The Asian tiger mosquito may be considered a small but mighty coloniser. Despite only measuring around 3mm, this invasive species has managed to spread across most of mainland France in just 20 years. With global warming creating perfect conditions for its proliferation, this tiny pest risks spreading diseases like zika, dengue and chikungunya now more than ever.
Spotted on French soil for the first time in 2004, the black-and-white striped insect can now be found in 71 of the country’s 96 mainland départements (administrative units). Health authorities have put these regions on red alert, meaning the species is both living and breeding in those areas.
Its spread has become a serious cause for concern. The mosquito is a vector for diseases like dengue, zika and chikungunya – viruses that have been the source of life-threatening epidemics in places like Brazil or La Réunion, where hundreds of thousands of people have been infected.
Concern is catching on with French politicians. France Unbowed (La France insoumise) MP François Piquemal (whose surname ironically translates to “stings badly”) said earlier this week that the spread of the tiger mosquito in France is a “political matter”, and called on the government to take action.
“Imported by neo-liberal globalisation … and a vector of serious diseases, it is having an impact on our lives,” Piquemal wrote in a tweet.
Now that temperatures in France are consistently on the rise due to global warming, creating the optimal conditions for the mosquito to proliferate, researchers are racing the clock to find a solution that will keep the spread of mosquito-borne diseases at bay.
The aedes albopictus world tour
Native to the forests of southeast Asia, the tiger mosquito, also known as Aedes albopictus, made its first appearance on the European continent in 1979. Having travelled all the way from China, the little insect made its first stop in Albania.
By laying their eggs on the walls of human-made objects containing stagnant water such as used tyres or containers, female mosquitoes enabled their offspring to hitch rides on ocean liners dealing in transcontinental trade. Adapted for survival through long periods of dryness thanks to their waterproof shell called a serosal cuticle, the eggs were able to latch on for months at a time. Once the ships reached their destinations and containers were offloaded, the eggs could hatch, given conditions that were hot and humid enough – which was the case in Albania.
“Albania was a Communist state at the time, and had strong [trade] relations with China. So the mosquito was transported from the east,” explains Anna-Bella Failloux, a medical entomologist at France's Pasteur Institute. “But the species never left the country, as it wasn’t trading with the rest of Europe.”
It would take another 11 years for the tiny tiger to reach the rest of the continent, starting with the Italian port of Genoa. Once there, its populations spread rapidly. The mosquito completely colonised Italy’s provinces in just 10 years’ time, and the country is now the most heavily infested in Europe.
In 2004, the first tiger mosquitoes were reported in France. Menton, a southern town just 11 kilometres from the Italian border, was home to the invasive species’ first breeding site. From there, it made its way across the country on trains, trucks, even cars.
Its spread in France “was not exponential”, says Pierre Tattevin, head of the infectious diseases department at Rennes University Hospital, “but it was continuous”. He says that since its arrival, “there have been about four or five new départments affected each year” and that the mosquito can now be spotted close to the English Channel, all the way north in Brittany.
Without globalisation, France wouldn’t be home to the tiger mosquito today. “Human activity allowed for the spread of this mosquito,” says Failloux, noting that it only spread rapidly throughout Europe after arriving in Italy, since Italy had greater exchange with other countries than Albania at the time.
It is also remarkably resistant compared to other species. “In under four decades, the tiger mosquito has conquered the tropical and subtropical world,” Failloux explains. “For its cousin, the aedes aegypti … it took 400 years.”
The aedes aegypti mosquito is commonly known as the yellow fever mosquito, but it can also transmit viruses like zika, dengue and chikungunya. Unlike the tiger mosquito, it can’t establish itself in non-tropical regions.
Super mosquito women
Of the 3,500 species of mosquitoes in the world, only 15% bite humans. And of those that bite, “three bear primary responsibility for the spread of human diseases”, according to National Geographic – the tiger mosquito being one of those three.
Acting as a vector, the tiger mosquito can carry serious viruses like dengue, zika and chikungunya. When the insect bites a human to get blood proteins for its eggs, it transmits the virus through its saliva. Female tiger mosquitoes are the only ones to bite, and do so every five to seven days, but “carry viruses for their entire life cycles”, explains Failloux. Under ideal conditions, the species can live for about three weeks.
What’s more, a female tiger mosquito is inseminated once in her life cycle, meaning she can “stock male sperm that she will use to fertilise her eggs all throughout her life”, says Failloux. And those eggs can withstand being dry “for months or even years” at a time, the entomologist explains.
For infectious diseases expert Tattevin, tiger mosquitoes pose a “huge” threat to society. Dengue, zika and chikungunya virus are all “major public health problems in countries where there have been epidemics, that’s what is worrying”, he says.
