2023-09-27 07:17:47
How is the virus transmitted?
West Nile virus is transmitted mainly by mosquitoes of the genus Culex. They carry the virus following biting an infected bird. Mosquitoes can transmit the virus to mammals, including humans and horses. These are epidemiological dead ends. Indeed, mosquitoes cannot become carriers following biting an infected mammal.
What species of birds can carry the virus?
250 species of birds can carry West Nile virus. The groups most affected are passerinesto which belong for example sparrows and corvids, and boys. When the virus first infects an avian population, it can cause high mortality. This was, for example, the case when the virus arrived in the United States in 1999. However, in Europe, infection of birds with West Nile virus is rare and mainly asymptomatic. It does not represent no risk for poultry farms.
What are the symptoms of the illness ?
Horses and human beings are the only mammals that can show symptoms. However, 80% of infections are asymptomatic.
In 20% of cases, horses and humans present a feverish form. It is characterized by fatigue, high fever, headaches and muscle and joint pain. It passes following a few days.
Finally, in less than 1% of casesthey can present a neurological form. It causes encephalitis and meningoencephalitis and can be fatal. Neurological following-effects can persist for several years or even throughout life.
Where does the virus come from and how has its geographic distribution evolved?
West Nile virus was discovered in 1937 in Uganda, in the district following which the virus was named. Since then, its range has expanded. According to the WHO, West Nile virus has a global distribution and is now present in Africa, Europe, the Middle East, North America and western Asia. The first cases of infected horses and humans in France were reported in the 1960s. These were sporadic cases reported around the Mediterranean. They became more common in Mediterranean countries from the end of the 1990s.
In 2022, recent infections of horses with West Nile virus were diagnosed for the first time on the Atlantic coast, in Gironde. In 2023, cases in humans and horses once more occurred outside regions near the Mediterranean. These have been reported in New Aquitaine, more precisely in the departments of Gironde and Charente-Maritime.
What are the causes of the extension of the virus’s distribution area?
The causes of the spread of the virus are multiple. These include international trade, global warming, urbanization and loss of biodiversity. These factors can modify the behavior of mosquitoes and birds carriers of the virus. This is particularly true for migratory birds which can modify their migratory routes.
What are the means of prevention for humans?
There does not exist no vaccine for humans or treatment. Only the symptoms can be treated. To protect yourself from the virus, it is advisable totake the usual measures once morest mosquito bites :
- Use repellents,
- Wear long, loose, light-colored clothing,
- Avoid going out at dusk, when Culex mosquitoes are most active.
Elderly or immunocompromised people are most likely to develop severe forms; they should particularly follow these recommendations.
It is also recommended toeliminate laying places : empty the cups and pots filled with water, cover the rainwater collection containers with a mosquito net and clean the gutters.
What are the means of prevention for horses?
There is a West Nile virus vaccine for horses. It is recommended vaccinate equines in areas where cases have been reported.
In 2023, these were Provence-Alpes-Côte d’Azur, Corsica and Nouvelle-Aquitaine.
How is the virus monitored?
West Nile virus surveillance is coordinated by the Directorate General of Food for animals and by the Directorate General of Health and Public Health France for humans.
ANSES is involved in the confirmation of avian and equine cases. Infections in wildlife are reported by the surveillance network for infectious diseases of birds and wild terrestrial mammals Sagir, which depends on the French Biodiversity Office. Surveillance of cases in horses is carried out in close collaboration with the epidemiological surveillance network for equine pathologies Respe, independent veterinarians and the departmental directorates for population protection (DDPP)
The identification of human cases is managed by the national reference center for arboviruses, that is to say pathogens transmitted by blood-sucking arthropods such as mosquitoes and ticks. This is supported by Inserm (National Institute of Health and Medical Research) and the IRBA (Army Biomedical Research Institute)
What is the role of ANSES?
The Virology unit of the ANSES animal health laboratory is a national and European reference laboratory for West Nile virus. It also carries out research to better understand the epidemiological situation and understand the factors influencing the virulence of the virus.
- The national reference mandate
The National Reference Laboratory (NRL) for West Nile Virus coordinates a network of 7 approved analysis laboratories. If an animal is suspected of being infected with West Nile virus, the laboratories in this network carry out the first serological analyses. These aim to find out whether the animal has actually been infected with the virus. The NRL does the confirmatory analyses. Molecular diagnosis, corresponding to the search for the viral genome in the blood, cerebrospinal fluid or target organs of infection such as the brain, liver or spleen, is carried out only by the NRL. When a case is proven, the LNR alerts the authorities, in particular the DGAL and the departmental directorate for the protection of the populations concerned.
The NRL also has the role of determining the reliability of the analysis methods used by the network’s laboratories. It regularly checks their detection capabilities by carrying out inter-laboratory proficiency tests using reference material that it provides.
- The European reference mandate
The Anses is European Union reference laboratory for equine diseases since 2008. It includes a section on mosquito-transmitted equine viruses. In addition to West Nile virus, this set of viruses includes Japanese equine encephalitis virus, Eastern equine encephalitis virus, Western equine encephalitis virus and Venezuelan equine encephalitis virus. West Nile virus is most common in Europe.
The role of the LRUE is to harmonize detection practices between the NRLs of the European Union countries and to carry out inter-laboratory tests with these laboratories. He can carry out analysis confirmations on request.
- Research to better understand the virus
ANSES conducts epidemiological investigations to have a global vision of horse infections with the virus, beyond symptomatic cases. These studies are carried out in France and internationally.
The Agency also carries out work to better understand the factors that determine the virulence of the virus. For example, she studies the molecular and genetic determinants which explain why certain strains are more virulent than others depending on the species infected.
Another subject of study, the effect of coinfection by a virus close to the West Nile virus, which is carried by the same species of birds and transmitted by the same mosquitoes: le virus d’Usutu. This co-infection, which has been reported in migratory birds and in a zoo in Germany but not in France, might influence the circulation of viruses. The research also aims to find out whether the infections were simultaneous or successive.
Comparison of the two viruses might also give avenues for the treatment of West Nile virus. Indeed, birds show symptoms when infected with the Usutu virus, but there are very few cases in horses and humans. The species with symptoms therefore seem reversed compared to West Nile virus. Understanding the cause might make it possible to better fight once morest it.
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