Second purebred case of Usutu virus detected in France, here's what we know - Edition du soir Ouest-France - 11/22/2022
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Second purebred case of Usutu virus detected in France, here’s what we know – Edition du soir Ouest-France – 11/22/2022

Written by Yannick Simonin, Virologist, Lecturer in Surveillance and Study of Emerging Diseases, University of Montpellier

The first case of Usutu virus had already been identified in France in 2016. A new case was confirmed in November 2022, in Landes. If Osoto is not one of the most severe mosquito-borne viruses that the French have to learn to deal with, it nonetheless deserves the attention of scientists and health authorities.

On November 10, 2022, a case of the Usutu virus was confirmed in the Landes arrondissement of New Aquitaine. This is the first case of arthropodia (a virus transmitted by phagocytic insects – in other words, they feed on blood – such as mosquitoes) identified in this region. It is also only the second case of Usutu virus infection listed in France. After the onset of flu-like symptoms (fever, headache, body aches), the patient fully recovers from infection.

Along with chikungunya virus, dengue virus, Zika virus and West Nile virus, Usoto is now one of the mosquito-borne viruses that has caused at least one case of infection in France – that is, in a person who has not traveled to a foreign country. If Usutu is not one of the most terrifying of these viruses that the French have to learn to deal with, it nonetheless deserves the attention of scientists and health authorities.

A virus originally infected in 2016

The first case of the Usutu virus was already identified in France in 2016, but studies revealed it only two years later. On November 10, 2016, a 39-year-old man was hospitalized for three days in neurology at Montpellier University Hospital due to sudden paralysis of one half of his face. The patient regained all of his faculties within a few weeks, without sequelae. Subsequent analyzes showed that he was infected by Usutu.

It was the work of our team of biologists from the University of Montpellier, Inserm and the University Hospital of Montpellier that made it possible to understand the origin of the disorders presented by this patient. We then analyzed 666 cerebrospinal fluid samples taken (and then frozen) from patients hospitalized in 2016 in Montpellier and Nîmes. Only one detection of the existence of the Usutu virus: his virus.

The most likely scenario is that this man, like the patient identified in Landes, was infected by a mosquito, after biting a bird, a reservoir for this virus. It is transmitted to humans mainly by mosquitoes ColexThis virus, common in France, has been actively circulating in our country since at least 2015, according to a study by ANSES (National Agency for Food Safety, Environment and Occupational Health).

From Swaziland to the capital, France

The Usutu virus was unknown until recently, but it has recently attracted the attention of the scientific community due to its high prevalence in Europe. This arbovirus belongs to a family flaviviride and sex flavivirusconsisting of more than 70 members.

Among these, we find some of the most dangerous arthropod-borne viruses for humans, such as Zika virus, dengue, yellow fever or West Nile fever. (West Nile virus).

Representation of the Usoto virus. (Source: Clé et al. Medicine and Sciences in press)

Usoto is named after the river of the same name, which is located in Swaziland, a small African country that shares a border with South Africa. It was first identified there in 1959.

Little is known about Osoto target cells. However, our team recently described his ability as others flavivirusto infect in the laboratory (in vitro cultures) not only cells of the nervous system, but also the nervous system of rodents.

. (Source: Salinas et al., PLoS Negl Trop Dis., 5 September 2017)

Birds as hosts, and mosquitoes as vectors

The natural cycle of Usutu transmission is a formative cycle, i.e. localized in a particular area. They mainly include bellbirds (such as the blackbird or magpie) and strigiform birds (such as the great gray owl) as “amplified” hosts, i.e. allowing active reproduction of the virus.

It is flower-loving mosquitoes (bird-biting) that act as vectors during transmission to humans. Various studies have shown the involvement of several species of mosquitoes in maintaining the Usutu cycle within birds, i.e. birds occupying the same niche.

. (Source: Clé et al. Medicine and Sciences)

Thus, the virus was isolated from mosquitoes Aedes albopictus (commonly known as tiger mosquito), Aedes caspiuis, Anopheles maculipennis, Culex quinquefasciatus, Culex perexiguus, Culex perfuscus, Coquillettidia aurites, Mansonia Africana And the Colex Babies. These different species are bird lovers, but they also bite humans.

