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 and 2014/110/EU on blood safety, blood establishments in EU/EEA Members States 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 weekly WNV epidemiological updates 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 addition, ECDC publishes maps highlighting areas where WNV outbreaks among equids and/or birds have been reported.

Weekly updates: 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 Member States. The case definition for probable and confirmed WNV human infections is defined in the Commission Implementing Decision (EU) 2018/945.

In line with the West Nile virus preparedness plan 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 Member States and EU neighbouring countries. The map highlight affected areas at NUTS 3 (Nomenclature of Territorial Units for Statistics 3) or GAUL 2 (Global Administrative Unit Layers 2) levels. The affected areas are marked in red. 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.

Weekly updates: 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 of the World Organisation for Animal Health (OIE).

Animal data (including outbreaks among equids and birds) are collected through the Animal Disease Notification System (ADNS) of the European Commission. While the report of equine encephalomyelitis due to WNV is mandatory at the EU level, the report of WNV infections among birds is voluntary. 

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

Following a One Health approach, the following maps aim at highlighting areas where WNV is circulating among humans and animals:

1) Distribution of WNV outbreaks among equids and/or birds;

2) Combined distribution of WNV infections among humans and outbreaks among equids and/or birds.