Vibrio anguillarum is a common pathogen that cause severe vibriosis disease in lumpfish (Cyclopterus lumpus). In this study, we developed a method to bio-encapsulate V. anguillarum bacterin in Artemia salina live-feed in order to orally immunize lumpfish larvae. We found that the highest amount of bacterin was detected at 3 hours A. salina post-inoculation. Bio-encapsulation was stable for 7 days at 4°C. We found that the bio-encapsulated bacterins were successfully delivered to the lumpfish larvae gut 6 hours after vaccination. Oral immunization of lumpfish larval using bio-encapsulated bacterins resulted in an increased gene immune response. However, the innate and adaptive gene expression was low, suggesting that vaccination of lumpfish larvae need booster immunization.
We orally immunized lumpfish larvae with bio-encapsulated V. anguillarum bacterin. One group was oral boosted after 3 months post primary immunization. A second group was oral and i.p boosted after 3 months and 4 months post primary immunization, respectively. A control group was mock immunized with the vaccine vehicle. The fish were challenged with V. anguillarum (106 CFU/fish) after 6 months post primary immunization.
Oral boosted fish showed a mortality delay in contrast to the control group. The oral and i.p. boosted group showed a relative percent survival (RPS) of 75.5% (p < 0.0001).
In conclusion, immunization of lumpfish larvae is not recommendable since the immune system of the fish is not mature enough to mount a full protective immune response. Also, oral immunization seems to be not effective to protect the fish against V. anguillarum challenge. Finally, i.p. immunization trigger a protective immune response in lumpfish against V. anguillarum. Oral immunization perhaps required the utilization of live attenuated vaccines since inactive vaccines do not trigger a protective immune response in larvae or mature fish.