F. Schaffner, Avia-GIS, Zoersel, Belgium & University of Zurich, Switzerland
W. van Bortel, Institute of Tropical Medicine, Antwerpen, Belgium
Invasive mosquitoes are a topical subject. Following the chikungunya outbreak in the Indian Ocean (2005-2007) and in Italy (2007), in which the invasive Aedes albopictus was recognized as vector, ECDC has disseminated recommendations and European countries have developed risk assessment and management plans for invasive mosquitoes and related mosquito-borne diseases. In 2009, Ae. albopictus was reported as newly introduced into Malta, Ae. japonicus was reported as established in Belgium, Switzerland and Germany and Ae. atropalpus was reported for the first time from Netherlands (see VBORNET NL1 for more details). The international used tyre trade is once more confirmed as an essential role-player for mosquito introductions and for the first time an invasive mosquito, Ae. japonicus, is widely spreading in central Europe. There are still big gaps in information on mosquito fauna from several regions or countries, and it may be that the known occurrences of invasive mosquitoes are just the ‘tip of the iceberg’.
The number of European countries/micro-states in which the Asian tiger mosquito Ae. albopictus has been observed at least once has risen to 17 i.e. 29% of the 59 countries of continental Europe. It is well established and spreading in Albania, Croatia, France, Greece, Monaco, Montenegro, Italy, San Marino, Slovenia, Spain, and the Vatican City, and the species could now be considered as established in southern Switzerland. North of the Alps, in Belgium and Germany, no established populations have been recorded so far and in The Netherlands the species is only observed inside greenhouses. The record in northern Switzerland was a misidentification of Ae. japonicus.It is also present in isolated foci in Bosnia and Herzegovina, but few details are available (see ECDC technical report ‘Development of Aedes albopictus Risk Maps’ for more details). The recent observation in Malta, where no homogenous population have been previously recorded, raises the possibility of spread of the species from Italy to northern Africa by ferry traffic. In addition, the establishment of homogenous populations in southern France, including the city of Marseille where people have relatives in tropical regions where dengue or chikungunya circulate, raises the risk of import of pathogens and local transmission in Europe. Moreover, mosquitoes may start to spread further north by road traffic along the Rhone valley.
Aedes aegypti and other invasive mosquitoes
Aedes aegypti was once found sporadically in Europe from the Atlantic coast (Britain, France, and Portugal) to the Black Sea. It was assumed to be eradicated for decades, despite some occasional records from Italy in 1971 and Turkey in 1984 and 2001, but new infestations have recently appeared in Madeira and southern Russia. Its establishment in areas with temperate climates is constrained by its intolerance to cold winters (no cold-tolerant diapausing eggs), but changes in climates mean that its re-establishment in Southern Europe is becoming more likely.
Aedes atropalpus is an invasive North American mosquito that has been reported from Italy (1996) and France (2003) from where it has been eradicated. Very recently the species was found to have become established in the Netherlands, in two used tyre yards. In all cases there is evidence of introduction by the international used tyre trade and preliminary modelling shows that climatic conditions in The Netherlands are not a limiting factor for further spread of this species either in the country or further within Europe.
Aedes japonicus was reported from northern France and Belgium in 2000 and 2002 respectively. Once again, these introductions are related to the used tyre trade. The species was eradicated from the French site, thanks to mosquito control measures. and it remains confined to two used tyre yards in Belgium. In contrast, Aedes japonicus was also very recently (2008) found to be widespread in Switzerland and southern Germany
Aedes triseriatus is not yet known to be established in Europe but it has been intercepted in a batch of used tyres imported from Louisiana (USA) into France, in 2004, and the risk of its establishment in Europe following repetitive introductions is significant.
The situation in European overseas territories is totallydifferent to that in continental Europe because many are in (sub)-tropical regions. Both Ae. aegypti and Ae. albopictus are active vectors of diseases (dengue, chikungunya) in these territories, and other mosquitoes are involved in vector-borne diseases outbreaks, like malaria and filariasis.
Figure 4. Distribution of invasive mosquitoes in Europe. Orange: Ae. albopictus (as for 2008)
Red: Ae. aegypti,
Blue: Ae. japonicus,
Dark green: Ae. atropalpus
Source for albopictus: Development of Ae. albopictus Risk Maps, ECDC tech. report, 2009
Surveillance and control programmes in Europe
A major objective during the coming years will be the control of sanitary and vectorial risk related to invasive mosquitoes. The efficiency of this control will arise from (1) the active risk assessment, based on entomological surveillance (presence or abundance of vectors) and disease surveillance (surveillance of introduced pathogens and early detection of autochthonous transmission), and (2) the application of efficient sanitary control measures including integrated vector control. To date, only a few countries implement such measures. France and the Italian region Emilia-Romagna (Northern Italy) have an action plan for chikungunya and dengue control. Several other countries implement active or passive invasive mosquito surveillance in order to detect invasion as early as possible: Belgium, Croatia, Czech Republic, Germany, Greece, Montenegro, the Netherlands, Serbia, Spain, Switzerland, and the UK. In some countries, mosquito abatement measures are also implemented: Albania, Croatia, France, Italy, Spain, Switzerland; but action plans are often limited in space and/or time. The few successes in eradication or effective control are reported from locations in Croatia, France, Italy and Switzerland, and can be attributed directly to the early detection and abatement before spread starts. (See also ECDC technical report)
First reports of invasive species have been collected in different ways (Table 1): many of them (10 items, 40 % of the cases) were acquired during specific active surveillance, 7 were based on nuisance complaints (among them one from a report in a newspaper), 4 were based on expert observations during occasional visits, 3 were acquired during local mosquito studies (occasional) or surveillance (regular), and 1 was acquired during a routine inspection for plant protection.
This underlines the role and efficiency of specific surveillance for the early detection of invasive species. In very few cases specific surveillance has failed to find a species which was then reported in another way. This was the case in Spain where the tyre trade was investigated but Ae. albopictus was only found to be established in a residential area, most probably after its introduction by road transport. Another such case was in Malta where some specimens where observed in villages not considered as possible ports of entry. Unfortunately sometimes the media translate surveillance into discovery of the species, or report the presence of the Asian tiger mosquito without any scientific confirmation. This occurred in Hungary (2001), United Kingdom (2007) and Austria (2008).
Table 1. Ways of report of introduction of invasive mosquitoes in Europe: country and date of report per species
| Ae. albopictus
Bosnia and Herzegovina, 2005
| Ae. aegypti
| Ae. atropalpus
| Ae. japonicus
| Ae. triseriatus
Among these first introductions, almost half of them (12/25) are most probably due to transport by international (often intercontinental) trade, mainly of used tyres, though in one case of Lucky Bamboo, and sometimes of unknown goods, most probably imported by ground traffic (road or ferry) originating from an infested area.
Evidence therefore suggests that movements of invasive mosquitoes are increasing, probably due to a rise in volumes of international trade and of movements of ground traffic from infested areas (e.g. from Italy to the Balkans during the last decade). Entomological surveillance of invasive mosquitoes has also been shown to be essential for early detection and efficiency of subsequent mosquito abatement measures.
Finally another important factor in the future will be the ability of authorities to adapt to and cope with changes of mosquito borne disease risk caused by proliferation of new vectors and the introduction of pathogens in a changing environment and climate. This adaptation wills of course first need to be directed towards mosquito surveillance and control methods, but attention should also be targeted towards disease surveillance and adjusting regulations for a global risk management.