Background. Ternopil region is endemic on leptospirosis. Its natural conditions (slightly alkaline or alkaline soils, air temperature, sufficient rainfall) contribute to the existence of major natural reservoir of the pathogen–mouse-like rodents. In the region, different serovariants of leptospira are exuded by rodents and farm animals. Objective. The materials of the Department of Highly Infectious Diseases of Ternopil Regional Laboratory Centre of the State Sanitation and Epidemiological Service of Ukraine, Ternopil Regional Laboratory of Veterinary Medicine, Clinic of Infectious Diseases of TSMU were studied. Leptospiras were detected by dark ground microscopy (DFM) of blood of patients, trapped rodents and examined farm animals. Results. The circulating of pathogens between different sources (rodents, animals) and annual disease incidence evidences that new leptospira serovar are carried onto endemic area mostly by farm animals; humans are infected from them through the environment sometimes in 3-5 years intervals; the further diffusion to the new areas of this pathogen serovars in all kinds of the examined mouse-like rodents is noticed. It is established that farm animals and rodents are competing reservoirs. To predict the future epidemiological situation of leptospirosis among the humans and to improve its diagnosis the constant monitoring of the population, infection and leptospira carriage among mouse-like rodent and farm animals and expanding of the panel of diagnostic leptospira strains including new pathogen variants in animals is necessary. Conclusions. The development of additional reservoirs in animals, with circulating of other pathogen serovars among them, such as mouse-like rodents, which were previously absent in the main natural reservoir, cause the change of etiological structure in human leptospirosis at the endemic areas. The range of human leptospirosis pathogens and its further spreading among all kinds of rodents increased during our research. The results of detection of leptospirosis pathogens among the various contingents which were studied evidence that the farm animals and rodents are competing reservoirs that cause human infection through environment.