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Background  Tularemia is a zoonosis caused by the  Francisella tularensis , a highly infectious Gram-negative coccobacillus. Due to easy dissemination, multiple routes of infection, high environmental contamination and morbidity and mortality rates,  Francisella  is considered a potential bioterrorism threat and classified as a category A select agent by the CDC. Tick bites are among the most prevalent modes of transmission, and ticks have been indicated as a possible reservoir, although their reservoir competence has yet to be defined. Tick-borne transmission of  F .  tularensis  was recognized in 1923, and transstadial transmission has been demonstrated in several tick species. Studies on transovarial transmission, however, have reported conflicting results.    Objective  The aim of this study was to evaluate the role of ticks as reservoirs for  Francisella , assessing the transovarial transmission of  F .  tularensis  subsp.  holarctica  in ticks, using experimentally-infected females of  Dermacentor reticulatus  and  Ixodes ricinus .    Results  Transmission electron microscopy and fluorescence in situ hybridization showed  F .  tularensis  within oocytes. However, cultures and bioassays of eggs and larvae were negative; in addition, microscopy techniques revealed bacterial degeneration/death in the oocytes.    Conclusions  These results suggest that bacterial death might occur in oocytes, preventing the transovarial transmission of  Francisella . We can speculate that  Francisella  does not have a defined reservoir, but that rather various biological niches (e.g. ticks, rodents), that allow the bacterium to persist in the environment. Our results, suggesting that ticks are not competent for the bacterium vertical transmission, are congruent with this view.

Francisella tularensis: No Evidence for Transovarial Transmission in the Tularemia Tick Vectors Dermacentor reticulatus and Ixodes ricinus