Aim: To perform a comparison of genetic characteristics of vaccine strain EV and its putative "virulent derivatives", obtained after passages through highly susceptible animals, in order to identify the strains-"revertants" and establish their possible origin.
Materials and methods: Yersinia pestis EV strains and its putative "virulent derivatives" were used in the study. Polymerase chain reaction and DNA-DNA hybridization were used for genetic analysis.
Results: Comparison of several genetic characteristics of vaccine strain EV and its putative "virulent derivatives" allowed to establish that virulent "revertants" are not derivatives of vaccine strain EV because they do not belong to East biovar, do not have ribotype characteristic for EV strain and contain pigmentation area, which is absent in vaccine strain.
Conclusion: Obtained results evidence against possibility of reversion of vaccine EV strain to virulent forms in organisms of highly susceptible animals and confirm its safety for vaccination.