Chikungunya in Réunion: a widespread epidemic requiring heightened vigilance
A severe chikungunya epidemic has been raging in Réunion since the beginning of 2025. At the beginning of April, two people aged 86 and 96 died, one of whom had comorbidities, and 14 newborns were in intensive care. We take stock of this little-known disease and this unprecedented epidemic.
Yannick Simonin, University of Montpellier
Chikungunya is a viral disease transmitted to humans by the bite of a mosquito that has been infected with the virus. Since early 2025, the island of Réunion has been experiencing a large-scale epidemic. The peak of the epidemic could occur in April or May.
Are chikungunya epidemics common in Réunion?
The situation currently affecting Réunion is unusual. The last major chikungunya epidemic on Réunion occurred twenty years ago, in 2005-2006. Prior to that, there had been very few isolated cases of chikungunya on the island.
Before this first major epidemic, Réunion was not an area where the chikungunya virus circulated, as Aedes aegypti, the main mosquito vector for this virus, is not found there. However, for many decades, the island had been home to another mosquito of the same family, Aedes albopictus, better known as the tiger mosquito. At that time, however, the tiger mosquito was not known to transmit this virus.
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However, in 2005-2006, to the great 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, allowing 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 outbreak of the 2025 epidemic?
The emergence of this type of epidemic is always very difficult to anticipate. However, certain conditions favor them, starting with the large numbers of tiger mosquitoes on Reunion Island. The southern hemisphere summer is hot and humid, creating an ideal environment for their reproduction.
Recent heavy rainfall has also contributed to water stagnation, which has increased the number of breeding sites for mosquitoes. This phenomenon was probably exacerbated by Cyclone Garance, which brought heavy rains on February 19 and 20, 2025.
Furthermore, in heavily urbanized areas of Réunion, artificial breeding sites such as buckets, used tires, flower pots, cans, clogged gutters, and other objects left outside and filled with stagnant water also play a role in feeding the laying and growth of eggs.
Finally, since the last major chikungunya epidemic occurred in 2005, the local population has very low immunity to this virus, allowing the pathogen to spread quite easily, particularly among low-income populations living in unsanitary conditions that are conducive to mosquito proliferation.
Where is chikungunya found in France and around the world?
The chikungunya virus is present in fairly large geographical areas where Aedes aegypti, the mosquito originally known to transmit this virus, is found in Asia, Oceania, Africa, and the Americas. It should be noted that the range ofAedes aegypti is expanding. Due to global changes, particularly climate change, there is a high probability that it will one day appear in Europe, along with the viruses it can transmit...
Thanks to a mutation that appeared twenty years ago, enabling it to adapt to the tiger mosquito (Aedes albopictus) present in Réunion, the distribution range of the chikungunya virus has changed significantly. Several strains of the chikungunya virus are now circulating, depending on whether they are adapted to Aedes aegypti or the tiger mosquito (Aedes albopictus).
In France, we are faced with the possibility of chikungunya transmission both in areas where Aedes aegypti is established, for example in the French West Indies (Guadeloupe, Martinique, Saint Martin, Saint Barthélemy) and French Guiana, but also in areas where the tiger mosquito (Aedes albopictus) is present, such as Réunion and mainland France.
Aedes albopictus is well established in many regions, particularly in the south, but also in the Île-de-France region and as far east as the eastern part of the country. In January 2024, it was considered to have established itself in 78 of France's 96 departments.
It should be noted that the risk is high in Mayotte, which, due in particular to its favorable humid tropical climate, is home to both species of mosquitoes.
Domestic cases of chikungunya—linked to infections within the country rather than to travelers who were already infected upon arrival—have already been recorded in mainland France in the past. The first identification of this virus in the Île-de-France region in 2024 perfectly illustrates its potential to take hold in Europe, and not just in Mediterranean regions.
Why do we hear more about dengue fever than chikungunya?
The chikungunya virus, like the dengue (or Zika) virus, is mainly transmitted by two large categories of mosquitoes of the genus Aedes: the tiger mosquito (Aedes albopictus) and Aedes aegypti.
