West Nile Virus
West Nile Virus (WNV) is a virus of the genus A recognised way of categorising closely related species of organisms. The genus is the first part of the Latin name of a species; for example, Homo Sapiens (human being) is part of the genus Homo Flavivirus, related to the viruses that cause Japanese encephalitis and yellow fever.
The virus is transmitted by mosquitoes, mainly of the Culex genus (in particular C. pipiens), and persists in nature through a mosquito-bird-mosquito transmission cycle. Mosquitoes get infected when they bite and extract the blood of an infected bird. Then, acting as vectors, they spread the virus to other animals.
Humans, equids (such as horses and donkeys) and other mammals are considered dead-end hosts as they do not further transmit WNV. When they are infected, the virus level in their blood is too low to infect other mosquitoes.
In humans:
- Around 80% of infections have no symptoms.
- 20% of WNV infections in humans may cause West Nile fever.
- Less than 1% affect the nervous system through the West Nile neuroinvasive disease.
In equids:
- Most WNV-infected equids show no clinical signs or only mild influenza-like illness with fever.
- Around 10% of horses displaying influenza-like symptoms also develop neurological signs, such as loss of coordination or seizures.
- In severe cases, horses may feel exhausted, lie down (being recumbent) and then be unable to get back up
In birds:
- WNV can cause several non-specific clinical signs, including weakness, low activity, dehydration and anorexia.
- Neurological signs may also develop and, in severe cases, WNV infection can be fatal.
- Certain bird species A subdivision of the genus, a species is a group of closely related and similar-looking organisms; for example, in the case of Homo sapiens (humans), the second part of the name (sapiens) represents the species, especially raptors and corvids, tend to be more sensitive to WNV infection and frequently develop clinical signs that may lead to fatal outcomes.
Latest
Since July 2025, EFSA and ECDC have been publishing monthly automated online reports, providing updates on human infections and outbreaks in birds and equids. Joint surveillance efforts help public health and veterinary authorities in Europe implement timely and effective control measures.
Milestones
2025
November
EFSA and ECDC published their fifth joint monthly report on WNV. As of 5 November, 1.096 human cases of WNV infection had been reported, with 95 deaths, along with 178 outbreaks in equids and 345 in birds.
While WNV transmission season is coming to an end, sporadic cases might still occur in the coming weeks.
October
EFSA and ECDC published their fourth joint monthly report on WNV. As of 3 October, 989 human cases of WNV infection had been reported, with 63 deaths, along with 127 outbreaks in equids and 217 in birds.
The report shows that Belgium detected WNV outbreaks in birds, marking the first detection of the virus in the country. Since Belgium had never reported locally acquired human cases, these outbreaks point to a likely recent introduction of the virus in the country.
September
EFSA and ECDC’s third joint monthly report updates WNV data at EU level. According to the report, as of 3 September, nine countries in Europe reported 655 human cases of West Nile Virus (WNV) infection, with 38 deaths.
During the same period, 72 outbreaks among equids and 114 outbreaks among birds were reported in eight countries.
August
EFSA and ECDC published their second joint monthly online report on WNV surveillance. As of 6 August, 202 human cases of WNV infection had been reported, along with 16 outbreaks in equids and 20 outbreaks in birds.
July
EFSA and ECDC published their first joint online report on WNV. Initial data for 2025 showed that, as of 2 July, five outbreaks of West Nile Virus (WNV) had been reported in equids and birds in the EU, while no human cases had been reported during the early season.
EFSA's role
As for other vector-borne diseases, EFSA and its Panel on Animal Health and Welfare provide independent scientific advice and scientific assistance on human health and animal health-related aspects of vector-borne zoonotic A term given to diseases and infections that can be transmitted between animals and humans diseases.
EFSA has also developed an interactive disease profile, which provides user-friendly and evidence-based information on WNV. The profile covers topics such as the geographical distribution of the virus, its characteristics and insights from experimental infection studies.
In 2024, EFSA launched the West Nile Virus dashboard, an interactive online tool for visualising data on WNV in animals, which are reported to EFSA in accordance with Directive 2003/99/EC on the monitoring of zoonoses and zoonotic agents. The dashboard is updated annually each December.
The annual One Health Zoonoses Report also covers data on WNV infections in animals (reported under Directive 2003/99/EC), as well as outbreaks in equids and birds (reported to the EU Animal Disease Information System, ADIS).
In the context of the ‘One Health’ approach, EFSA and ECDC conduct joint surveillance of WNV infections in humans, equids and birds, while also assessing the epidemiological situation in Europe.
The two agencies also joined forces on WNV through the VectorNet initiative, which started in May 2014. The initiative includes a common database on the presence and distribution of vectors and pathogens in vectors in Europe and the Mediterranean basin, maintained through a network of medical and veterinary experts and organisations.
EU framework
According to EU legislation, WNV infections in animals and humans must be notified by Member States to either national authorities or the European Commission. EU policy on communicable diseases focuses on surveillance, rapid detection and rapid response.
In accordance with Commission Implementing Regulation (CIR) (EU) 2020/2002, Member States must notify the Commission and the other Member States within 24 hours of confirmation of any primary outbreak of WNV in their territory, while secondary outbreaks should be notified to the Commission at the latest on the first working day of each week, covering the previous week. WNV outbreaks in animals are reported to the EU Animal Diseases Information System (ADIS).
In accordance with Directive 2003/99/EC, Member States must monitor viruses transmitted by arthropods – including WNV – based on the epidemiological situation (List B, Annex I of Dir. 2003/99/EC). Monitoring data on WNV in animals collected under this Directive are reported annually to EFSA.