An unprecedented number of dengue cases have been identified in mainland France since mid-summer. Indeed, nearly forty “indigenous” cases – in other words, infections contracted on national territory, without the patients having traveled abroad – had already been reported in mid-September, which is more than the cumulative total of the last 10 years.
The disease has also declared itself in departments where it had never been detected until now. While the number of infections will likely continue to rise in the days and weeks to come, which regions are affected? What are the most plausible causes of this exceptional situation? And why can this situation be worrying?
In the Alpes-Maritimes, the largest outbreak ever identified in metropolitan France
Currently, of the five departments affected by the disease, three are located in the Occitanie region and two in the Provence-Alpes-Côte d’Azur (PACA) region.
In Occitania, four cases have been identified in the same habitat, in the town of Salvetat Saint-Gilles, 20 km west of Toulouse, in Haute-Garonne; 3 cases have been reported in the town of Andrest and that, 15 km away, of Rabastens-de-Bigorre, in the Hautes-Pyrénées; 1 case was detected in Perpignan, in the Pyrénées-Orientales. The dengue virus had never been identified in these last two departments.
In the PACA region, the two departments affected are the Var, with six cases identified in Fayence and, above all, the Alpes Maritimes with 26 cases identified in three municipalities located less than 10 km away from each other: Saint-Jeannet, Gattières and La Gaude. The geographical and temporal proximity – they were declared over a month – of these new cases leans in favor of a single and same episode of circulation of the virus in these three municipalities. This outbreak is already the largest ever identified in metropolitan France. It is still active, with the last cases identified in early September. Other cases will likely be identified soon.
Until now, the number of autochthonous cases of dengue fever remained very limited in metropolitan France. It was estimated at less than thirty since the first case identified in 2010, with very often only a few cases identified per year.
The number of cases reported this year therefore already exceeds all the cases identified over the past 15 years.
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Note also that dengue is often asymptomatic (in about 70% of cases), it can go largely unnoticed. Furthermore, with the Covid-19 pandemic, the diagnosis of other viral diseases has probably been underestimated over the past two years. This may have concerned in particular various arboviruses, including dengue fever. The actual number of cases is therefore probably greatly underestimated.
Why such an explosion? While it was difficult to anticipate an increase in the circulation of dengue fever such as the one we are experiencing in 2022, in recent years several precursors have foreshadowed an increase in vector-borne diseases.
The vector mosquito has invaded our territory
The virus that causes dengue fever is transmitted by mosquito species belonging to the genus Aedeswhich includes the tiger mosquito (Aedes albopictus). However, since its first detection on our territory, in 2004 in Menton, this small black mosquito with a body and legs striped with white native to Southeast Asia and the Indian Ocean has not stopped increasing its French distribution area.
In less than 20 years, it has invaded 67 out of 96 metropolitan departments (compared to only 58 in 2020!). Its spread throughout the territory is inexorable.
The source of significant discomfort in the summer period (some tourist areas are infested with it), the tiger mosquito is capable of transmitting, in addition to the dengue virus, various viruses responsible for illnesses in humans, including Zika or chikungunya virus. Its extension on our territory increases the risk of spreading the diseases they cause.
Currently, among these three viruses responsible for tropical diseases, the dengue virus is the most present in metropolitan France, particularly in the southern regions, where the density of tiger mosquitoes is the highest.
A risk increased by environmental and climatic changes
Weather conditions, especially temperature, air humidity and precipitation, affect the geographic distribution and activity of mosquitoes. In particular, they promote their proliferation and their lifespan.
The summer we experienced, exceptional for its alternation of scorching heat and intense rainy episodes, particularly in the southern regions, led to a significant increase in the number of mosquitoes.
Indeed, if their eggs need water to complete their development process, they are nevertheless very resistant in the environment. In the event of a drought, they can remain viable for up to several years, allowing them to survive until the conditions for their hatching are favorable.
This explosion in mosquito populations has increased the risk of transmission of the dengue virus, itself already increased by the resumption of international trade and tourism.
Tourism and trade promote the spread of pathogens
Commercial or tourist exchanges, which have been growing exponentially for several decades, can facilitate the spread of viral diseases, and in particular diseases spread by vectors such as mosquitoes.