These viruses belong to a category of viruses called arboviruses, a name that comes from the English term " arthropod-borne virus," which means "virus transmitted by arthropods." Dengue fever is more widely known because it is the most common arbovirus worldwide, according to the World Health Organization (WHO).
This is also because the mortality rate associated with dengue fever is higher. Without appropriate treatment, it can reach 10% or even 20% (mainly due to hemorrhagic symptoms), but it most often ranges between 0.3% and 2.5%, depending on the study and country.
The mortality rate for chikungunya is much lower, at around 0.1%. The public health issue associated with chikungunya stems from its ability to cause chronic, disabling joint damage in many patients.
The categories of people most at risk of severe forms of chikungunya, which can take the formof neurological disorders such as encephalitis, are the elderly, particularly those already suffering from other diseases, immunocompromised individuals, and pregnant women.
Why is chikungunya not a harmless disease?
Chikungunya causes joint pain that can occur in the ankles, hips, shoulders, wrists, etc., and persist for a very long time, up to several years after infection, in nearly half of those infected. This situation can be very debilitating in everyday life for those affected.
In fact, chikungunya means "to become contorted" or "to walk bent over" in Makonde, a language spoken in Tanzania and Mozambique, where the disease was first described. This accurately describes the posture of patients with the disease, who may suffer from joint pain so severe that they are forced to adopt a hunched posture. Suffering from chronic pain and fatigue can be particularly debilitating, preventing patients from carrying out normal professional activities, for example.
It should be emphasized that there are no specific treatments for chronic damage caused by chikungunya, damage that remains poorly understood.
Is it due to the virus persisting in the body? Or is it caused by immune dysregulation leading to prolonged inflammation? The question remains open, even though research is progressing in this area.
Why is the new vaccine not recommended for the entire population?
It should be noted that we are fortunate to have a vaccine available, which is not the case for most mosquito-borne viruses. The chikungunya vaccine is very recent, having only been approved for marketing in the European Union in June 2024 (Ixchiq vaccine from Valneva).
In the context of the epidemic in Réunion, France, the Haute Autorité de Santé (HAS) estimated that
"The available results are sufficient to recommend it for populations at risk of severe and/or chronic forms, for whom the expected benefit is significant."
In practice, the HAS prioritizes people aged 65 and over, particularly those with comorbidities (high blood pressure, diabetes, cardiovascular, respiratory, renal, hepatic, and neurovascular diseases), as well as people aged 18 to 64 with comorbidities. At this stage, the HAS does not recommend the use of 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 doses are limited and there is a lack of data on its overall effectiveness, even though it is expected to be sufficiently effective based on the available data.
However, in the long term, the question of extending this vaccination program may arise in an attempt to curb the spread of the virus and protect the entire population of Réunion. It should be noted that other vaccines are also currently in development.
What are the ways to combat chikungunya, other than vaccination?
To protect against chikungunya, teams of vector control experts, commissioned by regional health agencies (ARS), are dispatched to the site as soon as a case of chikungunya is identified, in order to eliminate adult mosquitoes and larval breeding sites.
However, to limit the spread of mosquitoes upstream, it is important to inform the population so that they avoid creating artificial breeding sites, i.e., stagnant water where mosquitoes can lay their eggs and which is the primary source of mosquito proliferation.
Finally, it is important to limit the risk of bites by using mosquito nets, repellents, insecticides, and wearing light-colored, loose-fitting clothing that covers the skin.
How can we assess the risk of a chikungunya epidemic in mainland France?
In the opinion of the French Agency for Food, Environmental and Occupational Health & Safety (ANSES), "Tiger mosquitoes in mainland France: risk and impact of arbovirus disease" published in July 2024, the expert group of which I was a member clearly identified chikungunya as posing a potential epidemic risk in mainland France in the medium term (along with dengue fever and Zika), with the majority of departments now colonized by tiger mosquitoes.
An outbreak of chikungunya, like those of other arboviruses, also depends on the global circulation of the virus and air travel between countries where the virus is circulating.
Consequently, regular air travel between France and Réunion, but also, more generally, with the overseas departments and regions (DROM) and areas where chikungunya is actively circulating, could facilitate the introduction of the virus into mainland 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. Readthe original article.