Exotic viruses in France: a topic that is more relevant than ever
Usutu virus, Zika virus, chikungunya virus, dengue virus... In recent years, these exotic-sounding names have all made their way into the French media. And with good reason: responsible for diseases that until now only affected remote regions, these viruses are spreading beyond the areas where they have long been endemic and are now conquering our planet. France is not immune to this threat, either in its overseas territories or in mainland France, as evidenced by the ongoing spread of dengue fever and Usutu viruses around the Mediterranean arc. Which viruses should we be watching out for? What is the current situation? Here is what we have learned in recent years from the work of the surveillance networks currently in place and the research laboratories studying these viruses.
Yannick Simonin, University of Montpellier
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, midges, sand flies, or ticks.
In such cases, if the disease is caused by a virus, it is referred to as "arbovirus disease," and the virus involved is described as an "arbovirus" (from "arthropod-borne virus"). Since 2015, the World Health Organization (WHO) has been compiling an annual list of priority diseases. Updated in 2021, this list only includes viral diseases. Of the nine listed, three are arboviral diseases: Zika virus disease, Crimean-Congo hemorrhagic fever, and Rift Valley fever.
It is also important to note that most arboviruses are zoonoses. In other words, they originate in domestic or wild animals that carry the disease agent. This agent is then transmitted to humans when they are bitten by an arthropod vector that has previously fed on the blood of an infected animal.
What happens next depends in particular on the arbovirus transmitted. Some can pass from one human being to another, always via a vector. Others can also spread in parallel through other modes of transmission (the Zika virus can be transmitted by mosquitoes and sexually, for example). Some arboviruses cannot be transmitted between humans: humans are then said to be an "epidemiological dead end." This is the case, for example, with the West Nile virus and the Rift Valley virus.
Among the main contributors to the spread of arboviruses are mosquitoes, particularly the tiger mosquito (Aedes albopictus). Having recently invaded our territory, it has quickly established itself here. However, this species alone is capable of spreading several "exotic" viruses.
[Nearly 70,000 readers trust The Conversation newsletter to help them better understand the world's major issues. Subscribe today]
The tiger mosquito continues its rapid expansion
Catalyzed by international trade, the spread of the tiger mosquito (Aedes albopictus), a vector for several "exotic" viruses, has been very rapid.
Native to Asia, this small black mosquito with a white-striped body and legs was first detected in southern France in 2004, in Menton. Less than twenty years later, it is present in 67 of France's 96 metropolitan departments (compared to 58 in 2020). In the coming years, its territory will inevitably continue to expand.
Ministry of Solidarity and Health – Directorate General for Health
One of the distinctive features of this mosquito is its ability to transmit various viruses that cause diseases, the best known of which are probably the Zika virus, the chikungunya virus, and the dengue virus.
Dengue fever: an upward trend in indigenous cases
The dengue virus seems to have found fertile ground in the southern regions of our country. Although the number of dengue cases remains limited in mainland France, estimated at less than thirty in recent years, the upward trend is nevertheless confirmed.
This is evidenced by the increase in so-called "indigenous" cases, meaning that the disease was contracted on our territory, as opposed to imported infections, which occur in France but were contracted abroad during travel. The departments in the south and southeast of France, such as Hérault, Gard, Var, and Alpes-Maritimes, are the most exposed to the disease, particularly due to the combination of a high density of tiger mosquitoes and heavily urbanized areas.
Wikimedia Commons / Centers for Disease Control and Prevention
Given that dengue fever is largely asymptomatic (in 50% to 90% of cases, depending on the epidemic), and that in symptomatic individuals, the symptoms of the disease (fever, headache, muscle pain, etc.) can easily be confused with those of influenza or, more recently, COVID-19, the number of cases is certainly greatly underestimated.
Although the illness it causes is usually mild, the dengue virus can nevertheless lead to a potentially fatal form in about 1% of cases: dengue hemorrhagic fever, which is accompanied by bleeding in multiple organs. In addition, certain neurological disorders have also been reported.
Chikungunya is keeping a low profile
First identified in Tanzania in 1952, the chikungunya virus circulated for several decades in Africa, India, and Asia, as well as in the Indian Ocean. It was the epidemic that struck Réunion, Mauritius, Mayotte, and the Seychelles in 2005 that brought it to the attention of the French public.
The "disease of the bent man" (a possible translation of "chikungunya," a term derived from Makonde, a Bantu language spoken in Tanzania) is characterized in particular by the onset of fever and severe joint pain. These pains are very debilitating and often affect the hands, wrists, ankles, or feet. Headaches and muscle pain, as well as bleeding from the gums or nose, have been frequently reported. Recovery can take several weeks, and the pain can sometimes persist for several years.
The first two indigenous cases of chikungunya were detected in France in 2010, in the Var department. Since then, around 30 indigenous cases have been recorded, 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 region in 2017 with 17 cases recorded.
The chikungunya virus has been relatively uncommon in France in recent years, with three imported cases in 2021 and five so far this year, according to figures from Santé publique France. However, the virus remains under close surveillance, particularly because its spread by the tiger mosquito in temperate European regions cannot be ruled out.
