Due to the wide range of distribution, tendency to stationary persistence and high susceptibility of salmonid fish, vaccination against yersiniosis is an important part of prophylactic programs in many trout farms. The use of autovaccines prepared on the basis of isolates from a given fish farm is considered to be more effective than the use of commercial preparations. The aim of the study was to analyze cases of the disease in 2015 and to try to explain the failure of implemented prophylactic program based on autovaccination. Y. ruckeri strains isolated from one fish farm, from disease outbreaks in 2006, 2008 and 2015, as well as strains used for vaccine production were analyzed. Biotype evaluation, biochemical properties using API 20E assays, serotyping, and molecular identification were performed. All analyzed strains belonged to biotype 2. Two different profiles were obtained in API 20E tests. The first was obtained for vaccine strains and from disease cases in 2006 and 2008. The second was obtained for the strain isolated from the disease case in 2015. None of the profiles have been interpreted as Y. ruckeri in the APIweb. The identification of bacteria was confirmed by PCR. Disease strains and two vaccine strains belonged to the O1 serotype. Two of the vaccine strains have been qualified as O5 serotype. The obtained results indicate that the disease outbreak in 2015 was due to the emergence of a strain with increased pathogenicity and the use of an autovaccine based on strains belonging to the O5 serotype, considered to be less pathogenic. Immunization with the use of the O5 serotype does not provide complete protection against infection with other serotypes, so that fish remained susceptible to O1 serotype, which caused the disease in 2015. The results suggest the need for extending the diagnostics of strains used for the production of autovaccines.
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