Blood-feeding mosquitoes play an important role in the transmission of pathogens. Changes in climate and ecology as well as globalisation of international trade can influence their distribution (Reiter 1998; Becker 2008). During the last decades, various mosquito species, in particular Aedes albopictus have spread expeditiously from their tropical origin and colonised temperate zones. Some of these invasive species are capable to transmit chikungunya or dengue virus and autochthonous clinical cases caused by these viruses have been already reported from several European countries within the last few years (Gould et al. 2010; Gjenero-Margan et al. 2011).

Until recently, routine surveillance of mosquitoes in Germany was restricted to specific areas in the Southwest (Hesse, Rhineland-Palatinate, Baden-Wuerttemberg). Occasional findings of eggs and adults of A. albopictus along the Upper Rhine Valley have been reported in 2007 and 2011 (Pluskota et al. 2008; Werner et al. 2012) and the establishment of Ochlerotatus japonicus, another invasive species, has been recently discovered in wider areas of southern Germany (Becker et al. 2011). As a consequence, at the end of 2011, a surveillance network was initiated in close collaboration of scientific, traffic, and governmental institutions in order to assess the risk for introduction and spread of invading mosquitoes in Germany. The project focuses on selected possible hotspots that may act as introduction sites or stepping stones for invasive vectors. These sites include main international airports, harbours, and train stations with a high turnover of imported goods, as well as service stations and reloading sites at main motorways connecting Germany with countries in which A. albopictus and other invasive mosquito species have already been reported.

A total of 29 sites were selected (Fig. 1) and each site was equipped with one to three BG-Sentinel CO2 traps (Biogents) and three traditional ovitraps (Becker et al. 2010). In addition, to map the distribution of O. japonicus, a considerable number of cemeteries in southern Germany were sampled for mosquito larvae. From April 2012, all traps were inspected every second week. Until the end of the observation period in October 2012, invasive mosquito species were discovered repeatedly but only in traps placed in southern Germany. During the entire observation period, O. japonicus, eggs, larvae, and adults, respectively, were collected at various trapping sites, supporting previous observations that this species is widely distributed in southern Germany (Becker et al. 2010; Huber et al. 2012; Werner et al. 2012). In addition, in July and August 2012, A. albopictus adult females were discovered for the first time in Bavaria. The mosquitoes were found in traps located at two different service stations along the same motorway about 25 km apart from each other (Fig. 1). At the same time, two A. albopictus females were trapped at a motorway station in Baden-Wuerttemberg several hundred kilometres apart from the two infested sites in Bavaria. The finding prompted us to intensify surveillance, in that the period for inspection was reduced to weekly intervals and additional traps were set up in a perimeter of 200–500 m from the A. albopictus-positive traps according to European Centre for Disease Prevention and Control (2012) guidelines. During the intensified follow up, further A. albopictus adults were trapped at two of the three sites. In total, one male and 13 female mosquitoes were collected, comprising three to eight adults per infested site (Table 1). However, no A. albopictus oviposition was detected despite the distribution of considerable numbers of ovitraps and the fact that three out of 13 discovered females were gravid. The species A. albopictus was confirmed for all 14 specimens by PCR sequencing the mitochondrial cytochrome oxidase subunit 1 (CO1) gene (Folmer et al. 1994). To further determine whether the various mosquitoes represent siblings or independent introductions, a total of six microsatellite DNA markers were analysed according to previously published methods (Porretta et al. 2006; Kamgang et al. 2011). The results were not fully conclusive as only two of the markers were informative. However, the results were in favour of independent introductions at the three sites and the presence of siblings at the site in Baden-Wuerttemberg (Table 2).

Fig. 1
figure 1

Map of Germany showing the locations positive (n = 3) and negative (n = 26) for adult A. albopictus in the 2012 surveillance campaign. Numbers of positive sites refer to Table 1. BAV Bavaria, B-W Baden-Wuerttemberg

Full size image
Table 1 Inspections and findings of A. albopictus for each location per week, April–October 2012
Full size table
Table 2 Allele size variation of microsatellite markers from all A. albopictus trapped at the three different sites between July and October 2012
Full size table

Although the detection of A. albopictus (eggs or adults) in southwest Germany has been previously reported (Pluskota et al. 2008; Werner et al. 2012), the discovery of a considerable number of this invasive species concomitant in Bavaria and Baden-Wuerttemberg was unexpected. All mosquitoes were collected at service stations along main motorways connecting Germany with southern European countries either via Austria or Switzerland. Thus, it is likely that the mosquitoes originated from southern Europe and were introduced into Germany by transit road traffic. To our knowledge, there is no published report on the discovery of A. albopictus in Austria. However, there is information on a recent detection of A. albopictus larvae in September 2012 in the Lower Inn Valley in Tyrol (Franz Allesberger and Bernhard Seidel, personal communication), just 15–40 km apart from the infested sites in Bavaria. Thus, introduction from Austria to Bavaria cannot be excluded and intensified surveillance along the Inn Valley on both the German and the Austrian side is recommended.

All A. albopictus were collected in the second half of the trapping season suggesting that they were introduced into Germany in 2012 and not present before. Moreover, microsatellite DNA analysis suggests that the mosquitoes at the different sites represent independent introductions. Given the fact that we have discovered A. albopictus in traps at three out of seven service stations located along main motorways that connect Germany with southern European countries, it is evident that A. albopictus is regularly introduced into Germany. This however, poses the risk for establishment of this species at least in southern Germany, where conditions might be suitable for colonisation (Becker 2008).

The fact that relatively few adults and no other life stages of A. albopictus were found at the two locations in Bavaria indicates low levels of infestation. However, this might not be true for the infested site in Baden-Wuerttemberg where eight adults were collected and which is in close proximity to a site where A. albopictus was recently collected (Kampen et al. 2012). Thus, it will be important to continue intensified surveillance at all sites and beyond from early spring next year, to analyse whether A. albopictus is still present and whether control measures are indicated.

In conclusion, Germany is at risk for being colonised by A. albopictus due to regular and repeated introduction of this species by substantial ground vehicle transportation from southern European countries. To avoid establishment of this invasive mosquito species that poses a potential threat to public health, surveillance at least in southern Germany has to be expanded and suitable control measures should be in place.