Mechanisms Affecting the Acquisition, Persistence and Transmission of Francisella tularensis in Ticks

Abstract

Over 600,000 vector-borne disease cases were reported in the United States (U.S.) in the past 13 years, of which more than three-quarters were tick-borne diseases. Although Lyme disease accounts for the majority of tick-borne disease cases in the U.S., tularemia cases have been increasing over the past decade, with >220 cases reported yearly. However, when comparing Borrelia burgdorferi (causative agent of Lyme disease) and Francisella tularensis (causative agent of tularemia), the low infectious dose (<10 bacteria), high morbidity and mortality rates, and potential transmission of tularemia by multiple tick vectors have raised national concerns about future tularemia outbreaks. Despite these concerns, little is known about how F. tularensis is acquired by, persists in, or is transmitted by ticks. Moreover, the role of one or more tick vectors in transmitting F. tularensis to humans remains a major question. Finally, virtually no studies have examined how F. tularensis adapts to life in the tick (vs. the mammalian host), how tick endosymbionts affect F. tularensis infections, or whether other factors (e.g., tick immunity) impact the ability of F. tularensis to infect ticks. This review will assess our current understanding of each of these issues and will offer a framework for future studies, which could help us better understand tularemia and other tick-borne diseases.

A Francisella tularensis Chitinase Contributes to Bacterial Persistence and Replication in Two Major U.S. Tick Vectors

“…Chitin, a polymer of N-acetylglucosamine (GlcNAc), is a major component of the tick cuticle and is remodeled during the tick molting process. As such, it has been speculated that chitin may be used as an energy source by tick-borne bacterial pathogens [ 32 , 33 , 34 ]. Chitin cleavage can be performed by chitinases, and bacterial chitinases have been shown to promote bacterial persistence in marine environments and mammals [ 35 , 36 , 37 , 38 ].…”

Phase Variation of LPS and Capsule Is Responsible for Stochastic Biofilm Formation in Francisella tularensis

“…Therefore, the ability and exact role of biofilm in survival or virulence for these Francisella species has been questioned. If biofilm plays a role in persistence and transmission of Francisella remains unknown, largely because most biofilm studies have been completed with F. novicida (Tully and Huntley, 2020).…”

“…This divergence in environmental prevalence is also reflected in the associated arthropod vector. Namely, most tularemia cases in the United States are thought to occur from tick bites, while mosquitoes tend to be the drivers of European tularemia cases (Zellner and Huntley, 2019;Tully and Huntley, 2020).…”

A <em>Francisella tularensis</em> Chitinase Contributes to Bacterial Persistence and Replication in Two Major U.S. Tick Vectors

“…Chitin, a polymer of N-acetylglucosamine (GlcNAc), is a major component of the tick cuticle and is remodeled during the tick molting process. As such, it has been speculated that chitin may be used as an energy source by tick-borne bacterial pathogens [31][32][33]. Chitin cleavage can be performed by chitinases and bacterial chitinases have been shown to promote bacterial persistence in marine environments and mammals [34][35][36][37].…”