Improved ERM vaccination efficacy using combined vaccine administration methods

We have previously shown that immersion vaccination (30 sec) using the Aquavac Relera vaccine (containing formalin killed Yersinia ruckeri serotype O1 of both biotypes 1 and 2) provides the best protection (when compared to other commercial ERM vaccines on the Danish market) against infection following i. p. challenge using Y. ruckeri O1, biotype 2, which at present is the main bacterial pathogen in fingerling trout production in Denmark.

Despite a significant protection conferred by this vaccine (immersion) some mortality could be observed following challenge. We have therefore performed a study in order to elucidate if different vaccine administration methods (using Aquavac Relera) can improve protection and reduce mortality of exposed trout following challenge with this particular pathogen.

Rainbow trout (mean weight 7.8 g) reared at the Bornholm Salmon Hatchery under pathogen free conditions were vaccinated by 1) a single immersion (30 sec) and no booster, 2) a single immersion (30 sec) and a second immersion booster (30 sec) 410 degree-days later, 3) a single injection i.p. (0.1 ml) and no booster and finally 4) a single immersion (30 sec) and a booster injection i.p. (0.1 ml) 410 degree-days later.

Fish were compared to naïve and non-vaccinated control fish. All experiments were performed in duplicate. Fish were brought to our university facilities, acclimated for 14 days and then challenged by i.p. injection of Y. ruckeri O1, biotype 2 (8.5 x 106 cfu/fish). Control fish exhibited a mortality of 65 %, fish immersed only once showed a mortality of 36 %, fish immersed twice had 4 % mortality, fish injected once reached 2 % mortality.

Rainbow trout immersed once and subsequently boosted by injection showed no mortality at all. This superior protection offered by a combination of primary immersion vaccination and a booster vaccination by injection may lead to improved health and reduced drug application in Danish trout farms.