Linking Zoonosis Emergence to Farmland Invasion by Fluctuating Herbivores: Common Vole Populations and Tularemia Outbreaks in NW Spain

Water and mosquitoes as key components of the infective cycle of Francisella tularensis in Europe: a review

Francisella tularensis is the pathogen of tularemia, a zoonotic disease that have a broad range of hosts. Its epidemiology is related to aquatic environments, particularly in the subspecies holarctica. In this review, we explore the role of water and mosquitoes in the epidemiology of Francisella in Europe. F. tularensis epidemiology has been linked to natural waters, where its persistence has been associated with biofilm and amebas. In Sweden and Finland, the European countries where most human cases have been reported, mosquito bites are a main route of transmission. F. tularensis is present in other European countries, but to date positive mosquitoes have not been found. Biofilm and amebas are potential sources of Francisella for mosquito larvae, however, mosquito vector capacity has not been demonstrated experimentally, with the need to be studied using local species to uncover a potential transmission adaptation. Transstadial, for persistence through life stages, and mechanical transmission, suggesting contaminated media as a source for infection, have been studied experimentally for mosquitoes, but their natural occurrence needs to be evaluated. It is important to clear up the role of different local mosquito species in the epidemiology of F. tularensis and their importance in all areas where tularemia is present.

Strengthening the genomic surveillance of Francisella tularensis by using culture-free whole-genome sequencing from biological samples

Introduction

Francisella tularensis is a highly infectious bacterium that causes the zoonotic disease tularemia. The development of genotyping methods, especially those based on whole-genome sequencing (WGS), has recently increased the knowledge on the epidemiology of this disease. However, due to the difficulties associated with the growth and isolation of this fastidious pathogen in culture, the availability of strains and subsequently WGS data is still limited.

Methods

To surpass these constraints, we aimed to implement a culture-free approach to capture and sequence F. tularensis genomes directly from complex samples. Biological samples obtained from 50 common voles and 13 Iberian hares collected in Spain were confirmed as positive for F. tularensis subsp. holarctica and subjected to a WGS target capture and enrichment protocol, using RNA oligonucleotide baits designed to cover F. tularensis genomic diversity.

Results

We obtained full genome sequences of F. tularensis from 13 animals (20.6%), two of which had mixed infections with distinct genotypes, and achieved a higher success rate when compared with culture-dependent WGS (only successful for two animals). The new genomes belonged to different clades commonly identified in Europe (B.49, B.51 and B.262) and subclades. Despite being phylogenetically closely related to other genomes from Spain, the detected clusters were often found in other countries. A comprehensive phylogenetic analysis, integrating 599 F. tularensis subsp. holarctica genomes, showed that most (sub)clades are found in both humans and animals and that closely related strains are found in different, and often geographically distant, countries.

Discussion

Overall, we show that the implemented culture-free WGS methodology yields timely, complete and high-quality genomic data of F. tularensis, being a highly valuable approach to promote and potentiate the genomic surveillance of F. tularensis and ultimately increase the knowledge on the genomics, ecology and epidemiology of this highly infectious pathogen.

Vole outbreaks may induce a tularemia disease pit that prevents Iberian hare population recovery in NW Spain

Iberian hare populations have suffered severe declines during recent decades in Spain. Between 1970 and 1990s, a rapid increase in irrigation crop surface in NW Spain (Castilla-y-León region) was followed by a common vole massive range expansion and complete colonization of lowland irrigated agricultural landscapes from mountainous habitats. The subsequent large cyclic fluctuations in abundance of colonizing common voles have contributed to a periodic amplification of Francisella tularensis, the etiological agent that causes human tularemia outbreaks in the region. Tularemia is a fatal disease to lagomorphs, so we hypothesize that vole outbreaks would lead to disease spill over to Iberian hares, increasing prevalence of tularemia and declines among hare populations. Here we report on the possible effects that vole abundance fluctuations and concomitant tularemia outbreaks had on Iberian hare populations in NW Spain. We analysed hare hunting bag data for the region, which has been recurrently affected by vole outbreaks between 1996 and 2019. We also compiled data on F. tularensis prevalence in Iberian hares reported by the regional government between 2007 and 2016. Our results suggest that common vole outbreaks may limit the recovery of hare populations by amplifying and spreading tularemia in the environment. The recurrent rodent-driven outbreaks of tularemia in the region may result in a "disease pit" to Iberian hares: at low host densities, the rate of population growth in hares is lower than the rate at which disease-induced mortality increases with increased rodent host density, therefore, keeping hare populations on a low-density equilibrium. We highlight future research needs to clarify tularemia transmission pathways between voles and hares and confirm a disease pit process.

Viral Zoonoses in Small Wild Mammals and Detection of Hantavirus, Spain

We screened 526 wild small mammals for zoonotic viruses in northwest Spain and found hantavirus in common voles (Microtus arvalis) (1.5%) and high prevalence (48%) of orthopoxvirus among western Mediterranean mice (Mus spretus). We also detected arenavirus among small mammals. These findings suggest novel risks for viral transmission in the region.