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Chikungunya in Reunion: a large-scale epidemic requiring increased vigilance
Published: April 3, 2025 11:51am EDT
Author
Yannick Simonin
Virologist specializing in the surveillance and study of emerging viral diseases. University Professor, University of Montpellier
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A major chikungunya epidemic has been raging in Réunion since the beginning of 2025. In early 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 disease which remains little known and of this unprecedented epidemic.
Chikungunya is a viral disease transmitted to humans through the bite of a mosquito that has already been infected with the virus. Since early 2025, Reunion Island has been experiencing a large-scale epidemic . The epidemic peak could occur in April or May.
Are chikungunya epidemics common in Reunion?
The current situation in Réunion is atypical. Indeed, the last major chikungunya epidemic in Réunion was twenty years ago, in 2005-2006 . Before that date, few cases of chikungunya had been observed on the island and only very occasionally.
Before this first major epidemic, Réunion was not a zone of circulation of the chikungunya virus, because Aedes aegypti , the main mosquito vector of this virus, is not established there. On the other hand, the island had been home for many decades to another mosquito of the same family, Aedes albopictus , better known as the tiger mosquito . But at that time, the tiger mosquito was not known to be able to transmit this virus.
Yet 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 at 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 the adaptation of a virus to a new environment.
What factors contributed to the outbreak of the 2025 epidemic?
The occurrence of this type of epidemic is always very difficult to predict. However, certain conditions favor them, starting with the large presence of the tiger mosquito in Réunion. The southern summer is hot and humid, which creates an ideal environment for its reproduction.
Recent heavy rainfall has also contributed to waterlogging, which has increased the number of mosquito breeding sites. This phenomenon was likely also exacerbated by Cyclone Garance , which caused heavy rains on 19 and 20 February 2025.
Furthermore, in the highly urbanized areas of Réunion, artificial breeding sites, such as buckets, used tires, flower pots, cans, blocked gutters and other objects left outside and filled with stagnant water, also play a role in supporting egg laying and growth.
Finally, since the last major chikungunya epidemic occurred in 2005, the local population has very low immunity to this virus, which allows the pathogen to circulate quite easily, particularly among poor populations who live in unsanitary living conditions, which are conducive to the proliferation of mosquitoes.
What are the distribution areas of chikungunya in France and around the world?
The chikungunya virus is present in fairly large geographical areas where Aedes aegypti , the mosquito originally known for transmitting this virus, is present, in Asia, Oceania, Africa and America. It should be noted in passing that the distribution area of Aedes aegypti is expanding. Due to global changes, particularly climate changes, there is a high probability of seeing it appear one day in Europe and the procession of viruses that it can transmit...
Thanks to a mutation that appeared twenty years ago and which allowed it to adapt to the tiger mosquito ( Aedes albopictus ) present in Réunion, the distribution area of the chikungunya virus has been significantly modified. Several strains of chikungunya virus are now circulating, depending on whether they are adapted to Aedes aegypti or to the tiger mosquito ( Aedes albopictus ).
In the case of France, we are faced with possibilities of transmission of chikungunya both in areas where Aedes aegypti is established, for example in the French Antilles (Guadeloupe, Martinique, Saint-Martin, Saint-Barthélemy) or in Guyana, but also in those where the tiger mosquito ( Aedes albopictus ) is present, such as in Réunion but also in 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 France. As of January 2024, it is considered to have established itself in 78 of the 96 departments of mainland France.
Note that the risk is high in Mayotte which, particularly due to a favorable humid tropical climate, is home to both species of mosquitoes.
Indigenous cases of chikungunya—linked to infections within the country and not to travelers already infected upon arrival—have already been reported in mainland France. The first identification of this virus in the Île-de-France region in 2024 perfectly illustrates the potential for it to establish itself in Europe, and not just in the Mediterranean regions.
Why do we hear more about dengue fever than chikungunya?
The chikungunya virus, like that of dengue (or Zika ), is mainly transmitted by the two main 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 arthropod-borne virus which means “viruses transmitted by arthropods”. The reason we hear more about dengue is that it is the most widespread arbovirus in the world, 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 fluctuates between 0.3% and 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 stems from its ability to cause chronic, disabling joint damage in many patients.
The categories of people most exposed to severe forms of chikungunya, which can take the form of neurological disorders such as encephalitis, are the elderly, particularly those who already suffer from other pathologies, 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 can persist for a very long time , up to several years after infection, in nearly half of those infected. This situation can be very disabling in the daily lives of those affected.
Moreover, chikungunya means "bending" or "walking bent" in Makonde, a language spoken in Tanzania and Mozambique where the disease was first described. This aptly describes the posture of patients with the disease, who can suffer from joint pain so intense that they are forced to adopt a stooped posture. Suffering from chronic pain and fatigue can be particularly debilitating , preventing, for example, the ability to carry out normal work activities.
It should be emphasized that there are no specific treatments for chronic chikungunya infections, which remain poorly understood.
Is this a case of persistent virus activity in the body? Is it a case of immune dysregulation leading to prolonged inflammation? The question remains open, even though research in this area is progressing.
Why is the new vaccine not recommended for the entire population?
It's worth noting that we're fortunate to have a vaccine, which isn't the case for most mosquito-borne viruses. The chikungunya vaccine is very recent, having only been granted marketing authorization in the European Union in June 2024 (Valneva's Ixchiq vaccine).
In the context of the epidemic in Réunion, France, the High Authority for Health (HAS) has estimated that
"the available results are sufficient to recommend it to populations at risk of severe and/or chronic forms, for whom the expected benefit is significant."
In practice, the HAS is primarily targeting 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 now, the first people eligible for vaccination are the most vulnerable, because the available doses are limited and there is a lack of perspective on its overall effectiveness, even if we expect a sufficiently high effectiveness based on the available data .
However, in the long run, the question of expanding this vaccination program could arise in an attempt to slow the spread of the virus and protect the entire population of Réunion. It should be noted that other vaccines are also under development.
What are the means of combating chikungunya, other than the vaccine?
To protect against chikungunya , teams of vector control experts, mandated by the 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 breeding sites.
But to limit the spread of mosquitoes upstream, it is appropriate to inform the population so that they avoid creating artificial breeding sites, these stagnant waters which allow for laying eggs and represent 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, covering and loose clothing to limit the risk of bites.
How to assess the risks of a chikungunya epidemic in mainland France?
In the opinion of the French Food Safety Agency (ANSES), "Tiger mosquito in mainland France: risk and impacts of an arbovirosis" of July 2024, the group of experts of which I was a member clearly identified chikungunya as a potential epidemic risk in mainland France in the medium term (just like dengue and Zika), with the majority of departments now being colonized by the tiger mosquito .
A chikungunya epidemic, like those of other arboviruses, also depends on the dynamics of global circulation of the virus and air traffic between countries where the virus is circulating.
Consequently, regular air traffic between France and Réunion, but also, more generally, with the overseas departments and regions (DROM) and areas where chikungunya is actively circulating, could encourage the introduction of the virus into mainland France from the summer of 2025.
https://theconversation.com/chikung...leur-qui-exige-une-vigilance-renforcee-253495
