Ticks and Tularemia: Do We Know What We Don't Know?

Briana Zellner,Jason F Huntley

doi:10.3389/fcimb.2019.00146

Briana Zellner, Jason F Huntley

Open Access

https://doi.org/10.3389/fcimb.2019.00146

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Abstract

Francisella tularensis, the causative agent of the zoonotic disease tularemia, is characterized by high morbidity and mortality rates in over 190 different mammalian species, including humans. Based on its low infectious dose, multiple routes of infection, and ability to induce rapid and lethal disease, F. tularensis has been recognized as a severe public health threat—being designated as a NIH Category A Priority Pathogen and a CDC Tier 1 Select Agent. Despite concerns over its use as a bioweapon, most U.S. tularemia cases are tick-mediated and ticks are believed to be the major environmental reservoir for F. tularensis in the U.S. The American dog tick (Dermacentor variabilis) has been reported to be the primary tick vector for F. tularensis, but the lone star tick (Amblyomma americanum) and other tick species also have been shown to harbor F. tularensis. This review highlights what is known, not known, and is debated, about the roles of different tick species as environmental reservoirs and transmission vectors for a variety of F. tularensis genotypes/strains.

Highlights

Francisella tularensis (Ft), the causative agent of the zoonotic disease tularemia, can infect and cause lethal disease in over 300 species, including humans (Dennis et al, 2001; Keim et al, 2007)
A large number of complex studies have been performed to understand which tick vectors are infected with Ft, which ticks are most likely to transmit Ft, which Ft genotypes are most likely to be tick-transmitted, what tick life stage is the most infectious, or if Ft infections have impacts on ticks
The majority of Ft-tick studies have focused on D. variabilis which, in the U.S, has the widest geographic range (Figure 1) and is most often associated with human tularemia

Summary

INTRODUCTION

Francisella tularensis (Ft), the causative agent of the zoonotic disease tularemia, can infect and cause lethal disease in over 300 species, including humans (Dennis et al, 2001; Keim et al, 2007). This Gram-negative coccobacillus is divided into three subspecies: subsp. Type A strains, found solely in North America, are the most virulent for humans with a low infectious dose (

Francisella and Ticks

Host preferencea

Type B

Findings

CONCLUSIONS