Abstract
BackgroundTularemia 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.ObjectiveThe 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.ResultsTransmission 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.ConclusionsThese 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.
Highlights
Cultures and bioassays of eggs and larvae were negative; in addition, microscopy techniques revealed bacterial degeneration/death in the oocytes. These results suggest that bacterial death might occur in oocytes, preventing the transovarial transmission of Francisella
Ticks Are Not the Reservoir of F. tularensis reservoir, but that rather various biological niches, that allow the bacterium to persist in the environment
Tularemia is a zoonosis caused by Francisella tularensis, a highly infectious Gram-negative coccobacillus which has been isolated from over 250 species of mammals, birds, reptiles, amphibians, fish and invertebrates [1]
Summary
Tularemia is a zoonosis caused by Francisella tularensis, a highly infectious Gram-negative coccobacillus which has been isolated from over 250 species of mammals, birds, reptiles, amphibians, fish and invertebrates [1]. Multiple routes of infection, high environmental contamination and morbidity and mortality rates, F. tularensis is considered a potential bioterrorism threat and classified as a category A select agent by the Centers for Disease Control (CDC, Atlanta, Georgia, USA). For this reason, several studies have been carried out to evaluate variability of clinical features, feasibility and options for mass prophylaxis (i.e. vaccine), therapeutic approaches for treatment, development of improved diagnostic tests, including genome sequencing of several strains [2].
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