In recent years, there has been an increase in reported outbreaks of enteric redmouth disease (ERM) in ERM vaccinated farmed rainbow trout in a number of countries, including the United States and a range of European countries such as Denmark, Spain and the UK, which both affect animal welfare and cause considerable economic losses. These reported disease outbreaks have been associated with non-motile, virulent serovar O1 strains of Yersinia ruckeri, classified as biotype 2.

A standardised challenge model has been developed based on a newly isolated and highly virulent Y. ruckeri O1 biotype 2 strain obtained from an ERM disease outbreak in a Danish trout farm. This waterborne infection model gives us the opportunity to test and evaluate the effect of commercial and experimental vaccines against Y. ruckeri O1 biotype 2.

An experimental vaccine containing equal amounts of Y. ruckeri O1 biotype 1 and biotype 2 (whole cell bacterin) was developed, based on the most immunogenic strains from our collection. Rainbow trout were vaccinated by immersion (30sec) or bath (5min) in 1•10^9 CFU/ml, or given an intraperitoneal (i.p.) injection (5•10^8 CFU/fish).
The effect of the experimental vaccine has been compared to a state-of-the-art commercial ERM immersion vaccine (AquaVac® ReleraTM). Un-vaccinated and sham vaccinated rainbow trout were included as controls. Two months post vaccination the rainbow trout were challenged in duplicate with Y. ruckeri O1 biotype 2 by bath. No effect of the experimental immersion or bath vaccine was observed in the present study. However, full protection was achieved with i.p. injection of the experimental vaccine (p<0.0001, RPS=100%). Bath vaccination with AquaVac® ReleraTM induced a significant, partial protection relative to the control groups (p=0.002, RPS=58.5%).
It is suggested that the immunity induced by immersion and bath vaccination is inadequate for protection of the rainbow trout fry, since mortalities are still high in the vaccinated groups, which may explain the incidences of ERM disease outbreaks due to high virulent biotype 2 strains in farms, where the fish are primarily immersion vaccinated.