[Ecological interactions among Yersinia in their common reservoir, the rodent]

Plague, due to Yersinia pestis, is still active in various foci in the Americas, in Africa and Asia, whereas it has been absent from Europe since the end of the 18th century, after having killed the two-thirds of the continent's inhabitants within four centuries. Various hypothesis have been proposed to attempt to explain the spontaneous "eradication" of plague from Europe, including the improvement of hygiene and habitat, changes in the rat population and cross-immunity induced by other infections, such as salmonellosis, leprosy and other yersiniosis. The only Yersinia currently isolated in Europe are the species genetically related to Y. pestis, Y. pseudotuberculosis and Y. enterocolitica, which are less virulent and mostly enteropathogenic. Y. pestis and Y. pseudotuberculosis have a DNA relatedness of 90%, whereas it is of only 60% with Y. enterocolitica. Y. pseudotuberculosis has been used as efficient vaccine against plague. Present world epidemiological data show that Y. enterocolitica is progressively replacing Y. pseudotuberculosis. Experimental infection by Y. enterocolitica, inducing a transitory and spontaneously cured infection in the immunocompetent host, only inducing opportunistic infections in the immunodeficient host, promotes efficient immunity against plague. Thus, it seems likely that the emergence of some variants of Yersinia, less virulent than Y. pestis, but able to induce a long-lasting protective immunity against plague, have contributed to its eradication by a silent enzootic infection among the wild reservoirs of rodents.