Proof of the importance of these factors, during the Covid-19 pandemic, the number of cases of dengue infections from people returning from travel outside the Antilles decreased considerably, mainly due to the drastic drop in international air transport. With the clear recovery in air traffic this year, an increase in imported cases has been observed: from May 1 to September 2, 2022, Public Health France counted 165 imported cases of dengue fever in France, i.e. already as many as for the whole of France. year 2021…
These imported cases are as many risks of the appearance of “local” outbreaks, when the mosquito vector of the disease is present on the spot. And this, especially since dengue fever is the most common and widespread arbovirus in the world.
How is dengue transmitted?
Originating in Africa, the dengue virus is a arbovirus, from the English “ARthropod-BOrne VIRUS”, a virus transmitted by insects that feed on blood. It belongs to the genus flavivirussuch as Zika virus, or yellow fever and West Nile viruses, other arboviruses.
When a mosquito bites an infected person, it ingests the blood in which the dengue virus is present. The latter will then multiply in the body of the mosquito, which will transmit it to another individual during a new bite.
The first cases of dengue fever were recorded in the 18th century on the American continent. This condition is well known in many tropical and subtropical regions of the globe, from Africa to Asia via Latin America. The French overseas territories are not spared: Reunion, Guadeloupe or Martinique have suffered recurrent epidemics in recent years.
A disease that is often asymptomatic, but sometimes serious
Dengue fever is sometimes referred to as “tropical flu”, because its symptoms are most often flu-like: patients have fever, headaches, body aches… They can also sometimes develop a rash.
Manifestations of the disease occur 3-14 days after the mosquito bite, with an average of 4-7 days. The patient generally recovers spontaneously in a few days, but significant fatigue persists for several weeks.
Currently, there is no specific treatment for dengue fever. A vaccine exists (Dengvaxia), but it remains little used due to many limitations regarding its use. This vaccine has the disadvantage of increasing the risk of hospitalization and severe dengue in people not previously infected with the dengue virus. It is mainly prescribed to people living in endemic areas, and who have already been infected with the dengue virus.
The management of the disease consists mainly of treating the symptoms, in particular pain and fever. Aspirin and non-steroidal anti-inflammatory drugs are formally contraindicated due to the risk of bleeding.
The main problem associated with dengue fever is the risk of developing what is called severe dengue fever or dengue hemorrhagic fever, which can be complicated by a state of often fatal shock. Dengue hemorrhagic fever is manifested in particular by respiratory distress associated with multiple hemorrhages.
Fortunately, this severe form of dengue affects only a small percentage of infected people: between 1% and 5% of cases. Some people also develop severe neurological damage (such as encephalitis) but these complications are extremely rare.
Four different virus types, but no cross-immunity
There are four different types of dengue virus, which are differentiated from each other by subtle changes in their antigens (antigens are structures whose detection by the immune system leads to its reaction and the production of antibodies – it can s act of proteins, lipids, sugars, etc.).
These four dengue virus serotypes, named DENV-1, DENV-2, DENV-3 and DENV-4, although relatively similar, are different enough not to confer long-term cross-protection. In other words, if infection by one of the serotypes immunizes us against it, it does not offer protection against the other three. You can therefore be consecutively infected with the 4 dengue serotypes during your lifetime.
In mainland France, at least two serotypes, DENV-1 and DENV-3, have been identified this year. This means that the dengue virus has been introduced into the territory several times this summer.
Mosquito control as a means of prevention
After the discovery of cases of infection, the same scenario takes place: mosquito control operations are carried out near the cases detected, accompanied by awareness-raising actions with the public and health professionals (as well as during surveys door-to-door), carried out in collaboration with the ARS, Public Health France and mosquito control agencies such as Altopictus or the Interdepartmental Mosquito Control Agreement.
Currently, the best ways to avoid the spread of viruses such as dengue, transmitted by mosquitoes, is to limit the proliferation of these insects and to protect themselves from their bites.
To reduce the development of mosquito larvae, it is recommended to empty all containers of stagnant water, including flower pot cups and watering cans, and to cover rainwater containers, especially during periods of heavy rainfall. . To limit the risk of being bitten, it is recommended to use suitable repellents, and to wear loose and covering clothing.
Fortunately, in our country, the threat of arboviruses, including dengue fever, remains sporadic for the time being and the risk of an epidemic is limited. Strengthening surveillance networks currently remains one of the best strategies for combating these new threats that are difficult to anticipate.