Cases of dengue fever are skyrocketing in mainland France: what do you need to know?
An unprecedented number of dengue fever cases have been identified in mainland France since the middle of the summer. In fact, nearly forty "autochthonous" cases—in other words, infections contracted within the country, without the patients having traveled abroad—had already been reported by mid-September, which is more than the cumulative total for the last 10 years.
Yannick Simonin, University of Montpellier
The pupae of tiger mosquitoes, which will give birth to adults capable of transmitting dengue fever and other diseases, live in aquatic environments.
CDC/ Amy E. Lockwood, MS / Lauren Bishop
The disease has also appeared in departments where it had never been detected before. While the number of infections is likely to continue to rise in the coming days and weeks, which regions are affected? What are the most plausible causes of this exceptional situation? Should this situation be cause for concern?
[More than 80,000 readers trust The Conversation newsletter to help them better understand the world's major issues. Subscribe today]
In the Alpes-Maritimes department, the largest outbreak ever identified in mainland 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 Occitanie, four cases were identified in the same habitat, in the municipality of Salvetat Saint-Gilles, 20 km west of Toulouse, in Haute-Garonne; Three cases were reported in the municipality of Andrest and another 15 km away in Rabastens-de-Bigorre, in Hautes-Pyrénées; one case was detected in Perpignan, in Pyrénées-Orientales. The dengue virus had never before been identified in these last two departments.
In the PACA region, the two departments affected are Var, with six cases identified in Fayence, and, above all, Alpes Maritimes, with 26 cases identified in three municipalities located less than 10 km apart: Saint Jeannet, Gattières, and La Gaude. The geographical and temporal proximity of these new cases—they were reported within a month—suggests that this is a single outbreak of the virus in these three municipalities. This outbreak is already the largest ever identified in mainland France. It is still active, with the latest cases identified in early September. Other cases are likely to be identified in the near future.
Until now, the number of indigenous dengue infections has remained very limited in mainland France. It was estimated at less than 30 since the first case was reported in 2010, with often only a few infections identified each year. The number of cases reported this year therefore already exceeds the total number of cases identified over the last 15 years.
It should also be noted that dengue fever is often asymptomatic (in around 70% of cases), meaning it can go largely unnoticed. Furthermore, with the COVID-19 pandemic, the diagnosis of other viral diseases has likely been underestimated over the past two years. This may have been the case for various arboviruses, including dengue fever. The actual number of cases is therefore likely to be significantly underestimated.
Why such an explosion? While it was difficult to anticipate an increase in dengue fever cases such as the one we are experiencing in 2022, several warning signs in recent years pointed to a rise in vector-borne diseases.
The mosquito vector has invaded our territory.
Ministry of Solidarity and Health
The virus that causes dengue fever is transmitted by mosquito species belonging to the Aedes genus, which includes the tiger mosquito (Aedes albopictus). Since it was first detected in France in 2004 in Menton, this small black mosquito with a white-striped body and legs, native to Southeast Asia and the Indian Ocean, has continued to expand its range in France.
In less than 20 years, it has invaded 67 of the 96 metropolitan departments (compared to only 58 in 2020!). Its spread across the entire territory is inexorable.
Pxhere
A major nuisance during the summer months (certain tourist areas are infested with them), tiger mosquitoes are capable of transmitting not only the dengue virus but also various other viruses that cause disease in humans, including the Zika virus and the chikungunya virus. Their spread across our territory increases the risk of the diseases they carry spreading.
Currently, among these three viruses responsible for tropical diseases, the dengue virus is the most prevalent in mainland France, particularly in southern regions, where tiger mosquito density is highest.
A risk increased by environmental and climate change
Weather conditions, particularly temperature, humidity, and precipitation, affect the geographical distribution and activity of mosquitoes. In some cases, they increase their proliferation and lifespan.
The summer we experienced, which was exceptional due to its alternating heat waves and intense rainy spells, particularly in southern regions, led to a significant increase in the number of mosquitoes.
Although their eggs need water to complete their development process, they are nevertheless very resistant in the environment. In the event of drought, they can remain viable for up to several years, allowing them to survive until conditions become favorable for hatching again.
This explosion in mosquito populations has increased the risk of dengue virus transmission, which was already heightened by the resumption of international trade and tourism.
Tourism and trade promote the spread of pathogens
Trade and tourism, which have grown exponentially in recent decades, can facilitate the spread of viral diseases, particularly those spread by vectors such as mosquitoes.
