Chikungunya on Reunion Island: a major epidemic requiring heightened vigilance
A marked epidemic of chikungunya has been raging on Reunion Island since the beginning of 2025. In early April, two people aged 86 and 96 died, one of them with comorbidities, and 14 newborn babies were in intensive care. We take stock of this little-known disease and its unprecedented epidemic.
Yannick Simonin, University of Montpellier
Chikungunya is a viral disease transmitted to humans by the bite of a mosquito that has itself been infected with the virus. Since early 2025, Réunion Island has been experiencing a major epidemic. The epidemic peak could occur in April or May.
How common are chikungunya epidemics on Reunion Island?
The current situation in Réunion is atypical. Indeed, the last major chikungunya epidemic on Réunion was twenty years ago, in 2005-2006. Prior to this date, very few cases of chikungunya had been observed on the island, and only very occasionally.
Prior to this first major epidemic, Réunion was not a zone where the chikungunya virus circulated, as Aedes aegypti, the main mosquito vector of this virus, was not established there. On the other hand, the island had for many decades been home to another mosquito of the same family, Aedes albopictus, better known as the tiger mosquito. But then, the tiger mosquito was not known to transmit this virus.
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Yet in 2005-2006, much to the surprise of virologists, the first major chikungunya epidemic in Réunion was carried by the tiger mosquito. It was later discovered that the chikungunya virus had mutated on its surface, enabling it to better adapt to the tiger mosquito. The arrival of the virus on the island is thus a well-known and worrying example of a virus adapting to a new environment.
What factors contributed to the 2025 epidemic?
The appearance of this type of epidemic is always very difficult to anticipate. However, certain conditions are conducive to them, starting with the presence of large numbers of tiger mosquitoes on Reunion Island. The southern summer is hot and humid, creating an ideal breeding environment.
Recent heavy rainfall has also favored water stagnation, multiplying mosquito breeding grounds. This phenomenon was probably also amplified by cyclone Garance, which generated heavy rainfall on February 19 and 20, 2025.
In Réunion's highly urbanized areas, artificial breeding grounds such as buckets, used tires, flower pots, cans, blocked gutters and other objects left outdoors and filled with stagnant water also play a role in feeding egg-laying and growth.
Finally, since the last major chikungunya epidemic took place in 2005, the local population has a very low level of immunity to the virus, enabling the pathogen to circulate fairly easily, particularly among low-income populations living in insalubrious conditions, which are conducive to mosquito proliferation.
Where is chikungunya found in France and around the world?
The chikungunya virus is present in the vast geographical areas of Asia, Oceania, Africa and America where Aedes aegypti, the mosquito originally known for transmitting this virus, is present. Incidentally,Aedes aegypti 's range is expanding. As a result of global change, particularly climatic change, there is a high probability that it will one day appear in Europe, along with the host of viruses it is capable of transmitting...
Thanks to a mutation that appeared twenty years ago, enabling it to adapt to the tiger mosquito(Aedes albopictus) present on Reunion Island, the chikungunya virus' range has been significantly modified. Several strains of chikungunya virus are now circulating, depending on whether they are adapted to Aedes aegypti or the tiger mosquito(Aedes albopictus).
In the case of France, we're faced with the possibility of chikungunya transmission both in areas where Aedes aegypti is present, such as the French West Indies (Guadeloupe, Martinique, Saint-Martin, Saint-Barthélemy) and French Guiana, and in areas where the tiger mosquito(Aedes albopictus) is present, such as Reunion Island and mainland France.
Aedes albopictus is well established in many regions, particularly in the south of France, but also in the Île-de-France region and as far east as the east of France. In January 2024, it is considered to have established itself in 78 of France's 96 départements.
It should be noted that the risk is high in Mayotte, which is home to both species of mosquito thanks to its favorable humid tropical climate.
Indigenous cases of chikungunya - linked to infections on the territory and not to travellers already infected on arrival - have already been recorded in France in the past. The first identification of this virus in the Île-de-France region in 2024 is a perfect illustration of the possibility of its establishing itself in Europe, and not just in Mediterranean regions.
Why do we hear more about dengue than chikungunya?
Both the chikungunya and dengue (or Zika) viruses are mainly transmitted by the two main categories of mosquitoes in the Aedes genus: the tiger mosquito(Aedes albopictus) and Aedes aegypti.
