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West Nile virus risk assessment tool

Tools for public health

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.

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West Nile virus risk assessment tool - EN - [PDF-294.14 KB]

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.

Risk level

Description and triggers

Questions to be addressed

Surveillance activities

Suggested public health actions and interventions

0
  • Free area
  • No historical circulation of WNV

Is there any risk of WNV transmission in this area?

Obligatory:

  • Human: passive surveillance
  • Veterinary: passive surveillance for horses
  • SoHO: standard haemovigilance, biovigilance and post-transfusion/transplantation surveillance (not specific for WNV)

Health sector

  • Increase awareness amongst healthcare professionals about WNV so as it will be considered in the differential diagnosis of travellers returning from affected areas.
  • Education of travellers to affected areas on how to reduce the risk.
  • Ensure there are laboratory capabilities within the country for diagnosis.
  • Ensure SoHo donation authorities have implemented measures to prevent transmission through travellers returning from affected areas (see WNV and blood safety introduction to a preparedness plan).

1
  • Predisposed areas where the ecological conditions are suitable for WNV circulation.
  • No historical circulation of WNV is known.
  • The probability of a human outbreak is unknown but likely to be low.
  • Is there any risk of WNV transmission in this area in the season?
  • Would we detect WNV circulation if it occurred?

Obligatory:

  • Human: passive surveillance
  • Veterinary: passive surveillance for horses;
  • SoHO: standard haemovigilance- biovigilance and post-transfusion/transplantation surveillance (not specific for WNV)



Desirable:

  • Assess the risk of WNV transmission in the area.

Multi-sectoral collaboration and coordination

  • Consider drafting WNV preparedness plan

Health sector

  • Response as level above
  • Ensure there are laboratory capabilities within the country for diagnosis of WNV

Public communication

  • No specific action

Vector management response

  • No specific action.

2
  • Imperilled areas where the ecological conditions are suitable for WNV circulation
  • Past evidence of WNV circulation
  • The probability of a human outbreak is unknown.
  • To what extent has the virus infected local animal populations?
  • What is the prevalence of infection in animal populations?
  • Are passive/active surveillance systems (including laboratory diagnostics) in place to be able to identify horse and human infections with WNV?

Obligatory:

  • As above

AND:

  • Ensure timely detection and reporting of human cases by passive surveillance;
  • Develop and implement a surveillance plan including mosquito, bird and equid surveillance enabling the detection of WNV circulation.

Multi-sectoral collaboration and coordination

  • Develop a WNV preparedness plan, including surveillance activities and an integrated vector control plan.
  • Allocate resources necessary to enable emergency response (i.e. vector control, communication plan).
  • Establish close and regular exchange of information between all sectors as part of the WNV preparedness plan.

Health sector

  • Response as level above

    AND:
  • to assure an appropriate level of awareness among health care professionals;
  • to define roles and responsibilities, but also training courses, curricula, information and management recommendations;
  • availability of national guidelines for clinical management.

Public communication

  • Conduct public information campaigns during the mosquito season to strengthen use of personal protection measures against mosquito bites.

Vector management response

  • As part of the WNV preparedness plan: consider preparing vector control activities if entomological indicators suggest the need.
  • Allocate resources necessary to enable emergency response (i.e. vector control).
  • Implement larval control as part of the integrated vector control in the event of there having been WNV circulation in the previous year.

3a
  • Imperilled areas
  • Current surveillance findings (i.e. mosquito or bird screening) indicating WNV epizootic activity in the area in the second part of the season (i.e. August-September-October)
  • The probability of a human outbreak is low.
  • What is the geographic extension of the area where WNV is circulating?
  • What is the seasonal dynamic of WNV circulation?
  • What is the real risk for people?
 Obligatory:
  • As above



AND:

 

  • Ensure timely detection and reporting of human cases by passive surveillance.
  • As part of the surveillance plan: ensure the proper continuation of the surveillance activities.
 Multi-sectoral collaboration and coordination
  • Response as level above
  • Health sector
  • Response as level above

Public communication

  • Consider public information campaigns to support vector control response

Vector management response

  • Response as level above;

AND:

  • Implement public education programs focused on risk potential and personal protection, and emphasising residential source reduction.
  • Vector control focuses on larval control.

