2023-08-12 04:00:10
Usutu virus, Zika virus, chikungunya or dengue virus… In recent years, these exotic-sounding names have made a name for themselves in the French media. Image illustration Pixabay
And for good reason: responsible for diseases which until now were only rife in remote regions, these viruses are in the process of extracting themselves from the regions where they have long been endemic to set out to conquer our planet. France is not immune to this threat, neither in the overseas territories nor in the metropolitan regions, as evidenced by the current establishments of some of these viruses around the Mediterranean arc.
While the year 2022 has seen explode in metropolitan France the cases of “indigenous” dengue fever (in other words contracted in metropolitan France), and that an infection – also autochthonous – by the West Nile virus was detected for the first time in New Aquitaine, where is the situation? Which viruses should you watch out for first?
Here is what the work of surveillance networks and research laboratories studying these viruses has taught us in recent years.
Diseases transmitted from animals to humans
Many emerging infectious diseases are transmitted to humans via an animal “vector”, often a blood-sucking arthropod such as mosquitoes, culicoides midges, sandflies or ticks.
In such a case, if the disease is caused by a virus, it is called “arbovirosis”, and the virus involved is described as an “arbovirus” (from the English “arthropod-borne virus”, “virus transmitted by arthropods”).
In the list of priority diseases that the World Health Organization (WHO) has established each year since 2015, only viral diseases are included, including three arboviruses (out of nine diseases identified): Zika virus disease, Crimean-Congo hemorrhagic fever and Rift Valley.
An important point to underline concerning arboviruses is that most of them are zoonoses. In other words, they initially come from domestic or wild animals carrying the agent of the disease. This is then transmitted to humans, when the latter is bitten by an arthropod vector which has previously taken the blood of an infected animal. What happens next depends in particular on the transmitted arbovirus.
Some can pass from one human being to another, always via a vector. Others may also spread in parallel through other modes of transmission (the Zika virus can be transmitted by mosquitoes and sexually, for example). It also happens that certain arboviruses are not transmitted between human beings: we then say that humans are an “epidemiological impasse”. This is the case, for example, of the West Nile virus or the Rift Valley virus.
Among the main actors in the spread of arboviruses are mosquitoes, in particular the tiger mosquito (Aedes albopictus). Recently left to attack our territory, he quickly settled there. However, on its own, it is able to spread several “exotic” viruses.
The tiger mosquito continues its rapid expansion
Catalyzed by international trade, the expansion of tiger mosquito (Aedes albopictus)a vector of several “exotic” viruses has proven to be very fast.
Originally from Asia, this small black mosquito with a white striped body and legs was detected for the first time in the south of France in 2004, in Menton. Less than twenty years later, it is present in 71 metropolitan departments out of 96 (compared to 64 in 2021 and 58 in 2020). In the years to come, the extension of its territory will be inexorable.
In less than two decades, the tiger mosquito has invaded most of metropolitan France. Ministry of Solidarity and Health – General Directorate of Health
One of the peculiarities of this mosquito is its ability to transmit various viruses responsible for diseases, the best known of which are probably the Zika virus, that of chikungunya, or even that of dengue fever.
Dengue fever: an upward trend in indigenous cases
The dengue virus seems to have found a favorable playground in the southern regions of our country. Indeed, if the number of cases of dengue fever remains limited in metropolitan France, estimated at less than thirty in recent years, the upward trend is nevertheless confirmed.
This is evidenced by the multiplication of so-called “indigenous” cases, which means that the disease was contracted on our territory, unlike imported infections, which occur in France but were contracted abroad, during a trip. The departments in the south and south-east of France such as Hérault, Gard, Var or Alpes-Maritimes are the most exposed to the disease, in particular due to the combination of a high density of tiger mosquitoes and highly urbanized areas.
The year 2022 was exceptional in terms of dengue fever circulation in France: 66 cases were identified during nine episodes of indigenous transmission that affected 6 departments.
The tiger mosquito Aedes albopictus is recognizable by the white bands that streak its legs. It is the vector of the dengue and chikungunya viruses as well as the Zika virus.
