Arboviruses in Southern Europe: Current Status and Future Threats
Global warming is broadening the geographical range of the suitable habitat for most vectors. This, together with the increasing mobility of the human population and changes in land-use and land-cover may result in the establishment of local arboviral transmission cycles in temperate regions. In fact, in southern Europe we have already seen autochthonous cases of dengue fever (France, Croatia, Madeira, Catalunya and Murcia) and Chikungunya (Italy, France). Both diseases, together with Zika and yellow fever, are transmitted by mosquitoes of the Aedes genus, whose two most widespread species are Aedes aegypti and Aedes albopictus. Ae. aegypti feeds almost exclusively on humans and is therefore the more efficient disease spreader, making it the main culprit for the huge burden posed by these diseases in tropical regions. Ae. albopictus, on the other hand, feeds less often and on a variety of host species and does not, therefore, spread disease so effectively. However, its geographical range is much more extensive than that of Ae. Aegypti, which is only found in the tropics and is very sensitive to environmental conditions. Consequently, it is Ae. albopictus that is responsible for the ongoing arboviral colonisation of temperate regions. In fact, it is currently classified as one of the top 100 invasive species by the Invasive Species Specialist Group . It was first introduced in Europe around 1979 and it is now present in the whole Mediterranean basin, as can be seen in the map below:
By contrast, Madeira is the only part of Europe where Ae. aegypti is present although it has also been detected once in the Canary Islands.
Aedes mosquitoes adapt very well to urban settings in both tropical and temperate zones—unlike mosquitoes of the Anopheles genus (the vectors of malaria). This adaptability represents an additional threat to our highly urbanised societies since current projections estimate that by 2050 around two-thirds of the world’s population will live in urban areas (https://www.un.org/development/desa/en/news/population/2018-revision-of-world-urbanization-prospects.html )
In summary, the presence of Aedes albopictus mosquitoes in Catalunya has opened up the possibility of an Aedes-borne disease outbreak triggered by an imported case, that is, the arrival of an infected person travelling from an endemic country. As mentioned above, the main diseases transmitted by this vector are yellow fever, dengue, chikungunya and Zika. The global burden of yellow fever has been radically reduced by the development of a very efficient vaccine. Currently only rare (typically zoonotic-borne) self-limited outbreaks occur and the chance of importation is very low. Dengue and chikungunya, by contrast, have already demonstrated the potential threat they pose for southern Europe, and the same is true of Zika, as illustrated by the 2015 South American pandemic.
Although dengue, chikungunya and Zika are not the only arboviral threats posed by global warming and globalization in Southern Europe, ArboCat currently focuses on modelling the risk of outbreaks in Catalunya of these three diseases transmitted by the Aedes albopictus mosquito species because they represent the most serious and dramatic emergency we may have to deal with. The following is a description of the principal epidemiological aspects of these diseases.