These viruses belong to a category of viruses known as arboviruses, a name that comes from the English arthropod-borne virus meaning "virus transmitted by arthropods ". We hear more about dengue fever because, according to the World Health Organization (WHO), it is the most widespread arbovirus in the world.
This is also because the mortality rate associated with dengue fever is higher. If left untreated, it can reach 10% or even 20% (mainly due to haemorrhagic symptoms), but more often than not it ranges from 0.3% to 2.5%, depending on the study and the country.
The mortality rate for chikungunya is much lower, at around 0.1%. The public health problem associated with chikungunya is its ability to cause chronic, disabling joint damage in many patients.
The categories of people most at risk from severe forms of chikungunya, which can take the form ofneurological damage such as encephalitis, are the elderly, especially those already suffering from other pathologies, immunocompromised subjects and pregnant women.
Why is chikungunya not a harmless disease?
Chikungunya causes joint pain in the ankles, hips, shoulders, wrists... which can persist for a very long time, up to several years after infection, in almost half of those infected. This can be very disabling for those affected.
Indeed, chikungunya means "one who curls" or "one who walks hunched" in Makonde, a language spoken in Tanzania and Mozambique, where the disease was first described. This aptly describes the posture of patients suffering from the disease, who can experience such intense joint pain that they are forced to adopt a stooped posture. Suffering from chronic pain and fatigue can be particularly disabling, preventing, for example, normal professional activity.
It should be stressed that there are no specific treatments for the chronic damage caused by chikungunya, which is still poorly understood.
Is it a question of the virus persisting in the body? Or of immune dysregulation leading to prolonged inflammation? The question remains open, even if research is progressing in this field.
Why isn't the new vaccine recommended for the entire population?
It's worth pointing out that we're fortunate enough to have a vaccine, which is not the case for the majority of mosquito-borne viruses. The vaccine against chikungunya is a very recent development, having only been approved for marketing in the European Union in June 2024 (Valneva's Ixchiq vaccine).
In the context of the epidemic in La Réunion, France, the French National Authority for Health (HAS) estimated that
"The results available are sufficient to recommend it to populations at risk of severe and/or chronic forms, for which the expected benefit is significant".
In practice, HAS is targeting people aged 65 and over, particularly those with co-morbidities (hypertension, diabetes, cardiovascular, respiratory, renal, hepatic and neurovascular diseases), as well as people aged 18 to 64 with co-morbidities. At this stage, HAS does not recommend using the vaccine in pregnant women. The vaccine is also recommended for healthcare professionals and vector control professionals (i.e. those specializing in mosquito control).
For the time being, the first people eligible for vaccination are the most vulnerable, as the doses available are limited and there is a lack of hindsight regarding its overall efficacy, even if we expect it to be sufficiently effective on the basis of available data.
Eventually, however, the question may arise of extending this vaccination to try and curb the circulation of the virus and protect the entire population of Réunion. Other vaccines are also under development.
What other ways are there of combating chikungunya besides vaccination?
To protect against chikungunya, teams of vector control experts, appointed by regional health agencies (ARS), are dispatched as soon as a case of chikungunya is identified, to eliminate adult mosquitoes and breeding grounds.
But to limit the spread of mosquitoes upstream, we need to inform the population to avoid creating artificial breeding grounds - stagnant water that allows mosquitoes to lay their eggs and is their primary source of proliferation.
Finally, it's important to limit the risk of bites by using mosquito nets, repellents and insecticides, and by wearing light-colored, loose-fitting clothing.
How to assess the risk of a chikungunya epidemic in mainland France?
In the July 2024 opinion of the French Health Safety Agency (Anses), "Moustique tigre en France hexagonale: risque et impacts d'une arbovirose", the group of experts of which I was a member clearly identified chikungunya as a potential epidemic risk in mainland France over the medium term (just like dengue and Zika), with the majority of departments now colonized by the tiger mosquito.
A chikungunya epidemic, like those of other arboviruses, also depends on the dynamics of global virus circulation and air traffic between countries where the virus is circulating.
As a result, regular air traffic between France and La Réunion, and more generally with the French overseas departments and regions (DROMs) and active chikungunya circulation zones, could encourage the introduction of the virus into France as early as summer 2025.
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. Read theoriginal article.