title: Francisella tularensis: No Evidence for Transovarial Transmission in the Tularemia Tick Vectors Dermacentor reticulatus and Ixodes ricinus.
abstract: 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.
authors.affiliation: National Reference Laboratory for Tularemia, Istituto Zooprofilattico Sperimentale della Lombardia e dell'Emilia Romagna "Bruno Ubertini", Pavia, Italy.
authors.forename: Marco
authors.initials: M
authors.lastname: Genchi
authors.affiliation: National Reference Laboratory for Tularemia, Istituto Zooprofilattico Sperimentale della Lombardia e dell'Emilia Romagna "Bruno Ubertini", Pavia, Italy.
authors.forename: Paola
authors.initials: P
authors.lastname: Prati
authors.affiliation: National Reference Laboratory for Tularemia, Istituto Zooprofilattico Sperimentale della Lombardia e dell'Emilia Romagna "Bruno Ubertini", Pavia, Italy.
authors.forename: Nadia
authors.initials: N
authors.lastname: Vicari
authors.affiliation: National Reference Laboratory for Tularemia, Istituto Zooprofilattico Sperimentale della Lombardia e dell'Emilia Romagna "Bruno Ubertini", Pavia, Italy.
authors.forename: Andrea
authors.initials: A
authors.lastname: Manfredini
authors.affiliation: Department of Biology and Biotechnology (Lazzaro Spallanzani), Università degli Studi di Pavia, Pavia, Italy.
authors.forename: Luciano
authors.initials: L
authors.lastname: Sacchi
authors.affiliation: Department of Biology and Biotechnology (Lazzaro Spallanzani), Università degli Studi di Pavia, Pavia, Italy.
authors.forename: Emanuela
authors.initials: E
authors.lastname: Clementi
authors.affiliation: Department of Veterinary Science and Public Health (Divet), Università degli Studi di Milano, Milan, Italy.
authors.forename: Claudio
authors.initials: C
authors.lastname: Bandi
authors.affiliation: Department of Veterinary Science and Public Health (Divet), Università degli Studi di Milano, Milan, Italy.
authors.forename: Sara
authors.initials: S
authors.lastname: Epis
authors.affiliation: National Reference Laboratory for Tularemia, Istituto Zooprofilattico Sperimentale della Lombardia e dell'Emilia Romagna "Bruno Ubertini", Pavia, Italy.
authors.forename: Massimo
authors.initials: M
authors.lastname: Fabbi
mesh_terms: D000818:Animals; D001096:Arachnid Vectors / Q000382:microbiology*; D004269:DNA, Bacterial / Q000235:genetics; D003870:Dermacentor / Q000382:microbiology*; D005260:Female; D005604:Francisella tularensis / Q000235:genetics / Q000502:physiology* / Q000648:ultrastructure; D006168:Guinea Pigs; D054884:Host-Pathogen Interactions; D006801:Humans; D017404:In Situ Hybridization, Fluorescence; D018884:Ixodes / Q000382:microbiology*; D046529:Microscopy, Electron, Transmission; D009865:Oocytes / Q000382:microbiology; D010053:Ovary / Q000382:microbiology; D016133:Polymerase Chain Reaction; D012338:RNA, Ribosomal, 23S / Q000235:genetics; D014406:Tularemia / Q000175:diagnosis / Q000382:microbiology* / Q000635:transmission
chemical_list: D004269:DNA, Bacterial; D012338:RNA, Ribosomal, 23S