Mosquitoes also transmit the virus to horses. But this animal, like humans, is considered an accidental host: these species are susceptible to Usutu, but are considered epidemic “dead end”—that is, they cannot transmit the virus to their congeners.

Major death of birds

Usutu has been demonstrated in many species of birds, and several migratory species are thought to be responsible for introducing this virus to Europe. Others prefer to be responsible for its spread. Among the species vulnerable to infection by the Usutu is the common blackbird (Turdus merula), which has the highest known mortality rate.

In birds infected with Usutu, the infection leads to central nervous disorders. It manifests as prostration, confusion, motor incoordination, and weight loss. Autopsy often reveals inflammation of the hepatitis (enlarged liver) and spleen (enlarged spleen).

Lesions of the heart, liver, kidneys, spleen and brain of affected birds have also been reported. Usutu’s virulence makes it highly pathogenic to birds, in particular due to its overgrowth of a large number of tissues and organs. Osoto is the cause of massive bird deaths in different parts of Europe.

A virus was first discovered in Europe in 2001

The Yosoto virus was first detected in Europe in 2001, in Austria, on dead birds. Then it was reported in many European countries, in mosquitoes or birds.

The increase, in 2015, in the mortality rate of common blackbird birds in the departments of Haut-Rhin and Rhône, has alerted the authorities. Then investigations by ANSES and the National Bureau of Hunting and Wildlife (ONCFS) made it possible to identify the Usutu virus.

During the summer of 2016, a major Osoto epidemic affecting birds was recorded in Europe, with widespread activity of the virus in Belgium, Germany, France and for the first time in the Netherlands. This phenomenon reveals not only the continuing geographical spread of Usoto but also the emergence of new ecological niches.

Moreover, it has since been shown that Osoto virus spreads in mosquitoes. Colex Babies Camargue since at least 2015.

The recurrence of Usutu infection in different European countries indicates a continuing cycle of transmission in the affected areas. This observation could be explained either by the presence of infected wintering mosquitoes (the cold slows the organism down and it does not move until spring), or by the redeployment of the virus, by migratory birds from Africa.

Symptoms should be better described

The risk of transmission of Usutu virus from animals to humans was initially described in Africa. The first human case was reported in the Central African Republic in the 1980s, and the second in Burkina Faso in 2004. Symptoms in these two cases were moderate, including a rash and minor liver damage.

In Europe, there have been just under 80 cases of acute human infection with Usutu to date, mainly in Italy. In addition, more than 100 individuals were included who had antibodies against this virus, indicating that these individuals had been exposed to the pathogen.

Human infection is likely to be mostly asymptomatic, or with mild clinical expression. However, neurological complications such as encephalitis (inflammation of the brain, a part of the brain located in the cranial box) or meningoencephalitis (inflammation of the brain and its meninges and surrounding membranes) have been described, totaling about thirty cases in Europe.

Note that most of these neurological disorders are associated with a single Osoto strain called Europe 2 (out of 8 existing strains). Particular attention must be paid to the specific risks that these proportions can cause, which have not yet been identified in France.

Our team’s description of the atypical presence of facial paralysis, which was seen in the first French case, indicates that the symptomatic extent of Usutu virus infection is not fully known.

Virus extends its range

The recent history of outbreaks involving other lobeviruses calls for the scientific community to exercise extreme caution with regard to Usutu virus. Its range now extends to a large number of European countries. Epidemics of bird deaths due to this virus occur, and genetically very different strains are circulating at the same time. Lots of signs to alert.

Although very rare, some studies of serodispersal (the presence of antibodies in the blood directed against the virus) support the hypothesis that humans are more at risk of infection by Usutu than one might imagine.

Knowledge about the pathophysiology of this emerging virus is currently very unclear. In particular, ongoing work aims to better understand its biology and mechanisms associated with neuronal damage. In this context, research work accompanied by surveillance and prevention measures should be carried out in France, particularly in the regions most at risk.

The original version of this article was published on Conversation.

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