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

Department of Medical Microbiology and Immunology, University of Toledo College of Medicine and Life Sciences, Toledo, OH 43614, USA
Author to whom correspondence should be addressed.
Microorganisms 2020, 8(11), 1639;
Received: 29 September 2020 / Revised: 15 October 2020 / Accepted: 21 October 2020 / Published: 23 October 2020
(This article belongs to the Special Issue Epidemiology of Tularemia and Francisella tularensis)


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.
About half of US tularemia cases are associated with tick bite, and annual cases are slowly increasing. 

Tularemia, in aerosol form, is considered a possible bioterrorist agent that if inhaled would cause severe respiratory illness. It was studied in Japan through 1945, the USA through the 60’s, and Russia is believed to have strains resistant to antibiotics and vaccines. An aerosol release in a high population would result in febrile illness in 3-5 days followed by pleuropneumonitis and systemic infection with illness persisting for weeks with relapses. The WHO estimates that an aerosol dispersal of 50 kg of F. tularensis over an area with 5 million people would result in 25,000 incapacitating casualties including 19,000 deaths.