Experimental IPN virus infection of Atlantic salmon parr; recurrence of IPN and effects on secondary bacterial infections in post-smolts

Abstract

Infectious pancreatic necrosis (IPN) in Atlantic salmon ( Salmo salar L.) is a considerable problem in the Norwegian and Scottish fish farming industry. The disease is a main contributor to the post-smolt mortalities often observed in the first weeks after transfer to seawater. Asymptomatic carriers of infectious pancreatic necrosis virus (IPNV) are common, but the implications this has for the fish are largely unknown. In an experiment which lasted for 10 months, we aimed at investigating if vaccination, smoltification and seawater transfer may affect an IPNV carrier condition established in freshwater, and the possibility of reappearance of IPN in post-smolts. Vice versa, we wanted to see if this IPNV carrier condition could affect the protective effects of vaccines against bacterial pathogens, and the outcome of secondary bacterial infections. No changes in the IPNV carrier condition were registered during the period in freshwater, however a reactivation of the carrier condition occurred after transfer to seawater and 24% of the unvaccinated fish died due to IPN. The vaccine offered some unspecific protection against recurrence, and significantly fewer fish, 7%, died in the vaccinated group. In the challenge experiments with Aeromonas salmonicida and Vibrio salmonicida 15 weeks and 28 weeks after vaccination, respectively, the vaccine offered the same good protection against the bacterial pathogens in both IPNV carriers and in non-carriers. In the unvaccinated groups the time point of challenge with A. salmonicida concurred with the onset of IPN mortality in the IPNV carrier group, and significantly more fish in this group died compared to the control fish. The V. salmonicida challenge was performed several weeks after the IPN mortality had stopped. Mortality levels reached over 90% in both unvaccinated groups, but the fish started to die earlier and mortalities were significantly higher at day 8 till 12 after challenge in the IPNV carrier group, indicating that the fish could still be affected several weeks after the recurrence of IPNV. An asymptomatic carrier condition did not seem to have negative impact on the Atlantic salmon during the freshwater stage. However, we have demonstrated that an IPNV carrier state implies a risk of recurrence resulting in IPN mortality after seawater transfer. An oil-adjuvanted vaccine without IPNV antigens offered unspecific and significant protection against recurrence, and at the same time the specific protective effects against the bacterial pathogens were not impaired in the carrier fish.