The Unfolding Crisis: Chikungunya Grips Reunion

Reunion Island is currently battling a significant Chikungunya outbreak, with concerns mounting over the true extent of the epidemic. While official figures report over 33,000 confirmed cases since the start of 2025, along with six related deaths, some experts suggest the actual number of infections could be far higher, perhaps exceeding 100,000.

Gérard Coollon, director of the Regional Health Agency (ARS), voiced his concerns during a presentation of a mosquito control action plan in Saint-denis, stating that the number of affected residents likely surpasses official counts. This discrepancy highlights the challenges in accurately tracking the spread of the virus.

Underestimated Numbers: A Closer Look at the Data

The official data,as reported by Public Health France,indicates over 91,500 consultations related to Chikungunya since the beginning of 2025. However, entomologist Anne-Bella Failloux from the Pasteur Institute points out that these figures may not reflect the full picture. The current outbreak began in August 2024, and many infected individuals may not seek medical attention, leading to underreporting.

Coollon further elaborated on the issue, suggesting that the reported 6,000 to 7,000 new cases per week are likely inaccurate. He attributes this to the fact that not all patients are being tested. He estimates that general practitioners are seeing around 22,000 consultations weekly for symptoms consistent with chikungunya. Manuel Valls, the Minister of Overseas, during a visit to the island in early April, estimated that up to 70,000 people may have been affected, considering the unreported cases.

this situation mirrors challenges faced in other regions battling mosquito-borne illnesses. Such as, during the Zika virus outbreak in brazil in 2015-2016, studies estimated that the actual number of infections was substantially higher than the reported cases due to asymptomatic infections and limited testing capacity.

Peak Yet to Come: Concerns and Projections

Despite initial expectations of a mid-April peak, health authorities now believe the peak of the epidemic is still to come. This ongoing increase in cases is reflected in the rising number of sick leaves. Thierry Bies, director of health at the Reunion Social Security Caisse (CGSS), reported 12,186 sick leaves during the week of April 7-13, compared to 5,847 during the week of March 10, when the epidemic was less pronounced.

While a direct link between these sick leaves and Chikungunya cannot be definitively established, the significant increase suggests a strong correlation. The epidemic is described as “generalized and major” and “continues its progression,” according to France France.

Failloux notes that while the current outbreak is less severe than the 2005-2006 epidemic, which infected over 260,000 people and resulted in over 200 deaths, the situation remains concerning. During that earlier outbreak, approximately 30-40% of the population was infected. experts predict the peak of the current outbreak will occur towards the end of April.

Environmental Factors: Fueling the Mosquito Population

Several factors contribute to the rapid spread of Chikungunya on Reunion Island. Failloux highlights the impact of Cyclone Garance, which brought significant rainfall, creating numerous breeding grounds for mosquitoes. The island’s topography, characterized by numerous gullies, further complicates mosquito control efforts, as many larval habitats are tough to access and treat with insecticides.

The island’s climate and geography create ideal conditions for the proliferation of tiger mosquitoes, the primary vectors of Chikungunya. High temperatures and the abundance of breeding sites contribute to a large mosquito population, facilitating the transmission of the virus. The specific strain of the virus circulating on the island is an African genotype with a mutation that allows it to be efficiently transmitted by tiger mosquitoes, further exacerbating the situation.

The topography of the island and fairly high temperatures promote a very large quantity of mosquitoes, a framework conducive to maintaining this chikungunya epidemic.

Anne-Bella Failloux, entomologist, Pasteur Institute

Looking Ahead: When Will the Epidemic Subside?

Predicting the decline of the epidemic is challenging and depends largely on weather conditions. Failloux suggests that the onset of the southern winter in late June, with temperatures dropping to around 20 degrees Celsius, could slow down mosquito activity. At lower temperatures, mosquitoes are less active, feed less, and lay fewer eggs.

However, she cautions that current climatic disturbances make it difficult to guarantee a predictable decline. Unseasonal weather patterns could disrupt the natural cycle and prolong the epidemic.

Combating the Spread: Control Strategies in Action

Reunion island is implementing various control strategies to contain the Chikungunya outbreak. The ARS responds to reported cases by eliminating larval habitats around the patient’s home, educating residents about prevention measures, distributing repellents to priority groups, and identifying additional cases in the surrounding area. The ARS also utilizes insecticides to target adult mosquitoes and aquatic larvae in homes and gardens.

Failloux emphasizes that control methods have been developed and refined since the 2005 epidemic. One promising technique involves the release of sterile male mosquitoes. These mosquitoes are irradiated, rendering their sperm defective. When they mate with female mosquitoes, the resulting eggs are not viable, leading to a reduction in the mosquito population. This technique has shown promising results in reducing the risk of mosquito bites.

As the 2005 epidemic, means of struggle have been developed, and studies and research have been carried out to prepare for future epidemics.

Anne-Bella Failloux, entomologist, Pasteur Institute

Community Engagement: A Crucial Component

In addition to these measures, Failloux stresses the importance of educating the population about eliminating mosquito breeding sites. This includes removing standing water from containers, emptying saucers under flower pots, and clearing clogged gutters. She also advocates for vaccination, especially for individuals over 65 and those at high risk, provided the vaccine is readily available and effective.

Community engagement is crucial for the success of any mosquito control program. By empowering residents to take proactive steps to eliminate breeding sites, the island can significantly reduce the mosquito population and mitigate the risk of Chikungunya transmission.