2024 Olympics: increased surveillance for tiger mosquitoes and dengue fever!
Over the years, the dengue virus has become increasingly talked about in mainland France.
On July 8, 2024, the Occitanie Regional Health Agency (ARS) reported that it had identified the first case of indigenous infection of the year, in the Hérault department, near the city of Montpellier. The infected person had not recently traveled to a country where the virus is endemic.
Yannick Simonin, University of Montpellier
The situation is being closely monitored this year for two reasons: not only does climate change favor the proliferation of mosquitoes, but the risk of cases being imported from abroad is 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 fever, 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 studies and the four serotypes, this disease can be highly debilitating when associated with symptoms (high fever, pain, nausea, vomiting, retro-orbital, joint and muscle pain, etc.). Dengue can also manifest itself in a severe form. Potentially fatal in 1 to 5% of cases, it is accompanied by multiple bleeding, particularly gastrointestinal, cutaneous, and cerebral.
Every year, between 100 and 400 million people contract dengue fever worldwide, making it the most widespread arboviral disease (the adjective "arboviral" derives from the English "arthropod-borne virus," a viral disease transmitted by arthropod vectors, a branch of animals that includes insects and arachnids). It is estimated that approximately 500,000 people per year are hospitalized for severe forms of the disease, and that between 10,000 and 15,000 die from it.
The dengue virus is not endemic in France: local outbreaks (known as autochthonous) are always linked to the virus being imported by travelers from other geographical areas. Transmission occurs when a female tiger mosquito bites an infected animal or human (this is called a blood meal, which is necessary for egg laying). The virus infects the mosquito and multiplies in its body.
Once the mosquito's salivary glands are infected, each bite taken during a blood meal can infect another individual, enabling the cycle of arbovirus transmission in the human population. Consequently, as the number of imported cases increases, so does the risk of indigenous dengue transmission.
This risk also increases with the seasonal abundance of the tiger mosquito, which is growing and will peak between July and October, aided by rising temperatures and rainfall.
2024, a record year
Last June, the European Centre for Disease Prevention and Control (ECDC) issued a warning: this year, the risk of disease transmission by mosquitoes is particularly high. In Europe, "climate change is creating conditions that are more favorable to the spread of invasive mosquitoes in areas that were previously unaffected."
In addition, 2024 began with a worrying trend, with a much higher number of imported dengue cases than in previous years. Between January1and April 30, 2,166 imported cases of dengue were identified (compared to an average of 128 imported cases during the same period). These imported cases correspond to people returning from travel, mainly from the French West Indies: more than 80% of cases returned from Martinique or Guadeloupe, followed by Indonesia, Brazil, and Mauritius.
In addition, between May1and July 2, 719 additional imported cases were identified in mainland France (58% of which involved people returning from Guadeloupe or Martinique). For the record, a total of 2,524 imported cases were recorded for the whole of 2023.
This record number of imported cases has occurred very early in the season. It is attributed to the intensive circulation of the dengue virus in many tropical regions around the world, particularly in the Americas (more than 7.5 million cases since the beginning of the year, particularly in Brazil), Asia (particularly Malaysia and Thailand), 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 should expect the worst dengue season ever recorded in 2024, particularly due to the El Niño phenomenon. The Caribbean is not spared, and the high frequency of air travel between the Caribbean and mainland France naturally encourages the importation of dengue cases.
The main problem with importing such a large number of cases is the risk that they could cause local outbreaks of transmission in mainland France in the coming weeks and months in regions colonized by the tiger mosquito. However, many regions are now affected...
Indigenous broadcasts on the rise in France
Since 2010, there have been a few cases of dengue fever transmitted locally in mainland France each year. These transmissions, caused by the tiger mosquito, considered an invasive species, have increased dramatically since 2022, with record numbers of indigenous cases not only in France (66 cases in 2022), but also in Europe (130 cases in Italy, France, and Spain in 2023).
In 2022 and 2023, the cumulative number of dengue cases transmitted in our territory (110 symptomatic cases) was more than twice the cumulative total for the last 20 years!
Transmission clusters are multiplying, particularly in the south of France, but the north of the country is no longer spared from local transmission: a first cluster was identified last year in the Paris region.
This situation can be explained in particular by the spread of the tiger mosquito. Introduced to Menton in 2004, it had managed to colonize 20 departments by early 2014. Ten years later, it is now established in 79 departments in mainland France, and even in a few municipalities 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: amplifiers of transmission
The Olympic and Paralympic Games will be held between July 16 and September 8, right in the middle of the tiger mosquito's active season! We know that major international events, which generate large numbers of travelers from many different countries, are likely to encourage the importation of arboviruses.
Such conditions not only increase the risk of local dengue transmission, but also that of other arboviruses, such as those caused by the chikungunya or Zika viruses. With dengue circulating very actively around the world this year, the number of imported cases could skyrocket.
While it is always difficult and risky to predict the spread of these arthropod-borne viruses, we can expect the number of indigenous dengue cases to reach new highs in France in 2024. This is especially true given that the tiger mosquito is now well established in most of the areas where the events will be held, particularly in the Paris region.
Increasingly intense surveillance
The exceptional situation we are experiencing in 2024 risks overwhelming 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 severe strain for the past two years, approaching its maximum capacity in some areas. For example, the number of vector control interventions increased 30-fold between 2021 and 2023 between August and October in the Occitanie and Nouvelle-Aquitaine regions.
Those involved in vector control must therefore anticipate the scale of their actions each year based on the risk, despite the unpredictable nature of cases and episodes of indigenous transmission in terms of time and space.
However, despite the record numbers seen over the past two years, the number of dengue cases is likely to remain well below what we can expect. Climate change is likely to further amplify the phenomenon, as warmer temperatures not only increase the tiger mosquito's period of activity, but also promote the proliferation of arboviruses within it.
How can we fight back in 2024 and in the years to come?
To meet the challenges ahead, action will need to be taken on several levels. The organization and scale of surveillance and control tools will need to be consolidated. Local authorities will need to implement effective preventive control policies against the tiger mosquito. But that's not all: citizens themselves will need to get involved. As tiger mosquitoes only travel 100 to 200 meters from their breeding site, local action remains the most effective!
This year, the first adult tiger mosquitoes emerged at the end of March 2024 in the south of France. The pattern observed so far is no different from that of 2023. If tiger mosquitoes ruined your days last year, this year is likely to be much the same! Unless you and your neighbors take action to neutralize their larval breeding sites...
The fight against mosquito larvae is essential from April to October: as tiger mosquitoes lay their eggs almost exclusively in artificial containers, it is important to prevent them from accessing water by tightly covering rainwater tanks, insulating sand traps at the bottom of gutters with a strong mosquito net, emptying plant pots, buckets, saucers, etc. every seven days (or, failing that, pouring boiling water into them).
Finally, to limit the spread of mosquito-borne viruses, it is also essential to protect yourself from mosquito bites by installing mosquito nets on windows. When outdoors, be sure to wear long, loose-fitting clothing and use effective repellents on any exposed areas of the body.
The initial version of this article was co-written with Guillaume Lacour, medical entomologist and head of R&D, and Antoine Mignotte, research officer, both employees of Altopictus, a private vector control operator.
Yannick Simonin, Virologist specializing in the surveillance and study of emerging viral diseases. University Professor, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Readthe original article.