Dengue is the most common mosquito-borne disease. Though most people don’t get symptoms, those who do can experience high fever, head and body aches, nausea and rashes. These normally pass after one or two weeks, but in severe cases, the virus can be fatal. A vaccine for the virus exists but it is not widely used.
Zika is also a virus that is most often asymptomatic, but can pose a serious threat to pregnant women as it can cause congenital malformations in the child.
Chikungunya shares similar symptoms with dengue and zika, and while most go dissipate after a few days, some patients have reported long-lasting joint pain that can be debilitating. Cases of death or severe symptoms are rare, and usually related to pre-existing health problems.
In 2022, France’s health authorities recorded 65 indigenous cases of dengue fever, meaning that the people who contracted the virus had done so locally, on French soil, and not abroad.
So far in 2023, there have been no reported indigenous cases, but experts say that the country could become more vulnerable to infectious diseases spread by tiger mosquitoes as global temperatures increase, creating ideal conditions for them to proliferate.
And once the species has settled, it's “practically impossible to get rid of”, according to a press release published in April by the Institut Pasteur.
'More efficient' due to climate change
Global warming brought on by climate change means higher temperatures last longer. This not only draws out mosquito season, but also accelerates the mosquitoes' development, according to a 2020 study published by the French National Assembly.
“The tiger mosquito is much more efficient, active and comfortable at temperatures above 22 or 23 degrees Celsius,” Tattevin agrees, and acknowledges that global warming is creating ideal conditions for the mosquito to proliferate.
Entomologists carried out experiments to observe how climate factors drive the invasion of the species, and found that when temperatures increased, mosquitoes performed better. “They flew longer and farther, lived longer, were faster and reproduced more rapidly,” says Tattevin.
The experiments also showed that the tiger mosquito, unlike its cousin aedes aegypti who thrives in tropical climates, could withstand colder temperatures for longer periods.
Ideal weather conditions for tiger mosquitoes in France last from about May until September or October, but “when we reach the end of November and temperatures drop to around ten degrees, a female will lay its eggs, which will survive the winter season,” Failloux points out. “Once temperatures rise again in early spring, the eggs will hatch and develop into adult mosquitoes,” she says.
'Kill the mosquito'
Since the striped invader was first spotted in France 20 years ago, health authorities have warned of its dangers.
The French agency for food, environmental and occupational health and safety (Anses) published a flyer to inform the public on steps they can take to mitigate its spread at home. These include emptying and turning over any buckets, bowls, vases or other containers that could hold stagnant water, covering water-collection jerrycans with mosquito nets and cleaning gutters to prevent the mosquitoes from breeding. The government also urges people to wear long clothing and use mosquito repellent during high season.
People living in France can also report the presence of tiger mosquitoes using a dedicated platform set up by the agency.
For those having travelled to a country where zika, dengue or chikungunya exist, says Tattevin, it is vital to monitor symptoms. “If a person arrives in mainland France with any of the viruses, the first rule is to keep them under mosquito netting so that they can’t be bitten by another mosquito, which will go on to infect those around them,” he explains.
When it comes to large-scale solutions that could be implemented nationally, some are more promising than others. Spraying insecticides is one, but both Tattevin and Failloux agree that it is far from ideal. Tiger mosquitoes have become acclimated and even resistant to insecticides, meaning “higher doses are needed, which is toxic for our environment”, says Failloux.
Researchers are also looking into ways of sterilising male tiger mosquitoes to neutralise the potential for future offspring. Using the Sterile Insect Technique (SIT), male mosquito pupae (the stage before the insect becomes an adult) are exposed to gamma rays that break down the DNA inside their sperm. Once they are released into the wild and mate with a female, it becomes impossible for females to lay viable eggs. But “selecting and separating male pupae is not easy”, says Failloux, “and the method requires having access to labs where mosquitoes can be raised, which are expensive to finance”. The entomologist adds that sterilised males also lose their competitive drive to mate, “so if they aren’t the first to fertilise a female, a wild male could get there first”.
The most successful solution so far has been to contaminate males with the Wolbachia bacteria, which “makes it impossible for a mosquito to transmit a virus, particularly dengue”, says Failloux.
According to the World Mosquito Programme, a trial carried out on aedes aegypti mosquitoes in Indonesia led to a 77% reduction in the incidence of dengue and an 86% reduction in hospitalisations for dengue.
“It’s a very promising solution,” says Failloux, who believes that the only way forward is to “kill the mosquito”.
“That’s the only way we’ll stop these diseases from spreading,” she insists.
