Wolbachia prevalence in the vector species Culex pipiens and Culex torrentium in a Sindbis virus-endemic region of Sweden

Alexander Bergman,Jenny C Hesson

doi:10.1186/s13071-021-04937-6

Abstract

BackgroundWolbachia pipientis are endosymbiotic bacteria present in a large proportion of terrestrial arthropods. The species is known to sometimes affect the ability of its host to transmit vector-borne pathogens. Central Sweden is endemic for Sindbis virus (SINV), where it is mainly transmitted by the vector species Culex pipiens and Culex torrentium, with the latter established as the main vector. In this study we investigated the Wolbachia prevalence in these two vector species in a region highly endemic for SINV.MethodsCulex mosquitoes were collected using CDC light traps baited with carbon dioxide over 9 years at 50 collection sites across the River Dalälven floodplains in central Sweden. Mosquito genus was determined morphologically, while a molecular method was used for reliable species determination. The presence of Wolbachia was determined through PCR using general primers targeting the wsp gene and sequencing of selected samples.ResultsIn total, 676 Cx. pipiens and 293 Cx. torrentium were tested for Wolbachia. The prevalence of Wolbachia in Cx. pipiens was 97% (95% CI 94.8–97.6%), while only 0.7% (95% CI 0.19–2.45%) in Cx. torrentium. The two Cx. torrentium mosquitoes that were infected with Wolbachia carried different types of the bacteria.ConclusionsThe main vector of SINV in the investigated endemic region, Cx. torrentium, was seldom infected with Wolbachia, while it was highly prevalent in the secondary vector, Cx. pipiens. The presence of Wolbachia could potentially have an impact on the vector competence of these two species. Furthermore, the detection of Wolbachia in Cx. torrentium could indicate horizontal transmission of the endosymbiont between arthropods of different species.Graphical abstract

Highlights

Wolbachia pipientis are endosymbiotic bacteria present in a large proportion of terrestrial arthropods
Wolbachia was present in 96.5% of the Cx. pipiens population but could only be detected in two out of 293 Cx. torrentium individuals (0.68% prevalence, 95% CI 0.19–2.45%) (Table 1)
The other Cx. torrentium carried a Wolbachia whose wsp gene was highly similar (> 99.6% identity) to that of Wolbachia detected in several other insects, namely the spotted fritillary (Melitaea didyma; GenBank: MN322891.1), silverleaf whitefly (Bemisia tabaci; GenBank: AJ291379.1), azalea lace bug (Stephanitis pyrioides, GenBank: AB109622.1), Macrolophus pygmaeus (GenBank: FJ374283.1), and Amaurosoma flavipes (GenBank: JN601166.1), all of which carry Wolbachia from supergroup B

Summary

Introduction

Wolbachia pipientis are endosymbiotic bacteria present in a large proportion of terrestrial arthropods. Central Sweden is endemic for Sindbis virus (SINV), where it is mainly transmitted by the vector species Culex pipiens and Culex torrentium, with the latter established as the main vector. In this study we investigated the Wolbachia prevalence in these two vector species in a region highly endemic for SINV. One important biotic factor is the intracellular symbiont Wolbachia pipientis (Class: Alphaproteobacteria, Order: Rickettsiales), present in some nematode species and an estimated 40% of all terrestrial arthropods [5]. Northern Europe, the morphologically identical vector species Cx. pipiens and Cx. torrentium are enzootic vectors of both WNV and Sindbis virus (SINV), transmitting these viruses among birds [19,20,21,22]. In Sweden, SINV is considered endemic to the central and northern parts of the country [28,29,30]

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