First report on human-biting Culex pipiens in Sweden.

Culex mosquitoes are vectors of several bird-hosted arboviruses that cause outbreaks in Europe, such as Sindbis virus and West Nile virus. Recently, the human-biting form of Culex pipiens, Cx. pipiens biotype molestus, was found causing big nuisance in a housing cooperative in Gothenburg in southern Sweden, confirmed by molecular identification. This is the first report of human-biting Culex in Scandinavia, signalling increased risk of arbovirus infection in northern Europe.

Letter to the Editor

Mosquitoes of the genus Culex are the vectors of bird-hosted arboviruses that occasionally cause disease in humans, such as Sindbis virus (SINV) (Alphavirus) West Nile virus (WNV) and Usutu virus (USUV) (Flavivirus), all circulating in Europe. Seven potential vector species are recognised within the genus Culex in Europe, most of which show a preference for biting birds (ornithophilic). One of the species, the northern house mosquito Cx. pipiens, has been described as having two separate biotypes: Cx. pipiens biotype pipiens and Cx. pipiens biotype molestus, of which the latter fulfils certain criteria that separates it from the other: it does not hibernate in winter (homodynamic), it utilizes underground larval habitat (hypogeous), it can mate in confined spaces (stenogamous), it can lay its first egg batch without a blood meal (autogenous), and it feeds on humans (mammophilic). Culex pipiens biotype molestus has been reported from many central and southern European countries but never from Scandinavia [1-4]. Hybridisation of the two biotypes is quite common in the Mediterranean area, while hybrids are less commonly found in central Europe [3, 4]. Culex pipiens biotype molestus and potential hybrids are of high concern since they have the potential to serve as bridge vectors of these arboviruses from birds to humans, as they possess the vector competence of Cx. pipiens biotype pipiens [5] but an opportunistic feeding behaviour [6].

The last countrywide mosquito survey recognised three Culex species as endemic to Sweden: the ornithophilic species Cx. torrentium, Cx. pipiens biotype pipiens and Cx. territans that primarily feeds on ectothermic hosts [7]. Recently, an encounter with Cx. modestus that commonly bite man in central European countries has also been reported from southern Sweden [8].

In August 2016, one of the authors visited a private housing cooperative situated approximately 4 km south west of central Gothenburg, Sweden, that had experienced problems with mosquitoes biting tenants indoors. The housing cooperative is a property of approximately 4.5 ha with seven blocks of three-storey buildings. It is located in a residential area with similar buildings, intermixed with smaller park areas with deciduous bushes and trees. An online survey among the tenants showed that nearly 100 households (roughly a third of all flats in the cooperative), distributed throughout all the buildings and on all three floors of the cooperative, had problems with mosquitoes biting indoors. The earliest complaint of indoor biting was in winter/spring 2015, and since then the problem seems to have gradually increased, showing peaks in summer and autumn but also occurring during wintertime. The mosquitoes were reported to enter through open windows and balcony doors, as well as through the ventilation system on all three floors, and to be aggressive biters on humans during evening and night-time. Eleven female Culex mosquitoes were caught as they tried to bite humans, and Culex larvae were sampled in a catch basin just outside one of the affected buildings.

The eleven adult Culex females caught indoors and ten larvae from the outside catch basin were used for DNA extraction and molecular species identification. Since Cx. torrentium is the most common Culex species in Sweden, a first assay for separating Cx. torrentium and Cx. pipiens (s.l.) was run [9], identifying all 11 adults and ten larvae as Cx. pipiens (s.l.). The DNA was then amplified in a second PCR designed to distinguish between Cx. pipiens biotype pipiens and Cx. pipiens biotype molestus [10], resulting in identification of all 11 adults as Cx. pipiens biotype molestus (Fig. 1), and the ten larvae as Cx. pipiens biotype pipiens.

Fig. 1

Electrophoresis gel confirming the presence of Cx. pipiens biotype molestus in Sweden. PCR products from a diagnostic assay separating Cx. pipiens biotype pipiens and Cx. pipiens biotype molestus, from left to right: a 100 bp size marker, 1 Cx. pipiens biotype pipiens (control), 11 Cx. pipiens biotype molestus collected in Gothenburg

Thus, environmental characteristics (homodynamic and mammalophilic), as well as molecular identification confirm the first report of presence of Cx. pipiens biotype molestus in Sweden. As of yet, however, no larval habitat has been identified.

This is the first time that a species of Culex has been reported to cause nuisance in Scandinavia. The most common Culex species in Sweden, Cx. torrentium, is often infected with SINV and is also a very efficient enzootic vector of the virus among birds [11, 12]. For transmission to humans to occur, it is also necessary to have a competent bridge vector that feeds opportunistically on both birds and humans. The discovery of Cx. pipiens biotype molestus in a densely populated area Sweden is important, as the presence of a bridge vector may increase the transmission risk of endemic SINV, as well as the risk for introduction and establishment of WNV and USUV. In addition to its vector potential, Cx. pipiens biotype molestus causes severe nuisance to humans, biting usually during night-time.

The origin of the Swedish Cx. pipiens biotype molestus is unknown. Population genetics studies have shown that populations of Cx. pipiens biotype molestus from different European countries are more closely related to each other than to nearby Cx. pipiens biotype pipiens populations [1, 2, 4, 6]. Thus Cx. pipiens biotype molestus is considered by most authors to be introduced into an area, rather than evolving through an adaptation of local Cx. pipiens biotype pipiens populations. The ways of introduction of Cx. pipiens biotype molestus to Sweden, and its distribution, will be further investigated. These are important steps towards a realistic risk assessment for mosquito-borne virus transmission to humans in Scandinavia.