Awaiting a resurgence of the Zika virus
The Zika virus made headlines in 2015-2016. It caused a large-scale epidemic, mainly in Latin America. More than one million people were infected. The most serious effect associated with this virus is the development of microcephaly (reduced head circumference of the fetus) in infected pregnant women.
In France, two indigenous cases were identified in 2019 in the Var department, but the chains of transmission, particularly vector-borne, could not be clearly established.
Still on the World Health Organization's list of the 10 most dangerous diseases, this virus has mysteriously disappeared from the radar in recent years. However, its return to the viral spotlight is far from impossible: it has recently been in the news again in Thailand, and five travelers have developed the disease in Germany, the United Kingdom, and Israel after visiting this Southeast Asian country.
Although the mechanisms promoting the emergence of the Zika virus are poorly understood, seroprevalence studies (presence of antibodies in the blood) show that it is still actively circulating in certain areas (particularly on the African continent).
Monitoring it requires particular vigilance on the part of the scientific community in order to prepare for a potential reappearance.
Good old Culex isn't left behind either.
One of our "traditional" mosquitoes, Culex pipiens, found throughout France, is also capable of transmitting "exotic" viruses. This is particularly the case with the West Nile and Usutu viruses, two 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 and CIRAD, showed that these two viruses are becoming permanently established in certain areas of our territory, particularly in the greater Camargue region. They are regularly found not only in human blood samples, but also in animals such as birds (their natural reservoirs), dogs, horses, and mosquitoes.
So far, their impact on human health remains low: only one case of Usutu virus infection has been identified in France to date, in Montpellier, while around 30 cases of West Nile virus have been reported over the last five years. Nevertheless, it is important to monitor the spread of these viruses, as several strains with varying degrees of virulence are currently circulating, prompting caution. This is particularly true given that a major epidemic struck Europe in 2018, with more than 2,000 cases identified and more than 180 deaths reported.
Furthermore, Italy—like other southern European countries—reported early circulation of the West Nile virus this year, with nearly 150 cases identified by mid-August, revealing its increasingly permanent presence in the south of the continent.
Human activities and emergencies
The transmission of a disease from animals to humans does not necessarily lead to an outbreak, a large-scale epidemic, a pandemic, or the spread of the disease to new territories. Such events are the result of a combination of many factors.
The problem is that trade and tourism, which are growing exponentially in our hyperconnected world, can facilitate the spread of certain vectors and thus the risk of disease transmission.
This risk is further increased by environmental and climate change. Weather conditions, particularly temperature, humidity, and precipitation, affect the geographical distribution, activity, reproduction rate, and survival of these vectors, especially mosquitoes. In addition, climate change and human impact on the environment sometimes influence animal behavior, for example by altering 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, in order to intensify corn cultivation, massive land clearing was carried out, notably through the use of herbicides. This change in the environment led to a proliferation of rodents, some of which were carriers of the virus, causing the disease to reach epidemic proportions, particularly among agricultural workers. Thousands of people were infected. A similar situation was also observed in East Asia during the conversion of land for rice cultivation, with the Hantaan virus responsible for "Korean hemorrhagic fever."
Other factors contributing to the emergence of new diseases include socio-economic factors, such as the increase in the transport of goods and people, particularly via intercontinental air travel, and the ever-growing expansion of urban areas. High population densities, which facilitate the rapid transmission of diseases, and water supply difficulties linked to rapid urbanization contribute in particular to the proliferation of mosquitoes that are potential carriers of viruses.
Proof of the importance of these factors can be seen in the fact that during the COVID-19 pandemic, the number of imported "exotic" infections (i.e., those reported after travel) decreased significantly, mainly due to the drastic decline in international air travel. With the clear recovery of such traffic, however, an increase in these cases is expected this year.
This situation could have an impact on the emergence of outbreaks in France, as an infected person arriving on our territory could in turn transmit the disease to others, particularly if the vectors that transmit the disease are present.
Prevention, the first line of defense against "exotic" viruses
In the absence of effective antivirals or vaccines, as in the case of chikungunya or Zika, or when vaccines have certain limitations (as in the case of dengue, for which the only currently approved vaccine has the disadvantage of increasing the risk of hospitalization and severe dengue in people not previously infected with the dengue virus), the only solution is to anticipate the emergence of these pathogens.
The best way to achieve this is to set up appropriate, responsive networks, as close as possible to the field, in order to effectively study the interactions between animals, humans, and their various environments, using an approach known as One Health (human and environmental health).
Since the Covid-19 pandemic, national and international viral disease surveillance networks have been developed. Unfortunately, their capabilities remain well below what is needed to effectively monitor the circulation of high-risk viruses, not only in endemic countries, but also in countries where they are emerging.
The emergence and 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 behaviors, and our daily lives. This situation has made us acutely aware of the importance of monitoring and studying "new" viruses.
But beyond these previously "unknown" viruses, it is also essential to focus on "neglected" viruses, as they are responsible for diseases that are rampant far from our territories.
Propagation outside the African continent, and particularly in Europe, of the Monkeypox, the virus responsible for [monkeypox], is a good illustration of the challenges associated with such surveillance...
Yannick Simonin, Virologist, Associate Professor in Surveillance and Study of Emerging Diseases, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Readthe original article.