Wikimedia Commons / Centers for Disease Control and Prevention
But this number of cases is certainly largely underestimated, because dengue fever is largely asymptomatic (in 50% to 90% of cases depending on the epidemics). Moreover, in symptomatic people, the symptoms of the disease (fever, headache, muscle pain, etc.) can easily be confused with those of the flu or more recently of Covid.
If the disease caused by the dengue virus is most often mild, it can nevertheless lead to a potentially fatal form, in about 1% of cases. This form of dengue fever is called “haemorrhagic” because it is accompanied by bleeding in multiple organs. In addition, some neurological damage has also been reported.
The chikungunya is discreet
First identified in Tanzania in 1952, the chikungunya virus has circulated for several decades in Africa, India and Asia, as well as in the Indian Ocean. Moreover, it was the epidemic that struck the meetingMauritius, Mayotte and the Seychelles in 2005 which helped to make it known to the French public.
The “bent man disease” (possible translation of “chikungunya”, a term from Makonde, a Bantu language spoken in Tanzania) is characterized in particular by the onset of fever and severe joint pain. Very disabling, these often affect the hands, wrists, ankles or feet. Headaches and muscle aches, as well as bleeding gums or nose have been commonly described. The convalescence can last several weeks, and the pain can sometimes persist for several years.
The first two autochthonous cases of chikungunya were detected in France in 2010, in the Var. Since then, around thirty indigenous cases have been listed, including two major outbreaks, one in the city of Montpellier in 2014 (11 confirmed cases and 1 probable case) and the second in the Var in 2017 with 17 cases listed.
The chikungunya virus has been discreet in France in recent years, with 3 cases imported in 2021 et 5 for the current yearaccording to the count of Public health France. However, this virus remains closely monitored, in particular because its dissemination by the tiger mosquito in temperate European regions cannot be ruled out.
Waiting for a re-emergence of the Zika virus
The virus Zika had meanwhile hit the headlines in 2015-2016. It had been the cause of a very large epidemic, mainly in Latin America. More than a million people had been infected. The most serious damage associated with this virus is the development of a microcephaly (reduction of perimeter fetal skull) in infected pregnant women.
In France two indigenous cases had been identified in 2019in the department of Var, without the chains of transmission, in particular vectorial, having been clearly established.
Still appearing on the list of the 10 diseases most at risk established by the World Health Organization, this virus has however mysteriously almost disappeared from the radar for a few years. His return to the forefront of the viral scene is however far from being ruled out: he notably made people talk about him again very recently in Thailand, and 5 travelers developed the disease in Germany, United Kingdom and Israel, after having visited this Southeast Asian country.
Even if the mechanisms favoring the emergence of the Zika virus are little known, seroprevalence studies (presence of antibodies in the blood) show that it is still circulating actively in certain territories (especially on the African continent).
Its monitoring requires particular vigilance from the scientific community, in order to prepare for a potential reappearance.
The good old Culex is not left out
One of our “traditional” mosquitoes, Culex pipiens, present throughout France, is also capable of transmitting “exotic” viruses to us. This is especially the case with viruses. West Nile (virus du Nil occidental) and Usututwo very similar viruses that can occasionally cause severe neurological damage such as encephalitis (inflammation of the brain), meningitis (inflammation of the meninges) or even meningoencephalitis (inflammation of the meninges and brain) in humans.
A study conducted by our team and published in 2022, in collaboration with ANSES (National Agency for Food, Environmental and Occupational Health Safety) and CIRAD (Centre for International Cooperation in Agricultural Research for Development ), showed that these two viruses are in the process of settling permanently in certain areas of our territorymore particularly in the greater Camargue region.
They are found regularly not only in human blood samples, but also in animals such as birds (their natural reservoirs), dogs, horses and mosquitoes.
Many mosquitoes, including those of the Culex genus classically present in France, can transmit “exotic” viruses.
Shutterstock
So far, their impact on human health remains low: two infections with the Usutu virus have been identified in France, in Montpellier in 2016 and in the moors in 2022, while around thirty cases from West Nile have been counted. over the past five years.
