Some species of invasive mosquitoes of the genus Aedes, in addition to being a nuisance to citizens, also pose a threat to public health because they are potential vectors of diseases such as dengue, chikungunya, and Zika. Among them, the tiger mosquito (Aedes albopictus) is the most dangerous, because it can transmit numerous diseases, lives in close contact with us, and is spreading throughout Europe.
In Canton Ticino, a surveillance and control system for this species has been in place since 2000, which aims to contain its densities to reduce both nuisance and the risk of disease transmission. This surveillance system has been taken up in many other cantons under the supervision of the Swiss Mosquito Network (www.zanzare-svizzera.ch), supported by the Federal Office for the Environment. The surveillance system, in cooperation with the cantonal medical office, manages imported cases of these exotic diseases, preventing cases of local transmission. In many neighboring countries, however, numerous cases of local transmission of dengue or chikungunya are detected every summer.
In recent years, in addition to the tiger mosquito, two other invasive species, Aedes japonicus and Aedes koreicus, have arrived in Switzerland; however, these species are not considered to be a health risk, much less a source of great disturbance in urban areas. These species closely resemble the tiger mosquito: in their adult form they too are striped black and white and can confuse urbanites. The same applies to other life stages, such as their eggs, which form the basis of the system of surveillance traps (egg traps) distributed throughout the territory. This risks confusing the data of the surveillance system for the tiger mosquito, which is based mainly on a system of traps, very efficient and inexpensive, that detect the presence of invasive mosquitoes by laying their eggs.
Samples are analyzed in the laboratory by determining, through binoculars, the presence and densities of tiger mosquitoes. Failure to distinguish different invasive Aedes would result in ineffective data collection, undermining the validity of the surveillance system. Traditional methods, either breeding or molecular analysis, that can differentiate the eggs of invasive species, are, however, burdensome both in terms of time and cost, not allowing the analysis of the totality of samples, but only an extract of them, thus causing essential data to be lost for the success of the system. In samples where different species are present, there is a risk of not being able to identify all of them and define their densities.
A new technique that is yielding good results
The sector of Vector Ecology at SUPSI (Department of Environment Construction and Design) has succeeded in morphologically differentiating the outer structure of the eggs' membrane using a high-magnification microscope, thus being able to analyze quickly and at low cost all the samples that come from the surveillance system. This technique - illustrated in a paper published in the scientific journal Plos One - is not yet fully automated, but it has already yielded important operational answers: it is possible to quantify tiger mosquito densities with certainty, in case action is needed to limit their health risk. This system has also made it possible to detect the presence of tiger mosquitoes in new areas of Switzerland where co-presence with Aedes japonicus risks obscuring data regarding the tiger mosquito. For these cantons, it is important to determine the presence, or absence, of tiger mosquitoes, the only target for both nuisance and health risk.
SUPSI in collaboration with the World Health Organization is refining this technique on other mosquito species that have relevant health importance, such as Aedes aegypti, a mosquito also similar to the tiger mosquito, but a much more efficient vector and major culprit of dengue and chikungunya epidemics in tropical and sub-tropical countries. This species is already present in Europe, and its gateway is Cyprus.