Yersinia ruckeri (Y.r.) O1 biotype (bt) 2 causes outbreaks of Enteric Redmouth disease (ERM) in vaccinated, farmed rainbow trout around the world. Y.r. O1 bt 1 has been known since first described from the Hagerman Valley in Idaho in the 1950’s and outbreaks have now been controlled by vaccination for more than 35 years. Recent years have shown ERM outbreaks caused by Y.r. O1 bt 2 reported from ERM vaccinated farmed rainbow trout stocks in both Europe and the USA. It has been suggested that Y.r. O1 bt 2 is more virulent than bt 1. In order to test this hypothesis we have collected Y.r. O1 isolates from ERM outbreaks in Danish rainbow trout farms during 2011. Some of the isolates were obtained from vaccinated stocks, which subsequently developed ERM disease outbreaks. The Y.r. O1 isolates have been characterized and biotyped according to mobility and hydrolysis of Tween-80, and almost all isolates obtained in 2011 were double negative, which characterizes them as bt 2. Only one Y.r. O1 bt 1 strain was isolated, the rest of all the Y.r. O1 strains belonged to bt 2. Y.r. O1 bt 1 has been isolated from Danish rainbow trout farms for 30 years, but Y.r. O1 bt 2 has first been detected 12 years later and has caused many outbreaks of ERM which has led to an increase in the use of antibiotics.
In order to determine the virulence of the newly isolated Y.r. O1 strains, experimental infections were carried out. The mortalities of the newly isolated strains were compared to well characterized isolates of both bt 1 and 2.
In order to mimic a natural Y.r. infection, a bath infection model was applied. All Y.r. O1 isolates caused significant mortality compared to the un-infected control group. The Y.r. O1 isolates could be divided into two groups, regarding their virulence: A low virulence group causing less than 50 % mortality, and a high virulence group causing more than 50 % mortality. Interestingly, all tested bt 1 isolates grouped in the low virulence group together with a few bt 2 isolates. The highly virulent strains were exclusively Y.r. O1 bt 2 strains. The most virulent strain was obtained from a population of rainbow trout which had been immersion vaccinated with a commercial vaccine containing a bacterin of both Y.r. O1 bt 1 and 2 and received an oral booster vaccination 4 month later as recommended by the manufacturer.
The characterization of this highly virulent strain of Y.r. O1 bt 2 has made it possible to develop a standardized bath challenge model which is needed in order to test and develop new experimental vaccines which are expected to induce immunity against Y.r. O1 bt 2.