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Review
. 2020 Dec 23;9(1):26.
doi: 10.3390/microorganisms9010026.

Tularemia as a Mosquito-Borne Disease

Zakaria Abdellahoum  1 , Max Maurin  2   3 , Idir Bitam  1   4
Affiliations
Review

Tularemia as a Mosquito-Borne Disease

Zakaria Abdellahoum et al. Microorganisms. .

Abstract

Francisella tularensis (Ft) is the etiological agent of tularemia, a disease known for over 100 years in the northern hemisphere. Ft includes four subspecies, of which two are the etiologic agents of tularemia: Ft subsp. tularensis (Ftt) and Ft subsp. holarctica (Fth), mainly distributed in North America and the whole northern hemisphere, respectively. Several routes of human infection with these bacteria exist, notably through bites of Ixodidae ticks. However, mosquitoes represent the main vectors of Fth in Scandinavia, where large tularemia outbreaks have occurred, usually during the warm season. The mechanisms making mosquitoes vectors of Fth are still unclear. This review covers the inventory of research work and epidemiological data linking tularemia to mosquitoes in Scandinavia and highlights the gaps in understanding mosquitoes and Ft interactions.

Keywords: Francisella tularensis; Francisella tularensis subsp. holarctica; Scandinavia; Tularemia; arthropod vectors; mosquitoes.

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Conflict of interest statement

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Figures

Figure 1
Figure 1
Tularemia high-risk areas in Finland highlighted with purple color and in Sweden highlighted with blue color [54,57].
Figure 2
Figure 2
Cycle of F. tularensis transmission through mosquitoes. The yellow cycle illustrates mosquito contamination through a terrestrial animal reservoir (especially rodents); the Black cycle implies the acquisition of F. tularensis by the larval stage of mosquitoes and transstadial transmission to the adult stage (Pink dashed line), which subsequently becomes infectious. The red cycle illustrates an unconfirmed pathway of mosquito contamination through F. tularensis-contaminated flower nectar.
Figure 3
Figure 3
F. tularensis possible localizations inside mosquitoes and corresponding modes of transmission to humans. Mechanical contamination cycle via mosquito dejection: F. tularensis is ingested during a blood meal in an infected host ①, the bacterium ingested is located in the mosquito midgut ②, Francisella tularensis is then digested, crosses the Malpighian tube to be rejected onto a new host dermis during a blood meal ③. Mechanical contamination cycle via mosquito saliva: F. tularensis is ingested during a blood meal in an infected host ①, the bacterium stays hooked inside mosquito’s proboscis (i.e., the elongated appendage from the head), and then is reinjected to a new host via the contaminated saliva during a new blood meal ⑤. Biological contamination cycle: F. tularensis is ingested during a blood meal in an infected host ①, the bacterium migrates to the salivary glands and undergoes several multiplications ④, the bacterium is then injected into a new host via the contaminated saliva during a new blood meal ⑤.
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