Immunity to VHS virus in rainbow trout

Viral hemorrhagic septicemia virus (VHSV) is the rhabdovirus that causes most disease problems in farmed rainbow trout in Europe. Survivors of infection are usually immune to reinfection but as with other fish viruses, development of a modern recombinant vaccine has been complicated by the limited knowledge of the immune mechanisms and antigens involved in induction of immunity. Neutralizing and protective monoclonal antibodies recognize the envelope glycoprotein (G protein) which is the only viral protein known to he present on the surface of the virus particle. Immunoblotting analyses with monoclonal antibodies as well as with sera from immunized trout have indicated that protein conformation plays an important role in neutralization epitopes. The virus neutralizing activity often found in sera from convalescent trout is highly dependent on a poorly defined complementing activity in normal trout serum. Attempts to demonstrate involvement of the complement component C3 were not successful, but inhibition experiments indicated that the classical pathway for complement activation was needed. Being the target of neutralizing antibodies, the G protein is an obvious candidate for a recombinant vaccine. However, recombinant forms of the G protein expressed in Escherichia coli have been poorly immunogenic in fish, presumably due to incorrect protein conformation. Expression in insect cells has resulted in more potent products but, more recently, considerably higher levels of protection were found following vaccination with naked DNA encoding the G protein under the control of a CMV promotor. Genetic resistance to VHS would be a desirable alternative to vaccination but the time required to obtain this makes it a long-time goal. Results from breeding programs in France and Denmark nevertheless indicate that such a strategy may provide considerable improvement in resistance.