F. tularensis holarctica, Sweden | CDC EID

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Volume 17, Number 5–May 2011

Research
Transstadial Transmission of Francisella tularensis holarctica in Mosquitoes, Sweden
Jan O. Lundström, Ann-Christin Andersson, Stina Bäckman, Martina L. Schäfer, Mats Forsman, and Johanna Thelaus

Authors affiliations: Uppsala University, Uppsala, Sweden (J.O. Lundström, M.L. Schäfer); and Swedish Defence Research Agency, Umeå, Sweden (A.-C. Andersson, S. Bäckman, M. Forsman. J. Thelaus)

Suggested citation for this article

Abstract
In Sweden, human cases of tularemia caused by Francisella tularensis holarctica are assumed to be transmitted by mosquitoes, but how mosquito vectors acquire and transmit the bacterium is not clear. To determine how transmission of this bacterium occurs, mosquito larvae were collected in an area where tularemia is endemic, brought to the laboratory, and reared to adults in their original pond water. Screening of adult mosquitoes by real-time PCR demonstrated F. tularensis lpnA sequences in 14 of the 48 mosquito pools tested; lpnA sequences were demonstrated in 6 of 9 identified mosquito species. Further analysis confirmed the presence of F. tularensis holarctica–specific 30-bp deletion region sequences (FtM19inDel) in water from breeding containers and in 3 mosquito species (Aedes sticticus, Ae. vexans, and Ae. punctor) known to take blood from humans. Our results suggest that the mosquitoes that transmit F. tularensis holarctica during tularemia outbreaks acquire the bacterium already as larvae.


Outbreaks of tularemia are caused by the bacterium Francisella tularensis holarctica throughout the Northern Hemisphere and by F. tularensis tularensis in North America only. Routes of infection include transmission from blood-sucking arthropods and through contact with infected dead or live animals, as well as from aerosols, dust, and water (1). Two primary disease manifestations, ulceroglandular and glandular tularemia, are associated with vector-borne transmission of the bacterium (1). Traditionally, mosquitoes are considered the primary vectors of F. tularensis holarctica to humans in Russia and Scandinavia (2–4,5). Moreover, mosquito-borne transmission of tularemia may be becoming more common in central Europe; evidence shows that this infection has reemerged during the past decade (6,7).

The ulceroglandular form of tularemia is by far the most common in Sweden; most human cases occur in late summer and early fall and are assumed to be transmitted by mosquitoes (4,8). A total of 5,754 human cases of tularemia were reported during 1931–1993, and the incidence of infection varies greatly among these years, ranging from a few cases in some years to >2,700 cases during 1967 (8). In the Örebro area of central Sweden, widespread mosquito-associated tularemia outbreaks first occurred during 2000 and 2003 (4,5), after which human cases have continued to occur in this new area where tularemia is endemic (www.smi.se/statistik/harpest). However, how vector mosquitoes acquire the bacterium is still not clear.

The demonstrated ability of F. tularensis holarctica strains to survive in association with protozoa indicates that ubiquitous aquatic protozoa might be an important environmental reservoir for the bacterium (9–11). Moreover, mosquito larvae, mainly the species A. sticticus and other floodwater mosquitoes, exert a predatory effect on aquatic protozoan populations (12). These factors indicate that mosquito larvae may be exposed to F. tularensis in their natural aquatic environment. We investigated the natural occurrence of F. tularensis in mosquitoes hatched from larvae collected in an area where tularemia was endemic. Because of unknown mechanisms, the bacterium F. tularensis is extremely difficult to isolate directly from environmental samples. Thus, our study focuses entirely on molecular techniques.

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F. tularensis holarctica, Sweden | CDC EID


Suggested Citation for this Article
Lundström JO, Andersson A-C, Bäckman S, Schäfer ML, Forsman M, Thelaus J. Transstadial transmission of Francisella tularensis holarctica in mosquitoes, Sweden. Emerg Infect Dis [serial on the Internet]. 2011 May [date cited].
http://www.cdc.gov/EID/content/17/5/794.htm


DOI: 10.3201/eid1705.100426


Comments to the Authors
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Jan O. Lundström, Program for Population Biology and Conservation Biology, Department of Ecology and Genetics, Evolutionary Biology Centre, Uppsala University, Norbyvägen 18D, SE-75236 Uppsala, Sweden; email: jan.lundstrom@ebc.uu.se