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

<div><p>Background</p><p>Tularemia is a zoonosis caused by the <i>Francisella tularensis</i>, a highly infectious Gram-negative coccobacillus. Due to easy dissemination, multiple routes of infection, high environmental contamination and morbidity and mortality rates, <i>Francisella</i> 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 <i>F</i>. <i>tularensis</i> was recognized in 1923, and transstadial transmission has been demonstrated in several tick species. Studies on transovarial transmission, however, have reported conflicting results.</p><p>Objective</p><p>The aim of this study was to evaluate the role of ticks as reservoirs for <i>Francisella</i>, assessing the transovarial transmission of <i>F</i>. <i>tularensis</i> subsp. <i>holarctica</i> in ticks, using experimentally-infected females of <i>Dermacentor reticulatus</i> and <i>Ixodes ricinus</i>.</p><p>Results</p><p>Transmission electron microscopy and fluorescence in situ hybridization showed <i>F</i>. <i>tularensis</i> within oocytes. However, cultures and bioassays of eggs and larvae were negative; in addition, microscopy techniques revealed bacterial degeneration/death in the oocytes.</p><p>Conclusions</p><p>These results suggest that bacterial death might occur in oocytes, preventing the transovarial transmission of <i>Francisella</i>. We can speculate that <i>Francisella</i> 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.</p>