Even if the vaccines perform well in challenge tests, variable protective effect of IPN-vaccination is reported from the salmon farming industry ( Handout 2, 2004).
To counteract the serious problems and loss due to IPN, factors which contribute to this need to be identified. For development of effective vaccines it is especially important to gain more knowledge about how both the innate and acquired immune defence of the salmon act against different IPNV-infections. These may be due to an infection early in life leading to development of a symptom-free carrier state, later reactivation of the persistent virus infection into outbreak of IPN, and/or a new infection after transfer to sea. IPNV carriers are very common among farmed salmon, but it is unknown whether vaccination against IPN may influence the carrier condition or vice versa.
The project uses IPN-vaccinated and unvaccinated salmon and will investigate:
- The impact of the carrier condition of IPNV in pre- and post smolts.
- The risk a carrier condition represents by reactivation of an IPNV infection into new outbreaks, especially after the smolts are transferred to sea.
- The role IPNV strains with different ability to cause disease (high and low virulence) may have in this picture. See poster by/Kjersti Julin et al 2003 " Virulence studies of Norwegian field isolates of IPN virus.
Experimental IPNV challenge
In an extensive trial a covert carrier condition was established in groups of salmon challenged early in the freshwater phase with IPNV of high and low virulence. Groups of IPNV carriers and non-carriers were, 8 weeks before transfer to seawater, injected with vaccines with or without an IPNV component or with saline only. The experimental period was 5 months and all groups were observed for development of disease and mortality. See poster by/Sommer et al. " Vaccination of A.salmon carrying IPNV of different virulence".
Testing of samples
A large number of samples were collected from all groups during the challenge trial period, intending to reveal more of the interaction between the innate and acquired immune defence against different IPNV infections (both persistent and acute), with or without vaccination.
Head kidney tissue and cells (leucocytes) were isolated from all experimental groups at different time points. Cultivated cells that were collected before and after activation with immune stimulants are tested using a reporter-gen assay or a quantitative real-time PCR
Sera sampled throughout the experimental period are tested for antibodies that are IPNV neutralising or specific for recombinant IPNV-protein 2 in ELISA.
To gain more information on how the immune defence may influence the virus production and development of IPN in IPNV carrier salmon, the variations of virus amount in the differently treated groups during the pre- and post-smolt stage are investigated.