It is nevertheless important to monitor the propagation dynamics of these viruses, because several lines with varying degrees of virulence are currently circulating, which calls for caution. Especially since a major epidemic hit Europe in 2018, with over 2,000 cases identified and over 180 deaths reported. In 2022, southern Europe was again affected: Italy in particular recorded 723 cases and 51 associated deaths.
In France, at the end of July 2023, a first case of infection by the West Nile virus was detected in New Aquitaine, more precisely in the region of Bordeaux, which testifies to a tendency for the spread of this virus further north. of the Hexagon. Indeed, the previous cases of infection of this virus had been detected only around the Mediterranean (in the Provence-Alpes-Côte d’Azur and Occitanie regions).
Human activities and emergences
The passage of a disease from animals to humans is not necessarily synonymous with an epidemic outbreak, nor a large-scale epidemic, a pandemic or establishment in new territories. For this to happen, many events must come together.
The problem is that trade or touristwhich progress in a way exponential in our hyperconnected world, can facilitate the dissemination of certain vectors and therefore the risk of spreading diseases.
This risk is still increased by environmental and climatic changes. The weather conditions, in particular the temperatureair humidity and precipitation, affect the geographical distribution, activity, reproduction rate and survival of these vectors, especially mosquitoes.
Furthermore, changes in the climate and the impact of human beings on their environment sometimes influence animal behavior, for example by modifying the range of certain species, which can promote interactions between animals and humans.
Such environmental changes have led to outbreaks of Argentine hemorrhagic fever, caused by the Junin virus. In the 1950s, to intensify corn cultivation, massive clearing was carried out thanks, in particular, to the use of herbicides. This change in environment led to a proliferation of rodents, some of which carried the virus, pushing the disease to epidemic stage, especially among agricultural workers. Thousands of people were then infected. A similar situation has also been observed in East Asia when converting land for rice cultivation, with the Hantaan virus responsible for “Korean hemorrhagic fever”.
Among the other factors favoring the emergence of new diseases, let us mention in particular socio-economic factors, such as the increase in the transport of goods and people, in particular via intercontinental air transport, or the ever-increasing development of urban areas. The high population densities, which favor the rapid transmission of diseases, as well as the difficulties of water supply linked to rapid urbanization, contribute in particular to the proliferation of mosquitoes potentially carrying viruses.
Proof of the importance of these factors, during the Covid-19 pandemic, the number of cases of “exotic” infections imported (in other words, brought back from travel) decreased considerably, mainly due to the drastic reduction in air transport international. With the marked recovery of said traffic, an increase in these cases is, however, expected in 2023.
This situation could have an impact on the appearance of outbreaks of cases in France, because an infected person arriving on our territory can indeed in turn transmit the disease to other people, in particular if the vectors transmitting the disease are present. .
Prevention, the first weapon against “exotic” viruses
In the absence of an effective antiviral or vaccine, as in the case of chikungunya or Zika, or when the vaccine has certain limitations (as in the case of dengue fever, against which the only currently approved vaccine has the disadvantage increase the risk of hospitalization and severe dengue in people not previously infected with dengue virus), the only solution is to anticipate the emergence of these pathogens.
The best way to achieve this is to set up adapted and reactive networks, as close as possible to the field, in order to effectively study the interactions between animals, humans and their various environments, according to an approach qualified as One Health (“One health”, human and environmental).
Since the Covid-19 pandemic, national and international viral disease surveillance networks have grown. Their capacities unfortunately remain well below what is necessary to carry out effective monitoring of the circulation of high-risk viruses, not only in endemic countries, but also in countries where they are emerging.
The emergence, then the rapid spread in 2020, of the SARS-CoV-2 coronavirus, responsible for the Covid-19 pandemic, has had a major impact on our health, our behavior and our daily lives. This situation made us suddenly aware of the importance of monitoring and studying “new” viruses.
Beyond these hitherto “unpublished” viruses, it is also essential to look at “neglected” viruses because they are responsible for diseases raging far from our territories. The propagation outside the African continentand in particular in Europe, of the Mpox virus, formerly called monkeypox, has come to remind us of the issues related to such surveillance…
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