Immunization with viral antigens: infectious pancreatic necrosis.

Clinical IPN has traditionally been observed in brook trout (Salvelinus fontinalis) and rainbow trout (Oncorhynchus mykiss). However, during the past 10 years outbreaks of IPN have been reported frequently in farmed Atlantic salmon (Salmo salar L.) in Norway. Acute IPN with high mortality is observed in fry at start feeding and in smolt in the first six months after transfer to seawater. Today IPN is the most important infectious disease in Norway in farmed fish, giving economic losses of about 60 million USD yearly. A prophylactic strategy against this disease is strongly needed. Different strategies for developing IPN vaccines have been tested since the virus was first isolated in 1960. Vaccination with live virus has not been successful and is probably not an acceptable strategy for environmental risk reasons. Vaccination with inactivated virus has been tested in rainbow trout given by the oral route, by immersion and injection. Protection against challenge was obtained only by injection. IPN vaccines based on inactivated virus may be effective but are expensive. A subunit vaccine produced by fermentation is a more realistic strategy for fish vaccine production if the actual protective epitopes can be identified. Protective IPNV epitopes may include both B- and T-cell epitopes, but only B-cell epitopes have been examined so far. Epitope mapping with neutralising monoclonal antibodies indicates that the internal variable region of VP2 (aa 200-350) folds into an immunodominant structure including both serotype specific and conserved neutralisation epitopes that can renaturate spontaneously from E. coli-expressed recombinant VP2 (rVP2). Analysis with an IPNV group-A specific neutralising monoclonal antibody indicates that immunisation with recombinant protein containing the segment (aa 86-210) might induce protection against all IPNV serotypes. Subunit vaccines based on E. coli-expressed IPNV proteins have been tested in rainbow trout and Atlantic salmon. Vaccination by immersion in rainbow trout fry with bacterial lysate from E. coli expressing the IPNV Sp strain gene segment A induced protection against challenge with the IPNV Buhl strain. By injection of Atlantic salmon parr with partly purified E. coli-expressed rVP2 (N1 strain), increased resistance against IPN infection was demonstrated by challenge. In field trials it is shown that vaccination of pre-smolt with rVP2 included in a commercial oil/glucan adjuvanted multivalent bacterial vaccine gives protection against IPN in natural outbreaks, compared to fish vaccinated with the same vaccine without the IPNV component.