West Nile virus risk assessment tool


The main objective of this tool is to assist countries in determining the risk of West Nile virus human transmission in their territory. It is beyond the scope of this tool, and the mandate of ECDC, to detail response actions to be taken by Member States or provide clinical guidance. The risk assessment tool uses information gathered through the surveillance mechanisms described to ascertain the level of risk for human transmission of West Nile virus (WNV) within an area.


Risk levels

Seven possible levels of risk (level 0 – level 5) for transmission of WNV to humans are defined.

The risk assessment tool outlines, for each risk level:

  • questions to be answered in order to assess the risk for human transmission of WNV.
  • obligatory and desirable surveillance activities
  • suggested response measures

Definition of terms: ECDC has proposed common terminology for defining areas where arthropod-borne diseases, such as WNV, are being transmitted: 

  •  A risk area is an area where individuals are exposed to the risk (which can be small or large) of being infected with locally-acquired WNV infection. This is a generalised use of the term ’risk area’ to prevent the imprecision linked to this term due to its use to signify a specific level of risk in an area.
  • A predisposed area is a risk area where existing conditions might facilitate the transmission of WNV to humans, but the respective pathogen has not been detected. Conditions favouring transmission are receptivity and/or vulnerability of the area. The receptivity of an area is the presence and/or spread of arthropod vectors and the existence of other ecological and climatic factors favouring WNV transmission to humans. The vulnerability of an area means the proximity to areas where WNV infection is present or a frequent influx of infected birds and/or infective arthropods.
  • An imperilled area is a risk area where the pathogen has been detected in vectors, or transmission of the pathogen to animals has been detected, or the transmission of the pathogen to humans has occurred previously during a defined period.
  • An affected area is a risk area with ongoing transmission of WNV to humans. This means that there has been at least one autochthonous human WNV case as a result of local transmission in the area according to the agreed, standardised and disease-specific case definition. Under exceptional circumstances, a probable case can be used to determine transmission but only in specific and agreed situations when case confirmation cannot be performed within a reasonable time.

Related publications