The effect of oral vaccines against bacterial fish diseases has been a topic for debate in many years. Recently both M-cells and dendritic cells have been found in fish and it is therefore likely that antigens can be taken up from the intestine and induce immunity in orally vaccinated fish.
The objective for this project is to investigate whether oral vaccination of rainbow trout against Yersinia ruckeri O1 (biotype 1) causing Enteric Red Mouth disease (ERM) can protect rainbow trout against a subsequent experimental bath challenge with Y. ruckeri.
The rainbow trout were given oral vaccinations with AquaVacTM ERM Oral vet. (MSD animal health) or an experimental vaccine based on killed Yersinia ruckeri O1, (biotype 1) bacteria. Seven groups were studied: 1) Control group (no vaccination, no infection), 2) infected control, 3) experimental vaccine, 4) experimental vaccine w/ booster (4 months post primary vaccination), 5) AquaVac ERM (as a primary vaccine), 6) AquaVac w/ booster, and 7) one group with 10 fold increase (w/ booster) of the experimental vaccine in the feed.
The rainbow trout were bath challenged with 6.3 x108 CFU/ml Y. ruckeri 6 month post the primary oral vaccination. The challenge induced significant mortality in all infected groups except for the group which received the experimental vaccine in a ten times higher concentration. These results show that rainbow trout can induce specific immunity against Y. ruckeri after oral vaccination, and that high concentrations of dead Y. ruckeri bacteria are need in order to obtain significantly increased immunity against the disease. These results suggest that a high amount of the vaccine is digested in the stomach of the rainbow trout and therefore did not reach the intestine as immunogenic antigens.
The project is still ongoing, and samples have been taken for several immunological assays.