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Front. Cell. Infect. Microbiol. | doi: 10.3389/fcimb.2018.00089

Population genomics of Francisella tularensis subsp. holarctica and its implication on the eco-epidemiology of tularemia in Switzerland.

  • 1Biology, Spiez Laboratory, Switzerland
  • 2Division of Communicable Diseases, Federal Office of Public Health, Switzerland
  • 3Institute of Veterianry Bacteriology, Universität Bern, Switzerland
  • 4Swiss Tropical and Public Health Institute, Switzerland
  • 5Universität Basel, Switzerland
  • 6National Reference Center for Tick transmitted Diseases (NRZK), Switzerland
  • 7Medizinische Poliklinik, Kantonsspital Winterthur, Switzerland
  • 8Center for Biological Threats and Special Pathogens, Robert Koch Institut, Germany

Whole genome sequencing (WGS) methods provide new possibilities in the field of molecular epidemiology. This is particularly true for monomorphic organisms where the discriminatory power of traditional methods (e.g. restriction enzyme length polymorphism typing, multi locus sequence typing etc.) is inadequate to elucidate complex disease transmission patterns, as well as resolving the phylogeny at high resolution on a micro-geographic scale. In this study, we present insights into the population structure of Francisella tularensis subsp. holarctica, the causative agent of tularemia in Switzerland. A total of 59 Fth isolates were obtained from castor bean ticks (Ixodes ricinus), animals and humans and high resolution phylogeny was inferred using WGS methods. The majority of the Fth population in Switzerland belongs to the west European B.11 clade and shows an extraordinary genetic diversity underlining the old evolutionary history of the pathogen in the alpine region. Moreover, a new B.11 subclade was identified which was not described so far. The combined analysis of the epidemiological data of human tularemia cases with the whole genome sequences of the 59 isolates provide evidence that ticks play a pivotal role in transmitting Fth to humans and other vertebrates in Switzerland. This is further underlined by the correlation of disease risk estimates with climatic and ecological factors influencing the survival of ticks.

Keywords: Tularemia, Whole genome sequencing (WGS), Ticks, Francisella tularensis subsp. holarctica, Ecology, Epidemiology of infectious diseases, phylogenomics, canSNPs, core genome multilocus sequence typing, spatial statistics

Received: 21 Dec 2017; Accepted: 07 Mar 2018.

Edited by:

Jiri Stulik, Faculty of Military Health Sciences, University of Defence, Czechia

Reviewed by:

Prabhu B. Patil, Institute of Microbial Technology (CSIR), India
Jason Sahl, Northern Arizona University, United States  

Copyright: © 2018 Wittwer, Altpeter, Pilo, Gygli, Ackermann-Gäumann, Beuret, Karrer, Jacob, Grunow and Schuerch. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: PhD. Matthias Wittwer, Spiez Laboratory, Biology, Spiez, Switzerland,