These exotic viruses that threaten us

One of the long-term consequences of climate change is the risk of the emergence of new diseases considered "exotic", as they have hitherto been far removed from our territories.

Yannick Simonin, University of Montpellier

In recent years, several "autochthonous" cases of this type of exotic disease have been detected in our country. This term means that the disease was contracted on the territory where it occurred, as opposed to "imported" cases, where the disease was brought back from travel. This is an important distinction, as an autochthonous infection means that the virus is circulating in the country. Imported cases are not without risk, however, as an infected person can, if the vector is present, transmit the disease to others. Every year, there are hundreds of imported cases of arbovirosis in France.

Dengue fever, West Nile fever, tick-borne encephalitis... Which emerging diseases are the most closely monitored in our country, and by whom are they transmitted?

Diseases that often go unnoticed

Most of these emerging diseases are caused by viruses, and more specifically by arboviruses, i.e. viruses transmitted by arthropods ( arthropod-borne viruses). Whether insects (such as mosquitoes or sandflies, which resemble them) or mites (such as ticks), these "vectors" generally feed on blood, and infect their prey during their meals.

Another peculiarity is that it is animals that are initially infected by the viruses. Humans are infected in turn when an arthropod that has become infected by feeding on a domestic or wild animal then attacks it.

These viral diseases do not systematically cause illness. While the tiger mosquito(Aedes albopictus) transmits many human viral diseases, in most cases they are asymptomatic and go unnoticed. However, a significant proportion of people (between 20 and 30%) develop flu-like symptoms (fever, headache, joint and muscle pain), in some cases with an associated rash. Most often benign, they can, in a small proportion of cases, lead to sometimes severe complications.

The main threat: the tiger mosquito

Native to Southeast Asia and the Indian Ocean, the tiger mosquito has gradually colonized mainland France. It is now present in 51 départements, compared with 42 a year earlier. Northern France is no longer spared, and it is now also found in the Paris region. Specialists reckon that it will spread throughout mainland France within the next 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 recorded in the ^18th century on the American continent. Well-known in many parts of the world, including Africa, Asia and Latin America, this disease has recently become established in mainland France, particularly in the south of the country, where some 20 native cases have recently been reported (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 hemorrhagic fever. Potentially fatal, it manifests as 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 French West Indies, this virus is characterized by its propensity to cause persistent joint pain, which can last for several years after the initial infection. Occasional cases of ocular, neurological and cardiac complications have also been reported. To date, some thirty native infections have been recorded in metropolitan France, with the risk of localized epidemics appearing, as was the case in the Montpellier region in 2014 and in the Var region in 2017.

The Zika virus is another recently emerging virus. It hit the headlines 3 years ago, causing a massive epidemic in Latin America, mainly Brazil. The particularity of this virus, which originated in the Zika forest in Uganda, is its ability to cause severe neurological damage in newborns. These are characterized in particular by a sharp reduction in cranial perimeter (microcephaly). This malformation leads to insufficient brain growth, which in turn generates disorders of varying severity: epilepsy, cerebral palsy, learning disabilities, hearing loss, visual problems... A recent study published in the prestigious American journal Nature Medicine shows that three years after infection, children exposed to the Zika virus during pregnancy develop new neurological disorders.

Another feature of the Zika virus is its ability to be transmitted sexually (which is exceptional for arboviruses). In France, only the latter type of transmission has been demonstrated, and no native cases have yet been identified. This is probably because the tiger mosquito is a relatively poor vector for this virus, which is mainly transmitted by another mosquito, Aedes aegypti, which is very present in Latin America in particular, but has not yet established itself in France. It has, however, been identified on the island of Madeira, and its potential presence in Europe is being closely monitored.

Culex is not to be outdone

The tiger mosquito is not the only threat. The "local" mosquito(Culex pipiens), present throughout France, can also carry viruses that are potentially dangerous for humans.

This is particularly true of the West Nile virus. First isolated in the West Nile district of northern Uganda, it is capable of causing severe neurological damage in humans, including encephalitis and meningitis. Like its cousin, the Usutu virus, which is also spreading on our territory, the West Nile virus has as its natural reservoir certain bird species, not all of which have yet been clearly identified.

See also:
West Nile and Usutu viruses: will they take root in France?

In 2018, it spread widely across Western Europe, causing the largest epidemic ever recorded on the continent: 2,083 confirmed autochthonous human cases were reported (resulting in 181 deaths in a dozen countries), including France (with 27 recorded cases). According to the ECDC (European Centre for Disease Prevention and Control), this epidemic has affected more people than all the previous 10 years combined throughout Europe. This year, the West Nile virus has already been detected in 127 patients in Europe, including one case in Fréjus, France.

Ticks have equally exotic viruses

In addition to mosquitoes, other arthropods can transmit exotic viruses. Ticks are a case in point. These mites are better known to the general public for their ability to transmit bacterial diseases such as Lyme disease. However, they are also capable of spreading viral diseases such as TBEV, also known as tick-borne encephalitis. Predominantly found in northern Europe, this disease seems to be spreading steadily.

Even more problematic is the possibility that the Crimean-Congo hemorrhagic fever virus could take hold in mainland France. First described in the Crimea in 1944 and then in the Congo, this disease causes massive hemorrhaging associated with around 30% mortality. The virus was first identified in Europe in 2018, with one case identified in Spain in 2018. Monitoring its spread in Europe will undoubtedly be a major issue in the years to come.

Prevention is better than cure

For most of these emerging viruses, no curative treatment or vaccine is currently available for humans. Today, the most effective way to combat them is probably to attack the vectors that spread them. This is easier said than done, as many factors need to be taken into account.

This is particularly true of changes in environmental conditions brought about by human activity (particularly rising temperatures and variations in precipitation). By affecting the geographical distribution, activity, reproduction rate and survival of arthropods (particularly mosquitoes), they modify disease transmission.

Socio-economic factors are not to be outdone: for example, increased mobility, particularly via intercontinental air travel, favors the spread of infectious agents. Rapid urbanization also seems to be one of the factors accelerating the emergence of these new pathogens. In particular, it favours the proliferation of uncontrolled water storage sites, all of which are breeding grounds for mosquitoes with the potential to transmit viruses.

To reduce the development of mosquito larvae, we recommend emptying all containers of stagnant water (especially after watering). Finally, people at risk are encouraged to use suitable repellents and to wear loose-fitting, covering clothing to limit the risk of being bitten.

Improving surveillance

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

In order to limit the spread of potentially infected mosquito populations, mosquito control is carried out every year in certain European regions. The problem is that insecticide spraying can sometimes generate resistance in mosquitoes. What's more, their massive use in urban areas is not recommended, due to their toxicity.

Fortunately, the arbovirus threat in our country remains sporadic for the time being: strengthening surveillance networks remains the best strategy for combating these new, hard-to-anticipate threats.

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

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