E.-J. Scholte, National Centre for Monitoring of Vectors (CMV), Ministry of LNV, the Netherlands
M.A.H. Braks, National Centre for Public Health and the Environment (RIVM), the Netherlands
How nice would it be to have just one method for mosquito surveillance! One method that would provide useful answers to all the questions raised as rationale for mosquito surveillance like: ‘What is the spatial and temporal distribution of endemic mosquito species in a certain area? What are the relative abundances of nuisance species? When is the best time to start with mosquito control? Is mosquito abatement effective and sufficient? Where do invasive species occur and are they spreading or not? What geographical areas are high risk areas of mosquito-borne disease outbreaks due to high densities of potential mosquito vectors?’ Unfortunately, such a ‘one method’ does not exist. The choice of surveillance tools depends heavily on the information that is needed/wanted.
In areas where mosquito-borne viruses are (or recently were) circulating, such as northern Italy (Chikungunya, West Nile, Usutu), Finland and Sweden (Sindbis), or southern France (West Nile), vector surveillance is carried out with the aim to determine vector abundance and presence (incidence rates)/absence of the virus in the mosquito populations of the affected area. Surveillance is carried out by adult trapping (mostly using CO2-baited traps and gravid traps) that provide the samples for virus detection tests. Data for determining relative abundances of known and suspected vector species are collected by several methods, depending on the vector(s) and how these species are best collected. This could be either by adult trapping (e.g. Culex pipiens), larval dipping (e.g. Culex pipiens), or egg counts (oviposition traps, e.g. Ae. albopictus), or a combination. This type of mosquito surveillance is mostly restricted to relatively small areas.
Regarding detection of invasive exotic species, most emphasis is placed on Aedes albopictus, Ae. japonicus, and (for some Mediterranean areas) Ae. aegypti. Important lessons were learnt from the rapid spread of the invasive exotic Ae. albopictus in Italy. It prompted countries such as Italy itself, Albania, Belgium, Croatia, Czech Republic, France, Germany, Greece, Montenegro, the Netherlands, Serbia, Spain, Switzerland, and the UK to start mosquito surveillance to detect invasion as early as possible, making use mostly of oviposition traps and passive surveillance as tools for detection. In this case, strategies for trap placements depend on the most likely way of introduction like imported used tires and Lucky Bamboo, and road traffic originating from infested areas. In the case of surveillance activities at used tire companies, collection of larvae is often used. Occasionally, other invasive mosquito species are detected in these surveillance activities, such as Ae. atropalpus in Italy, France, and very recently also in the Netherlands. In cases where invasive species have become established in large areas, such as Ae. japonicus in Switzerland and Ae. albopictus in Italy, surveillance methods are based on known preferred breeding sites that are easily accessible (e.g. cemeteries for Ae. japonicus in Switzerland).
In addition to their vector potential, mosquitoes can be a notorious nuisance. Several areas in Europe experience mosquito nuisance for certain periods of the season. In those cases, surveillance is mostly used to determine the locations and timing of mosquito control activities, and to determine control activities efficacy afterwards (validation). Examples of large areas are the rice field areas in Greece and Italy, the wetlands of southern Sweden, the floodplain of the river Rhine (southern Germany), the French Mediterranean coast, some deltas in north-eastern Spain and south-western Portugal, and floodplains in Hungary, Poland and Serbia. In all of these cases surveillance is based on larval densities, measured by standardized ‘larval dipping’ methods, associated with adult trapping by human landing collection and CO2-baited traps for assessing the abatement’s efficiency.
In the situation where little is known about the spatial and temporal distribution of endemic mosquito species, surveillance should include a combination of both egg, larval and adult collections. This, however, is very labour intensive and thus expensive, so countries often have to choose a single approach to reduce costs. In Belgium for example, mosquito biodiversity is studied using adult traps only (MODIRISK project), placed randomly in the country (cross-sectional). The sampling strategy was specifically designed to prepare for spatial modelling and to test a variety of sampling strategies to contribute to the establishment of cost-effective mosquito surveillance protocols. The Netherlands applies a similar strategy in their nationwide mosquito surveillance.
The VBORNET project will be a unique opportunity to discuss the standardization of mosquito surveillance protocols. In addition VECMAP a recently started ESA funded project aims at providing a set of standardized software tools and user-tailored services for mosquito surveillance and control activities (see further in this newsletter).