Today’s frequent use of non-medical treatment methods against salmon louse (Lepeophtheirus salmonis) introduces severe welfare challenges for Norwegian salmon farming. Finding new salmon louse treatments not involving extensive handling such as a salmon louse vaccine, can provide a significant improvement. Parasites are, however, far more complex than bacteria and viruses, having several mechanisms to evade the host’s immune system. Therefore, vaccines against ectoparasites are extremely challenging to make. Nevertheless, much effort has been done to develop a salmon louse vaccine, where the most promising vaccine candidates tested are expressed in the louse intestine. However, vaccines with intestinal antigens require an enhanced antigen concentration to be effective demanding stronger adjuvants, and the side effects perhaps becomes too severe for commercialization. By analysing the lice’s gut content, we have found that the reason for this may lie in the louse’s intestinal function, where digestive enzymes are secreted into the intestinal lumen degrading blood components and probably also vaccine induced antibodies. Antigens acting outside the louse at the skin interface may therefore have a greater potential as vaccine candidates.
One of the most important features allowing a parasite to settle onto a host is the secretion of immunomodulatory factors. The salmon louse secretes such factors into their mouth tube, and these are deposited onto the host skin during lice feeding where they dampen inflammatory responses and kills immune cells. Such host modulative proteins can thus have great potential as vaccine candidates since antibodies against these proteins do not need to encounter the louse’s digestive system to have an inhibitory effect. Such a vaccine must induce the secretion of antibodies into the salmon’s mucus, which can here bind to and inhibit the action of secreted lice components and might enable the salmon to resist salmon louse infestations fully or partially. However, as immune suppressive proteins have the potential to generally dampen immune responses producing an immune suppressed salmon, we have mutated the antigens in such a way that they are left non-functional but still folded correctly so that the induced antibodies can recognize native proteins. By implementing these antigens into DNA vaccines, mutants have been produced and tested on a larger scale, where we have modified the antigens and included flagellin as a molecular adjuvant for enhanced antibody production. Also, many of the proteins are relatively small proteins, and these are not always the best at inducing antibody responses. Though, by increasing the size of one antigen by injecting a construct encoding four proteins connected by a linker, the serum antibody response was enhanced, and specific antibodies could also be detected in mucus.