Detection of Francisella tularensis in voles in Finland.

Francisella tularensis is a highly virulent intracellular bacterium causing the zoonotic disease tularemia. It recurrently causes human and animal outbreaks in northern Europe, including Finland. Although F. tularensis infects several mammal species, only rodents and lagomorphs seem to have importance in its ecology. Peak densities of rodent populations may trigger tularemia outbreaks in humans; however, it is still unclear to which extent rodents or other small mammals maintain F. tularensis in nature. The main objective of this study was to obtain information about the occurrence of F. tularensis in small mammals in Finland. We snap-trapped 547 wild small mammals representing 11 species at 14 locations around Finland during 6 years and screened them for the presence of F. tularensis DNA using PCR analysis. High copy number of F. tularensis-specific DNA was detected in tissue samples of five field voles (Microtus agrestis) originating from one location and 2 years. According to DNA sequences of the bacterial 23S ribosomal RNA gene amplified from F. tularensis-infected voles, the infecting agent belongs to the subspecies holarctica. To find out the optimal tissue for tularemia screening in voles, we compared the amounts of F. tularensis DNA in lungs, liver, spleen, and kidney of the infected animals. F. tularensis DNA was detectable in high levels in all four organs except for one animal, whose kidney was F. tularensis DNA-negative. Thus, at least liver, lung, and spleen seem suitable for F. tularensis screening in voles. Thus, liver, lung, and spleen all seem suitable for F. tularensis screening in voles. In conclusion, field voles can be heavily infected with F. tularensis subsp. holarctica and thus potentially serve as the source of infection in humans and other mammals.