Transstadial Transmission of Francisella tularensis holarctica in Mosquitoes, Sweden

In Sweden, human cases of tularemia caused by Francisella tularensis holarctica are assumed to be transmitted by mosquitoes, but how mosquito vectors acquire and transmit the bacterium is not clear. To determine how transmission of this bacterium occurs, mosquito larvae were collected in an area where tularemia is endemic, brought to the laboratory, and reared to adults in their original pond water. Screening of adult mosquitoes by real-time PCR demonstrated F. tularensis lpnA sequences in 14 of the 48 mosquito pools tested; lpnA sequences were demonstrated in 6 of 9 identified mosquito species. Further analysis confirmed the presence of F. tularensis holarctica–specific 30-bp deletion region sequences (FtM19inDel) in water from breeding containers and in 3 mosquito species (Aedes sticticus, Ae. vexans, and Ae. punctor) known to take blood from humans. Our results suggest that the mosquitoes that transmit F. tularensis holarctica during tularemia outbreaks acquire the bacterium already as larvae.