[Populations of pathogenic Leptospira of varying virulence in nature]

The study of geographically remote populations of Norway rats (Rattus norvegicus) revealed that in one of these populations a highly virulent population of Leptospira copenhageni, serogroup icterohaemorrhagiae, and in another population of rats a faintly virulent population of these microorganisms circulated simultaneously. At the same time in vitro experiments with Leptospira cultures showed the absence of the constant probability of sharp changes in the level of their virulence in time.