Olympic Games 2024: tiger mosquito and dengue fever surveillance stepped up!
Over the years, the dengue fever virus has been making more and more headlines in mainland France.
On July 8, 2024, the Agence régionale de santé (ARS) d'Occitanie declared that it had identified the first case of autochthonous infection this year, in the Hérault region, near the city of Montpellier. The patient had not recently travelled to a country where the virus is endemically circulating.
Yannick Simonin, University of Montpellier
The situation is under particular scrutiny this year, for two reasons: not only does climate change favor the proliferation of mosquitoes, but the risk of importing cases from abroad is also very high, particularly during major international gatherings such as the Olympic Games.
What do you need to know about this threat, and how can you protect yourself?
Dengue, mostly asymptomatic but sometimes fatal
The dengue virus is transmitted by tiger mosquitoes(Aedes albopictus), an invasive species that has become well established in Europe. Most often asymptomatic, between 50% and 90% depending on the studies and the 4 serotypes, this disease can be highly incapacitating when associated with symptoms (high fever, pain, nausea, vomiting, retro-orbital, joint and muscle pain...). Dengue fever can also take a severe form. Potentially fatal in 1 to 5% of cases, it is accompanied by multiple bleeding episodes, notably in the gastrointestinal tract, skin and brain.
Every year, between 100 and 400 million people worldwide contract dengue fever, making it the most widespread arboviral disease (the adjective "arboviral" derives from "arthropod born virus", a viral disease transmitted by arthropod vectors, a phylum of animals including insects and arachnids). It is estimated that around 500,000 people a year are hospitalized for severe forms of the disease, and between 10,000 and 15,000 die.
The dengue virus is not endemic in France: any outbreaks of local cases (known as autochthonous) are always linked to the importation of the virus by travellers from other geographical areas. Transmission occurs when a female tiger mosquito bites an infected animal or human (this is known as the blood meal, and is necessary for egg-laying). The virus infects the animal and multiplies in its body.
Once the mosquito's salivary glands are infected, each bite taken during a blood meal can contaminate another individual, enabling the transmission cycle of arboviruses in the human population. Consequently, the greater the number of imported cases, the greater the risk of indigenous transmission of dengue fever.
This risk also increases with the seasonal abundance of the tiger mosquito, which is on the rise and will peak between July and October, favored by rising temperatures and rainfall.
2024, a record year
Back in June, the European Centre for Disease Prevention and Control (ECDC) warned that the risk of mosquitoes spreading disease is particularly high this year. In Europe, "climate change is creating more favorable conditions for the spread of invasive mosquitoes in previously unscathed areas".
What's more, 2024 began on a worrying trend, with the number of imported dengue cases much higher than in previous years. Between January1 and April 30, 2,166 imported cases of dengue fever were identified (compared with an average of 128 imported cases over the same period). These imported cases correspond to people returning from travel, mainly from the French West Indies: over 80% of cases came from Martinique or Guadeloupe, followed by Indonesia, Brazil and Mauritius.
Between May1 and July 2, a further 719 imported cases were identified in mainland France (58% of which involved people returning from Guadeloupe or Martinique). For the year 2023 as a whole, 2,524 imported cases were recorded.
This record number of imported cases comes very early in the season. It is attributed to the intensive circulation of the dengue virus in many tropical regions of the globe, notably in the Americas (more than 7.5 million cases since the start of the year, particularly in Brazil), Asia (Malaysia and Thailand in particular) and West Africa (more than 30,000 cases identified in Burkina Faso).
This situation comes as no surprise: the Pan American Health Organization (PAHO) had warned that Latin America and the Caribbean could expect the worst dengue season on record in 2024, due in part to the El Niño phenomenon. The French West Indies have not been spared, and the high frequency of air travel between the Antilles and France naturally favors the importation of dengue cases.
The main problem with importing such a large number of cases is the risk that, in France over the coming weeks and months, they will give rise to local outbreaks of transmission in regions colonized by the tiger mosquito. Many regions are now concerned...
