Dengue in France in 2024 - A year of record numbers of imported and autochthonous cases

Dengue is a mosquito borne infection which is frequently imported to Europe, and when conditions are right, can also be locally transmitted, giving rise to local or so called “autochthonous” cases [1]. The importation of dengue has increased in recent years due to a rise in international travel and when viraemic returned travellers are bitten by European Aedes mosquitoes, single local cases and even outbreaks have been reported.

Dengue is mainly transmitted by infected Aedes mosquitoes, with humans and monkeys serving as pathogen reservoirs. The dengue virus, an RNA virus of the Flaviviridae family, infects certain monocytes and macrophages and multiplies in them. Between 40 and 80 percent of dengue cases are asymptomatic or have mild flu-like symptoms. Other cases present with cause high fever and severe headaches, back and joint pain (dengue has been termed ‘breakbone fever’) [2]. However, re-infection can lead to more severe courses with haemorrhages and symptoms such as abdominal pain, vomiting, tachypnoea, fatigue, weakness, plasma leakage and organ failure, which is known as ‘severe dengue’ and can be fatal [3]. Diagnosis of dengue is based on travel history and specific tests such as the NS1 antigen test or PCR, while treatment is based on the elimination of symptoms and requires intensive care in severe cases. Dengue is a notifiable infection in many countries.

The Aedes albopictus mosquito has become established in many temperate areas in Europe and is a major vector of the dengue virus in France [4]. The transmission cycle (Fig. 1) begins when the mosquito ingests the virus from a viraemic human, the virus spreads via the midgut cells to the mosquito's salivary glands, and then the virus is passed on to the next human through a bite. In addition to this direct form of transmission to humans, there is also the possibility of vertical transmission of the virus from an infected female mosquito to her offspring, which supports the spread of the virus [3]. The spread of Aedes albopictus in France is favoured by factors such as human mobility, trade, including the transport of used tyres, urbanisation and climatic changes. Urban and peri-urban areas also offer many artificial water containers such as flowerpots, gutters and buckets in which mosquitoes can breed [4]. Mild winters and higher temperatures promote the spread of the vector due to the accelerated replication of the virus within the mosquito and faster larval development [5,6]. The ideal temperature range for Aedes albopictus is between 28° and 32° degrees Celsius, while extremely low or much higher temperatures can limit the survival of the mosquito [7].

Fig. 1.

The dengue virus cycle in humans and mosquitoes (from Guzman et al., 2016) [8].

The A. albopictus mosquito spread throughout France and Europe in the 1990s through the global trade in used tyres. The autochthonous transmission of dengue in France is closely connected to imported cases from other countries and regions, especially the French overseas territories [9,10] as these are preferred destinations for French travellers. In 2024, the majority of all imported dengue cases in France originated in the French overseas territories [10]. As of the 5th November2024, some 4074 imported cases of dengue have been reported in France in 2024. Of these, 1803 cases occurred after 1 May, the start of the period when the mosquito vectors are active and when more data is collected, including details about country or region of origin. This data shows that 54.5 % of imported cases between 01.05.2024 and 05.11.2024 originated in the French overseas territories of Guadeloupe and Martinique alone [10] (Fig. 2, Fig. 3).

Fig. 2.

Pie chart showing distribution of imported dengue cases in France by countries of origin between 01.05 and 05.11.2024 (Data from Sante publique, 2024) [10].

Fig. 3.

Map showing the countries of origin of imported dengue cases in France between 01.05 and 05.11.2024 (Data from Sante publique, 2024) [10].

There were 85 autochthonous or locally transmitted cases of dengue reported in mainland France in 2024. This is the highest number of autochthonous cases in France reported to date, higher than the previous peak of 66 cases in 2022 [11,12]. All the autochthonous cases occurred in the south of the country, in the regions o Auvergne - Rhône - Alpes, Occitanie, and Provence - Alpes - Côte d’Azur. This has been the trend in most years. However, an exception took place in 2023, with 3 cases in Paris, the capital city of France, situated in the northern part of the country 13.) (Fig. 4). The transmission season for dengue goes from June to October and reaches its maximum in September (Fig. 5)

Fig. 4.

The annual number of autochthonous cases reported in France between 2012 and 2024 showing the peak incidence of cases in 2024. Original graphic using figures from Santé publique France, Géodes, dengue, 2012–2023 and Santé publique France, chikungunya, dengue et zika, 2024) [11], [12].

Fig. 5.

The monthly number of autochthonous cases reported in France in 2024, based on data published in Bulletins in the period July–November [13].

