Yersinia ruckeri O1 biotype 2 (Y.r. bt2) has recently caused several outbreaks of Enteric Redmouth disease (ERM) in ERM-vaccinated farmed rainbow trout in Denmark, Finland, Spain, the UK and the USA. Recently, we have isolated a very virulent Y.r. bt2 strain from an ERM outbreak in a Danish rainbow trout stock which had been vaccinated against both Y.r. bt 1 and 2 with a commercial “state of the art” immersion vaccine and boosted with a commercial ERM oral vaccine 4 month later. The newly isolated Y.r. bt2 strain has been used to develop a standardized challenge model which gives us the opportunity to test and evaluate the effect of both commercial and experimental vaccines against Y.r. bt2 in a waterborne infection experiment, mimicking a natural infection.
We have developed an experimental immersion and bath vaccine containing equal amounts of the most immunogenic Y.r. bt 1 and 2 strains among the isolates obtained from ERM outbreaks in Denmark as well as other countries. The effect of the experimental vaccine is compared with a “state of the art” commercial vaccine which also contains Y.r. bt 1 and 2 in equal amounts.
Groups of rainbow trout were immersion vaccinated in a 1:10 dilution of the vaccine for 30 seconds as recommended by the manufacturer. As a positive control groups of trout were bath vaccinated for 5 minutes. Two months later was all groups bath challenged with Y.r. bt 2 in duplicate.
The challenge data obtained in the present study have indicated that neither a “state of the art” commercial nor the experimental immersion ERM vaccine provided significant protective immunity against the virulent Y.r. bt2 infections. IP. injection of the experimental vaccine without adjuvant induced full protection.
Significant mortality were seen in all immersion and bath vaccinated groups. The insufficient immunity induced by immersion and bath vaccination explains why ERM disease outbreaks are observed in fish farms where all the rainbow trout are properly vaccinated against ERM.