Native transmission on the rise in France
Since 2010, a few indigenous cases of dengue fever have been transmitted every year in mainland France. These transmissions due to the tiger mosquito, considered an invasive species, have increased drastically since 2022, with record numbers of native cases occurring not only in France (66 cases in 2022), but also in Europe (130 cases in Italy, France and Spain in 2023).
Over the years 2022 and 2023, the cumulative number of dengue cases transmitted on our territory (110 symptomatic cases) represented more than twice the cumulative total for the last 20 years!
Transmission outbreaks are multiplying, particularly in the south of France, but the north of the country is no longer spared local transmission: last year, the first cluster was identified in the Paris region.
One of the main reasons for this situation is the distribution of the tiger mosquito. Introduced in Menton in 2004, it had managed to colonize 20 départements by early 2014. Ten years on, it is now present in 79 departments in mainland France, and even in a few communes in Belgium.
This rapid and increasing spread of colonized territories has been made possible by the mosquito's ability to adapt to temperate climates, as well as by the intensity of human dissemination of its eggs (laid in containers) and fertilized females (all transported in vehicles).
The Olympic Games, transmission amplifiers
The Olympic and Paralympic Games will be held between July 16 and September 8, at the height of the tiger mosquito's activity! We know that major international events, which generate very large flows of travellers from many different countries, are likely to encourage the importation of arboviruses.
Such conditions not only increase the risk of local dengue transmission episodes, but also those of other arboviroses, such as those caused by the chikungunya or Zika viruses. With dengue circulation very active around the world this year, the number of imported cases could soar.
While it is always difficult and risky to anticipate the circulation of these arthropod-borne viruses, we can expect the number of native cases of dengue fever to break new records in France in 2024. All the more so as the tiger mosquito is now well established in the vast majority of areas where the events will be held, particularly in the Paris region...
Surveillance under increasing strain
The exceptional situation we're facing in 2024 risks saturating the existing vector control system, from the health professionals responsible for epidemiological surveillance to the mosquito control operators in charge of insecticide interventions.
Active between May and November, this system had already been under considerable strain over the past two years, approaching maximum capacity in some areas. For example, the number of vector control operations between August and October in the Occitanie and Nouvelle-Aquitaine regions increased 30-fold between 2021 and 2023.
Every year, therefore, vector control operators have to anticipate as accurately as possible the scale of their actions in relation to the risk, despite the random nature of the occurrence of cases and episodes of autochthonous transmission in time and space.
And yet, despite the record numbers of the last two years, the number of dengue cases is probably still well below what we can expect. Indeed, climate change is likely to amplify the phenomenon even further, with warmer temperatures not only increasing the period of activity of the tiger mosquito, but also favoring the multiplication of arboviruses within it.
How can we fight, in 2024 and in the years to come?
To meet the challenges ahead, we need to act on several levels. The organization and size of surveillance and control tools will need to be consolidated. City councils will have to implement genuine preventive policies against the tiger mosquito. But that's not all: citizens themselves will have to get involved. As the tiger mosquito only travels 100 to 200 metres from its nesting site, local action is still the most effective!
This year, the first adult tiger mosquitoes emerged in southern France at the end of March 2024. The dynamics observed so far are no different from those of 2023. If the tiger mosquito spoiled your day last year, this year is likely to be just like the last! Unless you and your neighbors take action to neutralize its larval breeding grounds...
Controlling mosquito larvae is essential from April to October: as the tiger mosquito lays its eggs almost exclusively in artificial containers, it must be prevented from gaining access to water, by hermetically covering rainwater tanks, insulating downspout sand traps with solid mosquito netting, emptying pots, buckets and saucers every 7 days (or, failing that, pouring boiling water into them), etc.
Finally, to limit the spread of mosquito-borne viruses, it's also essential to protect yourself against mosquito bites, by installing mosquito nets on windows. When outdoors, be sure to wear long, loose-fitting clothing, and use effective repellents on uncovered parts of the body.
The initial version of this article was co-authored with Guillaume Lacour, medical entomologist and R&D manager, and Antoine Mignotte, research officer, all employees of Altopictus, a private vector control operator.
Yannick Simonin, Virologist specializing in the monitoring and study of emerging viral diseases. University Professor, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Read theoriginal article.