Proof of the importance of these factors can be seen in the fact that during the COVID-19 pandemic, the number of dengue infections among people returning from trips outside the Caribbean decreased significantly, mainly due to the drastic reduction in international air travel. With the sharp recovery in air traffic this year, an increase in imported cases has been observed: from May1 to September 2, 2022, Santé publique France counted 165 imported cases of dengue fever in France, already as many as for the whole of 2021...
These imported cases pose a risk of "local" outbreaks, as long as the mosquito that carries the disease is present in the area. This is especially true given that dengue fever is the most common and widespread arbovirus disease in the world.
How is dengue transmitted?
Native to Africa, the dengue virus is an arbovirus, from the English "ARthropod-BOrne VIRUS," a virus transmitted by blood-feeding insects. It belongs to the Flavivirus genus, like the Zika virus, or the yellow fever and West Nile viruses, other arboviruses.
When a mosquito bites an infected person, it ingests blood containing the dengue virus. The virus then multiplies in the mosquito's body, which will transmit it to another individual when it bites again.
The first cases of dengue fever were recorded in the18th century on the American continent. This disease is well known in many tropical and subtropical regions of the world, from Africa to Asia and Latin America. France's overseas territories are not spared: Réunion, Guadeloupe, and Martinique have experienced recurring epidemics in recent years.
A disease that is often asymptomatic, but sometimes serious
Dengue fever is sometimes referred to as the "tropical flu" because its symptoms are most often flu-like: patients have fever, headaches, body aches, etc. They may also sometimes develop a rash.
Symptoms of the disease appear between 3 and 14 days after the mosquito bite, with an average of 4 to 7 days. The patient usually recovers spontaneously within a few days, but severe fatigue persists for several weeks.
Frederick Murphy/CDC
Currently, there is no specific treatment for dengue fever. A vaccine exists (Dengvaxia), but it remains little used due to numerous limitations regarding its use. This vaccine has the disadvantage of increasing the risk of hospitalization and severe dengue in people who have not previously been infected with the dengue virus. It is mainly prescribed to people living in endemic areas who have already been infected with the dengue virus.
Treatment of the disease mainly consists of managing symptoms, particularly pain and fever. Aspirin and non-steroidal anti-inflammatory drugs are strictly contraindicated due to the risk of bleeding.
The main problem associated with dengue fever is the risk of developing what is known as severe dengue or dengue hemorrhagic fever, which can lead to shock, often with fatal consequences. Dengue hemorrhagic fever is characterized by respiratory distress associated with multiple hemorrhages.
Fortunately, this form of dengue fever affects only a small percentage of infected individuals: between 1% and 5% of them are affected. Some individuals may also develop severe neurological disorders, such as encephalitis, but these complications are extremely rare.
Four different types of viruses, but no cross-immunity
There are four different types of dengue virus, which differ from each other by subtle modifications in their antigens (antigens are structures whose detection by the immune system triggers its response and the production of antibodies—they can be proteins, lipids, sugars, etc.).
These four serotypes of dengue virus, named DENV-1, DENV-2, DENV-3, and DENV-4, although relatively similar, are sufficiently different that they do not confer long-term cross-protection. In other words, infection with one serotype immunizes us against that serotype, but does not protect us against the other three. It is therefore possible to be infected with all four dengue serotypes during one's 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 country several times this summer.
Mosquito control as a means of prevention
After cases of infection are discovered, the same scenario unfolds: mosquito control operations are carried out near the detected cases, accompanied by awareness-raising campaigns targeting the public and healthcare professionals (as well as door-to-door surveys), conducted in collaboration with the ARS, Santé publique France, and mosquito control agencies such as Altopictus orthe Interdepartmental Mosquito Control Agreement.
Currently, the best ways to prevent the spread of viruses such as dengue fever, which is transmitted by mosquitoes, are to limit the proliferation of these insects and to protect yourself from their bites.
To reduce the development of mosquito larvae, it is recommended to empty all containers of standing water, including flower pot saucers and watering cans, and to cover rainwater receptacles, especially during periods of heavy rainfall. To limit the risk of being bitten, it is recommended to use appropriate repellents and to wear loose-fitting, covering clothing.
Strengthening surveillance networks remains one of the best strategies for combating these new threats, which are difficult to anticipate.
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.
Yannick Simonin, Virologist, Associate Professor in Surveillance and Study of Emerging Diseases, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Readthe original article.