Dengue, "West Nile", chikungunya, zika … These exotic viruses that threaten us


In recent years, several "autochthonous" cases of this type of exotic diseases have been detected in France. Their mode of transmission is closely monitored.

Yannick Simonin, author of this article, is a virologist and lecturer in surveillance and study of emerging diseases at the University of Montpellier. The original version of this article was published on The Conversation website, of which franceinfo is a partner.

One of the long-term consequences of the upheaval of our climate is the risk of emergence of new diseases considered as "exotic", because so far far from our territories.

In recent years, several "indigenous" cases of this type of exotic diseases have been detected in our country. This term means that the disease was contracted in the territory where it was declared, unlike the "imported" cases, where the disease was reported from travel. The distinction is important because an indigenous infection means that the virus circulates on the territory. Imported cases, however, are not without risk, because an infected person can, if the vector is present, transmit the disease to others. However, every year hundreds of imported cases of arboviruses are listed in France.

Dengue, West Nile, tick-borne encephalitis … What are the most watched emerging diseases in our country, and by whom are they transmitted?

Most of these emerging diseases are caused by viruses, specifically arboviruses, that is, viruses transmitted by arthropods. arthropod-terminal virus). Whether they are insects (such as mosquitoes or sandflies, which resemble them) or mites (such as ticks), these "vectors" usually feed on blood, and infect their prey during their meals.

Another special feature is that animals are initially infected by viruses. In turn, humans become infected when an arthropod that has become infected by feeding on a domestic or wild animal then attacks it.

These viral diseases do not always make people sick. So, if the tiger mosquito (Aedes albopictus) transmits numerous human viral diseases, they are in most cases asymptomatic and go unnoticed. However, a significant proportion of people (between 20 and 30%) develop symptoms that can be likened to influenza (higher or lower fever, headache, joint and muscle pain) with, in some cases, a rash associated skin. Most often benign, they can, in a small proportion of cases, lead to sometimes severe complications.

Ministry of Solidarity and Health

Native to Southeast Asia and the Indian Ocean, the tiger mosquito has gradually colonized metropolitan France. It is now present in 51 departments, against 42 a year earlier. The north of France is no longer spared, and it is now found in the Paris region. Experts believe that it should extend over the entire city in a few years …

Among the main emerging viruses transmitted by the tiger mosquito is the Dengue virus, a disease of African origin whose first cases were listed in the XVIIIe century on the American continent. Well known in many parts of the globe, such as Africa, Asia and Latin America, this disease has recently become established in metropolitan France, particularly in the South, where some 20 indigenous cases have recently been identified. (including 4 in 2018).

The main problem associated with dengue fever is the risk of developing what is known as severe dengue fever or dengue haemorrhagic fever. Potentially fatal, it is manifested in particular by respiratory distress associated with multiple hemorrhages. Fortunately, this form only affects a small percentage of those infected (around 1%).

Another virus transmitted by the tiger mosquito is the Chikungunya virus, isolated in the early 1950s on the Makonde Plateau in Tanzania. Well known in the West Indies, the particularity of this virus is its propensity to cause persistent joint pain, which may last several years after the initial infection. Some occasional cases of ocular, neurological and cardiac complications have also been reported. So far, about thirty autochthonous infections have been identified in the metropolis, with the risk of occurrence of localized epidemics, as was the case in the Montpellier region in 2014 and in the Var in 2017.

Zika virus is another emerging virus that has emerged recently. It hit the headlines three years ago, causing a massive epidemic in Latin America, mainly in Brazil. The peculiarity of this virus, originating from the Zika forest in Uganda, is its ability to cause serious neurological damage in the newborn. These are particularly characterized by the sharp reduction of the cranial perimeter (microcephaly). This malformation leads to insufficient growth of the brain, which in turn generates more or less severe disorders depending on the severity of the injury: epilepsy, cerebral palsy, learning disorders, hearing loss, visual problems … A study recently published in the prestigious American magazine Nature Medicine shows that three years after infection, children exposed to the Zika virus during pregnancy have new neurological disorders that appear.

