About the seasonal surveillance of West Nile virus infections

Most West Nile virus (WNV) infections in humans are acquired through mosquito bites, but human infections can also occur via blood transfusion. According to the Commission Directives 2004/33/EC(link is external) and 2014/110/EU(link is external) on blood safety, blood establishments in EU/EEA countries should apply temporary deferral criteria for donors of allogeneic blood donation for “28 days after leaving a risk area of locally acquired West Nile Virus unless an individual Nucleic Acid Test (NAT) is negative”.

To support the competent authorities responsible for blood safety in the implementation of the EU blood safety directives, ECDC publishes a weekly WNV epidemiological update during the WNV transmission season (usually between June and November), including the geographical distribution of human cases in the EU/EEA and EU neighbouring countries. In the monthly epidemiological update, ECDC provides an enhanced analysis of the human and animal cases of WNV infection and a timely assessment of the risk for the EU/EEA.

WNV infections in humans 

The ECDC updates are based on available data reported through the European Surveillance System (TESSy) by the national public health authorities of EU/EEA countries. The case definition for probable and confirmed WNV human infections is defined in the Commission Implementing Decision (EU) 2018/945(link is external).

In line with the West Nile virus preparedness plan(link is external) in Europe and the ECDC risk assessment tool:

• an affected area is defined as an area with at least one locally acquired human WNV infection meeting the EU case definition (Commission Implementing Decision (EU) 2018/945);
• at least the first case detected in an area should be confirmed according to EU case definition (Commission Implementing Decision (EU) 2018/945). Only under exceptional circumstances a probable case might be used to determine an affected area.

Based on this, ECDC publishes a weekly map of the distribution of WNV infections in humans by affected areas in the EU/EEA countries and EU neighbouring countries. The map highlight affected areas at NUTS 3 (Nomenclature of Territorial Units for Statistics 3)(link is external) or GAUL 1 (Global Administrative Unit Layers 1)(link is external) levels. The affected areas are marked in red and newly affected regions (first case(s) reported in this region in 2024) in orange. Areas where no human cases have been reported are coloured in dark grey and countries not covered by the weekly updates are coloured in light grey.

WNV outbreaks among equids and birds

ECDC publishes data on outbreaks of WNV infections among equids and birds, defined as one or more equid/ bird infected with WNV. Equine or bird cases are defined according to the Terrestrial Animal Health Code(link is external) of the World Organisation for Animal Health (OIE).

Animal data (including outbreaks among equids and birds) are collected through the Animal Disease Information System (ADIS)(link is external) of the European Commission. The report of equine encephalomyelitis due to WNV and the report of WNV infections among birds is mandatory at the EU/EEA level. (link is external)

The distribution of human cases covers EU/EEA countries and EU neighbouring countries, whereas outbreaks among equids and birds covers only EU/EEA countries.