3b

Imperilled areas

  • Current surveillance findings (i.e. mosquito or bird screening) indicating WNV epizootic activity in the area, in the first part of the season (May-June-July)
  • The probability of a human outbreak is low to moderate.

Idem as level above

Obligatory:

  • As above

AND:

  • Human: active/enhanced surveillance in the area with confirmed virus circulation.
  • Ensure timely reporting of human cases by passive/active surveillance.
  • As part of the surveillance plan: increase the mosquito and bird surveillance activities.

Multi-sectoral collaboration and coordination

  • Response as level above

Health sector

  • Response as level above

Public communication

  • Response as level above
  • AND:
  • Implement public information on personal protection and source reduction

Vector management response



AND:

 

  • Response as level above
  • Increase effort for public information on personal protection and source reduction continued larval control
  • If surveillance indicates virus circulation is increasing initiate ground adult mosquito control in areas at high risk for humans or in hot spot sites (if known)
4

Imperilled areas

  • WNV-specific IgM detected in local non-vaccinated horse(s) or WNV isolated from local horse.
  • The probability of a human outbreak is high.
 What is the geographic extension of the area where WNV is being transmitted to horses (as humans are likely to follow soon)?

Obligatory:

  • As above

AND:

  • Human: active/enhanced surveillance in the area with confirmed virus circulation
  • Ensure timely reporting of human cases by passive/active surveillance
  • As part of the surveillance plan: increase the mosquito, bird and horse surveillance activities.

Multi-sectoral collaboration and coordination

  • Create/establish multi-sectoral outbreak response team

Health sector

  • Response as level above
  • Increase awareness among health professionals

Public communication

  • Conduct public information campaigns to strengthen use of personal protection measures against mosquito bites and source reduction.

Vector management response

  • Response as level above
  • If surveillance indicates virus circulation is increasing, initiate ground adult control in high-risk areas for humans or in hot spot sites (if known).

5

Affected area

  • At least one human case detected (i.e. probable or confirmed human case according to EU case definition).
  • Outbreak ongoing
  • What is the geographic extension of the ’affected area’ where WNV is being transmitted to humans?
  • What is the risk of increasing numbers of human cases?

 

Obligatory:

  • As above

AND:

  • Human: active/enhanced surveillance in the area with confirmed virus circulation.
  • Ensure timely reporting of human cases by passive/active surveillance.
  • As part of the surveillance plan: continue the surveillance activities.

Multi-sectoral collaboration and coordination

  • Regular meetings of multi-sectoral outbreak emergency response team
  • Establishment of geographical boundaries of affected area.

Health sector

  • Response as level above
  • Increase awareness among health professionals
  • Safety of SoHO: implement EU directive for blood, tissues, cells and organ safety in affected area, quantitative risk assessment of transfusions/transplantations, implement deferral policy, +/- NAT screening, +/- inactivation techniques, evaluate impact of measures implemented on blood supplies.

Public communication

  • Response as level above;
  • AND:
  • Increase public information campaigns in order to reach most people at risk.

Vector management response

  • Response as level above;
  • AND:
  • Intensify ground adult mosquito control with multiple applications in areas of high risk of human cases.
  • Enhance risk communication.
  • Monitor efficacy of spraying on target mosquito populations.

If a large area is involved coordinate the programme through an emergency unit with all authorities involved.

 

Related publications

Risk assessment

West Nile virus risk assessment tool

​West Nile virus (WNV) is an emerging pathogen whose ecology and epidemiology extend across multiple interfaces. As the epidemiology and transmission cycle of WNV is complex, assessing the risk of WNV being transmitted to humans is not always straightforward. Therefore, the WNV risk assessment tool has been developed to provide operational guidance in support of the risk assessment process.

west-nile-virus-risk-assessment-tool.pdf - EN - [PDF-610.44 KB]