorus, the heart seems to have a minor role in virus multiplication. Virus was also recovered from cohabiting fish, demonstrating that covertly infected cod may represent a reservoir of infectious IPNV for surrounding fish populations. Expression analysis of selected cod immune genes showed that i.p. injection of IPNV induced gene expression of ISG15 and LGP2, markers for the innate antiviral defence, while expression of markers for the inflammatory response (interleukins IL-1 beta, IL-8, IL-10) was not significantly increased.