The post-COVID travel boom combined with a warm summer have led to dengue outbreaks in Italy and France.
Dengue typically affects tropical regions — but the disease is currently surging in parts of southern Europe, spreading among people there and reaching areas where it had not been recorded previously.
The mosquito-borne disease, which can cause fever, headaches and fatigue, and kills as many as 40,000 people each year, is not endemic to mainland Europe. Most incidences or small outbreaks originate from travellers who are infected abroad and bring the virus back. But this year, a combination of warm weather conditions and an increase in the number of imported cases has sparked a surge in local infections carried by tiger mosquitoes (Aedes albopictus), which inhabit southern Europe.
“It’s a situation that warrants an awful lot of attention,” says Patricia Schlagenhauf, an epidemiologist at the University of Zurich, Switzerland.
Nature asked researchers how worrying the problem is and whether the threat could grow in future.
Why are we seeing so many dengue cases in Europe this year?
Several factors are contributing to the rise in locally transmitted instances of dengue, researchers say. The resumption of international travel after the COVID19 pandemic has brought more travellers back from dengue-endemic areas. As of 27 October, France reported 1,414 imported instances of dengue; in 65% of cases, individuals had arrived from Martinique (550) and Guadeloupe (369). By contrast, only 217 imported cases were reported in 2022 and 164 in 2021.
The presence of A. albopictus mosquitoes capable of transmitting the virus means that when infected travellers are bitten after they return, these insects now carry the virus in their bloodstream and can transmit the disease to other people they bite.
These mosquitoes thrive in temperatures between 15 °C and 35 °C and can breed in small amounts of standing water. “It’s enough to have a container under your plants on your balcony with water in it. That’s an ideal place for the adults to breed,” says Schlagenhauf.
“At the moment, you have all of those conditions in southern Europe. You have a lot of travellers coming back, you have a local mosquito capable of transmitting the virus, and you also have the right climate and temperature,” she adds.
Where are the worst outbreaks currently?
As of 25 October, 105 locally transmitted cases have been reported in southern Europe, including 66 in Italy, 36 in France and 3 in Spain, according to the European Centre for Disease Prevention and Control.
The local outbreak in Italy, which has Europe’s largest A. albopictus population, centred around the Lombardy and Lazio regions and included 28 cases in Rome.
In France, the number of locally transmitted cases so far is lower than in 2022. But the outbreaks have now extended into the Auvergne-Rhône-Alpes and Île-de-France regions, areas that had not reported any locally transmitted cases before.
Because 50–90% of individuals are asymptomatic, the actual incidence of dengue could be higher than reported. “Through a door-to-door survey, we identified several autochthonous cases who had consulted their GP, but their doctors did not prescribe the [right] biological tests for dengue fever,” says Frédéric Jourdain, an epidemiologist at the French national public health agency, who is based in Montpellier. “There is still a need to raise awareness among health professionals.”
Could dengue become endemic in Europe?
Instances of local transmission in Europe tend to be sporadic and seasonal, peaking in late summer and early autumn, and decreasing during the winter months. “I don’t see that changing any time soon on the European continent, I do not see it moving into an all-year-round problem,” says Christina Frank, an infectious-disease epidemiologist at the Robert Koch Institute in Berlin.
For dengue to become endemic in Europe, the virus would have to establish itself in the local mosquito population. This means that an infected female mosquito has to pass the virus on to its eggs, so that when those hatch, the offspring are already infected. “From a scientific point of view, it is not impossible,” says Jourdain. “But such a thing is quite rare” and would usually occur in areas with much larger and year-round outbreaks.
“What we are observing in Europe is an emerging issue. But it’s clearly different from the dengue burden in tropical areas. So, it’s going to remain an imported pathology, a nonendemic [disease],” says Jourdain. “Will we have a [dengue] virus that is more adapted to Aedes albopictus? For me, it’s still a research question.”
What is the role of climate change in this?
Europe’s long and warm summer this year has created the ideal conditions for A. albopictus mosquitoes to thrive and colonize large urban areas.
The mosquitoes, which can also transmit viruses such as chikungunya and Zika, have “a longer period of activity and they can build up bigger populations if the summer is longer”, says Frank. “And they can also probably live a little further north than otherwise.”
“The time lag between human cases can reduce because processes in the mosquito can speed up, and there is more time for an outbreak to build up,” she adds.
The species is currently found in 20 European countries, and is expected to establish itself in northern and western Europe over the next few decades, including in Belgium, the Netherlands, parts of Germany and the southernmost regions of the United Kingdom. “It is an extraordinarily adaptive mosquito” because of its genetic diversity, says Didier Fontenille, a medical entomologist at the Institute of Research for Development in Montpellier. But it does take several years to build up a population of adapted mosquitoes, he adds.
What is the global trend in dengue infections?
Dengue is on the rise worldwide, with more than 4.2 million cases reported in 79 countries as of 2 October. The Americas are experiencing one of the highest incidences since 1980. Chad reported its first-ever outbreak in August, and Bangladesh — where dengue is endemic — has had nearly seven times more cases this year than it had in 2018.
“What that triggers is essentially bigger populations that are infected and higher risk of importation of these viruses into well-connected areas,” says Moritz Kraemer, an epidemiologist at the University of Oxford, UK. “The more often we see importations occur, the more possibilities there are for local transmission,” he adds.
Current global efforts to combat dengue outbreaks include introducing mosquitoes infected with Wolbachia, a bacterium that prevents the insect from transmitting viruses. The approach yielded promising results when tested on Aedes aegypti mosquitoes in Yogyakarta, Indonesia, and three cities in Colombia. But because the A. albopictus mosquito naturally carries two strains of Wolbachia, the situation is more challenging.
Source : Nature