Considering the potential for dengue to become endemic in France, it is important to consider the protective measures for travellers and for the local French population. Public Health authorities recommend individual protection against mosquito-bites such as long, light clothing and mosquito repellents. However, long-term adherence to these measures is difficult. Permethrin is an insecticide with which clothes can be impregnated, offering protection up to four weeks, which might increase adherence to preventive measures. The use of mosquito nets is questionable as it fails to take the diurnal activity of the mosquitos into account. For travellers to endemic areas, a new vaccine Qdenga has recently been approved by the European Medicine Association and is available in many European countries with varying guidelines for its indication [14].

To reduce the risk of dengue in the local community, measures include the elimination of potential incubation sites, such as standing water in coasters, gutters, vases and buckets [15]. In 2023, the French Health authorities undertook fumigation in Paris, in response to a traveller with dengue virus [16]. Specifically for the Paris Olympics in 2024, health authorities not only drew attention to the individual measures to protect visitors from a dengue-infection during their stay, but also focused on travellers from dengue-endemic regions encouraging them to adhere to mosquito-bite prevention as they could infect the local mosquito population with dengue. In traditional dengue endemic areas, such as Brazil and Indonesia, extensive measures to combat mosquito borne diseases have been taken. some of these areas opted for Wolbachia programs, where bacteria-infected mosquitoes harm the local dengue-mosquito population – however, this is currently not implemented in France.

Mass gatherings, such as the 2024 Olympic Games in Paris, raised a concern about the possible spread of infectious diseases. In France the potential spread of dengue was a concern, as this major event attracts millions of visitors and athletes [17]. Of the 204 countries participating in the Olympics, 74 are frequently endemic and 49 are sporadically endemic for dengue. Many of these are smaller countries, such as Aruba and other Caribbean islands, with only a few athletes attending the event [18], but it only takes one infected person to start the transmission. There were circa 11.2 million visitors to Paris during the games, of which 15.2 % were international tourists, amounting to around 1.7 million people and 10,500 athletes [19,20]. Most visitors came from the USA, UK or Germany. Reducing the potential of a dengue outbreak in this context requires prevention measures that are difficult to implement as A.albopictus is day active and the games were held outdoors and not in indoor stadiums [21,22]. Personal protection methods, and education campaigns, as well as informing visitors, who can get the first symptoms after they return home, where the virus can spread further is important. The course of the number of dengue cases in 2024 was very similar to the previous year, when a sporting event, the Rugby Worlc Cup took place in late summer. No clear correlation between the Olympic Games and the number of dengue cases in France was seen.

What about the future? Climate projections anticipate increased temperature and precipitation in many areas, including France. This may result in an increased range of A. albopictus throughout France, particularly if high summer temperatures above 30° Celsius are present. Vector Capacity (VC) is a mathematical approximation of the efficiency of vector-borne disease transmission. The threshold value for vector transmission of dengue is 0.2 VC and is an indication of the possibility of a dengue outbreak. Currently, Paris has a VC below this threshold, and Nice, a city in southern France, has a VC that is slightly above 0.2 for the months of July and August. However, when future climactic scenarios are considered, this situation changes. Even under a conservative scenario, Paris will also have a VC above the threshold in July and August, and Nice will have a higher and broader VC curve. A more extreme projection predicts both Paris and Nice having VCs well above the outbreak threshold, and over an extended period of time [23], [24]. All projections anticipate an increase in vectorial capacity, and therefore an increased threat of outbreaks and transmission of dengue in France in the future. In addition, A.albopictus is known for its adaptability to urban habitats. The spread of A.albopictus, particularly in southern France, is not only favoured by climatic conditions, but also by increasing urbanisation and the associated creation of breeding sites in urban areas [25].

In conclusion, the year 2024 was a significant year for dengue in France with record numbers of imported and autochthonous cases. The intensive public health preparations for the Olympic games with respect to vector control, reflect the concern of French authorities with regard to the threat of dengue transmission, particularly in Paris where mosquito spraying programmes were initiated. As of late November 2024, no locally transmitted cases of dengue were reported in Paris which suggests success of these prevention strategies. Nevertheless, France has reported unprecedented numbers of dengue cases in 2024 reflecting French travel patterns and preferred destinations. The record number of autochthonous cases in France (n = 85) as of November 2024 is the largest annual number ever recorded in France reflecting successful transmission of the dengue virus, particularly in the South of France by the locally established Aedes albopictus. Climate change predictions suggest that wider regions of France will be susceptible to dengue in the future. This implies that surveillance and increased prevention campaigns are crucial to slow down the inexorable rise in dengue cases in France.