Yersiniosis in Atlantic salmon is caused by Yersinia ruckeri ( Y. ruckeri). Since 2015 it has mainly been a problem in larger fish (> 1kg), causing elevated mortality over 30 % at cage level. The outbreaks occurring i n late sea phase have exclusively been caused by Y. ruckeri serotype O1b. Serotype O2 has also been found in a few freshwater sites but seems to be less virulent than O1b. So far, there is no evidence of O1a in Norwegian salmon industry.
Norwegian salmon farmers have since 2016 started using injection vaccination of smolts to secure long-term protection also after sea transfer. Efficacy after injection vaccination , assessing RPS, and safety, using the Speilberg scoring system, have been obtained in controlled laboratory studies. Field surveys have been used to collect safety and efficacy data in a commercial setting, with focus on survival and disease detection as well as antibody responses.
Laboratory challenge studies with different Y. ruckeri serotypes have been performed in Atlantic salmon, using different administration methods and challenge models. Results from these studies will be presented and discussed.
The data shows high consistency between results from laboratory studies and field surveys. The level of protection against yersiniosis is overall high, including in sites with elevated challenge due to outbreaks in adjacent unvaccinated cages. There have been no registered outbreaks on fish groups vaccinated with this vaccine by injection since the start in 2016. The water-based vaccine gives high level of protection against yersiniosis in Atlantic salmon up to slaughter (18 months post vaccination) and is safe to use in co-injection.