There has been a growing number of cases of dengue and chikungunya in mainland France in recent years: in 2022, there were 66 autochthonous cases of dengue fever, i.e. people infected locally, which is as many as the total number of autochthonous cases over the previous ten years. Dengue fever, like chikungunya or Zika, is an arboviral disease, meaning that it is caused by a virus transmitted by arthropod vectors, in this case mosquitoes.

At the request of the Directorate General for Health, ANSES conducted an expert appraisal to estimate the likelihood of an epidemic of mosquito-borne arboviral disease occurring in mainland France, along with its health, social and economic impacts. Given the increase in cases of dengue fever in recent years, the expert appraisal focused on viruses transmitted by the tiger mosquito. It drew on a consultation with stakeholders in the field and a literature review.

A fairly high risk of an arboviral disease epidemic in the next five years

Until now, all the autochthonous cases of viruses transmitted by the tiger mosquito in mainland France have been localised outbreaks, in which it has always been possible to trace the source of infection. The experts currently estimate that taking all viruses into consideration, the probability of an arboviral disease epidemic occurring within the next five years is between 6 and 7, on a scale of 0 to 9. "We call it an epidemic when it is not possible to link all the infected people to a particular outbreak. This means that transmission has been able to circumvent the control system", explains Émeline Barrès, from ANSES's Risk Assessment Department, one of the two coordinators of the expert appraisal.

The occurrence of an epidemic depends on a number of factors: the presence of tiger mosquitoes in the area, climatic conditions favourable to their reproduction (in particular an accumulation of hot days over a given period, together with rainfall), the arrival of infected people from areas where viruses are circulating, and the effectiveness of measures to control tiger mosquitoes and virus transmission.

Risk of saturation of the vector control system

If an epidemic were to occur, the means of preventing and controlling arboviral disease could quickly become saturated: "Some of the vector monitoring and control players we interviewed for the expert appraisal told us that they would have been overwhelmed if more cases had occurred in the last few years", says Véronique Raimond, a health economist in ANSES's Social Sciences, Economics & Society Department, the other coordinator of the expert appraisal.

Managing cases of arboviral disease, whether imported or autochthonous, requires considerable material, financial and human resources. As an example, surveillance and monitoring protocols require all contacts with the infected person over the previous 10 days to be traced and the places they visited checked, in order to eliminate any tiger mosquitoes present. Based on current resources, an increased number of cases would lead to a deterioration in the quality of operations and in the working conditions of the people involved in surveillance. In this situation, priorities would need to be established. ANSES published an opinion in March 2024 proposing criteria for prioritising vector control measures.

The Agency recommended adapting resources to the needs of prevention, surveillance and control of arboviral diseases. It also called for the introduction of an interministerial plan to combat these diseases, in order to better identify and coordinate the players involved, including public engagement.

Risk of strain on the healthcare system in the event of a major epidemic

The experts believe that the healthcare system would come under strain in the event of a major epidemic, i.e. if numerous cases were to occur over a wide area. There is also a risk of saturation if this epidemic were to coincide with another, as was the case in 2020 in the French Caribbean, where there was an epidemic of dengue fever at the same time as COVID-19. Moreover, there could be a risk if healthcare services, particularly general practitioners and emergency services, were already saturated, as may be the case during the mosquito's period of activity.

In order to better prepare for the consequences of an arboviral disease epidemic in mainland France, it is vital to fully exploit the experience of the French overseas territories. The experts also stress the importance of training healthcare workers to recognise the risk factors and warning signs of severe forms of arboviral disease.

Economic impacts and wider social inequalities

While an epidemic of arboviral disease in mainland France is unlikely to have a major impact on the overall economy, certain sectors could be weakened. Tourism is likely to be the main industry affected if an arbovirus were in circulation, even on a small scale, with a drop in visitor numbers in the areas concerned.

Epidemics are also known to widen social inequalities. In the event of an arboviral disease epidemic, ANSES calls on the authorities to pay attention to ensuring that the most underprivileged parts of the population have access to information, preventive measures and treatment.

To encourage universal engagement in vector control and disease prevention initiatives, the Agency recommends getting the general public and all stakeholders involved in developing strategies to combat arboviral diseases at local and regional level.        
Furthermore, since the presence of the tiger mosquito is encouraged by climate disruption and rising temperatures, control of arboviral diseases should form part of a broader approach to combating climate change and other global changes.