Chikungunya, dengue, Zika – what are the risks in mainland France?

In the twenty years since the tiger mosquito was first reported in mainland France in 2004, it has conquered four-fifths of the country. How has it been able to do so?

Louis Lambrechts: The tiger mosquito, originally from South-East Asia, is particularly invasive. It is able to survive our winters because when the temperature gets too cold its eggs can enter diapause, a sort of state of hibernation. And although it arrived in Europe before the recent acceleration in climate change, the changing climate has helped it to spread. Longer summers provide time for more breeding cycles, enabling the mosquitoes to proliferate more quickly across an area that is expanding increasingly northwards.

What viruses can these mosquitoes transmit?

L. L.: Tiger mosquitoes can spread dengue, an intense form of tropical influenza, and chikungunya, which causes debilitating joint pain that can last for months or even years. Luckily, these two viruses are not endemic, meaning they have no permanent reservoir in mainland France, so most of the tiger mosquitoes circulating in our regions do not carry them. But the mosquitoes can be infected with one of these viruses when feeding on the blood of an infected person, for example a traveler returning from a tropical or subtropical region, and then pass the virus on by biting someone else. This is not such a rare occurrence. In 2025, for example, there were at least 1,073 imported cases of chikungunya,(1) which gave rise to 788 locally acquired cases. The health authorities managed to curb transmission through an effective surveillance strategy and swift mosquito control measures. But every new reported case raises fears that the virus will take hold in mainland France.

The same is true of Zika, a disease that can lead to severe neurological and autoimmune complications and serious congenital malformations in babies born to infected mothers. Although the tiger mosquito is not the main vector for Zika (which is mainly transmitted by its cousin Aedes aegypti), it can spread the virus if there are imported cases. The tiger mosquito has also been shown to be compatible with the Usutu and West Nile viruses, which mainly affect birds but can also infect humans.

You recently published some reassuring research on Zika. Can you tell us more?

L. L.: We compared two variants of the virus – the Asian and African lineages. Back in 2021, we noticed a paradox: although the African strain is more transmissible and virulent, all the major outbreaks have been caused by the Asian strain. It appears that, by chance, the former strain is confined to wild animals in African forests, where the mosquitoes belong to a subspecies of Aedes aegypti that is not very effective in transmitting the virus to humans. The key discovery revealed in the study that we published in October 2025 is that the African strain is more transmissible because of its multiple genetic mutations. It is unlikely that the Asian variant will acquire as many mutations spontaneously and become more infectious. This research, conducted as part of a project funded by MSDAVENIR, is reassuring, but there is still a risk that the African virus, which in theory is more dangerous, will emerge from the forests and contaminate humans. If this happens, it could cause severe, rapid outbreaks.

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In conclusion, what is the risk for us living in mainland France of being infected by Zika, chikungunya or dengue?

L. L.: The risk is very real. Tiger mosquitoes are now found in almost all French regions, and climate change is enabling viruses to replicate and spread more quickly. Several Institut Pasteur studies have also demonstrated the strategies used by the mosquitoes to expand their range. Colleagues have shown that the tiger mosquito has acquired the ability to transmit chikungunya just as effectively at 20°C as 28°C. And my team and I recently discovered that the dengue virus is changing the eating habits of mosquitoes so that they take smaller, more frequent meals and can therefore bite more people. All these findings show that every reported case needs to be monitored closely.