Another special feature of the Zika virus is its ability to be transmitted sexually (which is exceptional for arboviruses). In France, only this last type of transmission has been highlighted, and no autochthonous case has been identified for the moment. Probably because the tiger mosquito is a pretty bad vector for this virus, which is transmitted mainly by another mosquito, Aedes aegypti, very present in Latin America in particular, but which is not yet established on our territory. It has nevertheless been identified on the island of Madeira and its potential location in Europe is closely monitored.

Ministry of Solidarity and Health

The tiger mosquito is not the only threat present on the territory. The "local" mosquito (Culex pipiens), present on the whole of the metropolis, is also carrier of viruses potentially dangerous for the human being.

This is mainly the case of the West Nile virus (West Nile virus). Isolated for the first time in the West Nile district of northern Uganda, it is capable of causing severe neurological damage in humans, such as encephalitis or meningitis. Like its cousin Usutu virus, also expanding on our territory, the West Nile virus has as natural reservoir some bird species, which are not yet all clearly identified.

In 2018, it spread widely in Western Europe, causing the largest epidemic ever recorded on the continent: 2,083 confirmed indigenous human cases were reported, resulting in 181 deaths in a dozen countries, including France (with 27 cases listed). This epidemic has totaled more affected individuals than the total of the previous 10 years across Europe according to the European Center for Disease Prevention and Control (ECDC). This year, the presence of West Nile virus has already been demonstrated in 127 patients in Europe, including a case identified in France, Fréjus.

Beyond mosquitoes, other arthropods can transmit exotic viruses. This is particularly the case with ticks. These mites are best known to the general public for their ability to transmit bacterial diseases such as Lyme disease. However, they are capable of spreading viral diseases such as TBEV, also known as tick-borne encephalitis. Present mainly in the north of Europe, this affection seems in continuous extension.

Even more problematic is the possibility of seeing the Crimean Congo haemorrhagic fever virus settle in metropolitan France. Described for the first time in Crimea in 1944 then in Congo, this disease causes massive haemorrhages associated with about 30% mortality. This virus has been identified for the first time in Europe in 2018, with a case identified in Spain in 2018. Monitoring its spread in Europe will undoubtedly be an important issue in the years to come.

For most of these emerging viruses, no cure or vaccine is currently available to humans. Today, the most effective way to fight against them is probably to attack the vectors that propagate them. Easier said than done, because many factors are to be taken into account.

This is particularly the case for changes in environmental conditions caused by human activity (in particular the increase in temperatures and variations in precipitation). By affecting the distribution, activity, reproductive rate and survival of arthropods (including mosquitoes), they alter the transmission of disease.

Socio-economic factors are not left out: for example, the increase in mobility, particularly via intercontinental air transport, promotes the spread of infectious agents. Galloping urbanization also seems to be one of the accelerating factors in the emergence of these new pathogens. Indeed, it promotes in particular the multiplication of uncontrolled water storage, which are as many breeding places for mosquitoes potentially vectors of viruses.

To reduce the development of mosquito larvae, it is recommended to empty all containers with standing water (especially after watering). Finally, exposed populations are encouraged to use suitable repellents and to wear loose, covering clothing to limit the risk of stinging.

Faced with the emergence of these exotic diseases, most of the countries concerned, including France, have set up active surveillance networks. They bring together experts with different skills: veterinarians, clinicians, entomologists, researchers … This is the case, for example, of the SAGIR epidemiological surveillance network, which examines the circulation of pathogens in wild birds and terrestrial mammals.

In order to limit the spread of populations of potentially infected mosquitoes, mosquito control takes place every year in certain European regions. This is particularly the case in the Camargue, one of the French areas most exposed to the threats of arboviruses. This action is controlled by the Interdepartmental Agreement on Mosquito Control. Problem: Insecticide sprays generate a lot of resistance in mosquitoes. In addition, their massive use in urban areas is not recommended because of their toxicity.

Fortunately, in our country the threat of arbovirus remains sporadic for now: the strengthening of surveillance networks is currently the best strategy to fight against these new threats difficult to anticipate.The Conversation

Yannick Simonin, Virologist, assistant professor in surveillance and study of emerging diseases, University of Montpellier

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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