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Introduction and General Information

West Nile virus and infection caused by the virus must be addressed regionally. In the USA, surveillance, decision making and any actions taken to reduce the risk of WNV infection are taken primarily at State level, with action as required at a county level.

This page is designed to support decision makers in determining the appropriate actions to be taken for the control of West Nile virus, to ensure that all relevant factors are considered, and to maximise the effectiveness of actions taken.

Published Guidelines linked in Wildpro

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Is West Nile Virus Present?

It is important to understand that there is no single universally applicable arbovirus surveillance system and that "in any given jurisdiction, surveillance systems should be tailored according to the probability of arbovirus activity and available resources." (D147)

Determining that WN virus is present in a given geographical region (country, state, county or smaller area) depends on the detection of the virus in at least one bird, mosquito, horse, human or other animal in that region.

  • A strong indication of the presence of WN virus in an area can also be given by the detection of antibodies to the virus in a resident of the area who has not travelled outside the area. This may be a human, horse, sentinel bird, captive bird, resident (non-migratory) free-living adult bird, hatching-year bird (hatched in the area) or other resident animal. 
  • In the USA in 1999-2001 the presence of dead crows (Corvus brachyrhynchos - American Crow) was an effective indicator of WN virus activity in an area; data from 2002 found that while this continued to be the case in northern regions in some southern regions the presence of dead blue jays (Cyanocitta cristata - Blue jay) was a more sensitive indicator. (D147)

Information on the species which have been identified as definitive hosts and vectors for WN virus is provided in:

Detection of the presence of WN virus requires active and/or passive surveillance measures.

Further information on virus surveillance in mosquitoes, birds, horses and humans is provided in:

Associated techniques linked from Wildpro

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Is West Nile Virus Infection a Problem?

Whether or not WN virus is a problem in a given location depends on:
  1. The actual risk of illness posed by the virus in that area or the occurrence of illness (number and severity of cases of illness in humans / horses / birds); and
  2. The public attitude towards risk or incidence of infection (public perception of whether there is a problem).

The risk of human illness can be quantified by a range of surveillance tools including:

  • Geographical range of virus activity based on the number and distribution of dead birds.
    • Data from the last few years of WN virus activity in the USA has shown that counties with a high density of dead crow reports, and those which reported WN virus-infected dead birds early in the transmission season were more likely to report WNV infection cases in humans subsequently. (D147)
  • Vector-associated factors: size of mosquito population, species composition of mosquito population, physiological age of mosquitoes, rate of infection of mosquitoes (MIR - Minimum infection rate).
  • Climatic factors: these affect mosquito survival and development and the rate of extrinsic virus development (time taken from ingestion of virus by a mosquito in a blood meal until the virus is present in the salivary glands of the mosquito and can be inoculated into a new host at a subsequent blood meal).
  • Demographic factors: the number of elderly people in the population, the quality of housing in relation to the presence of mosquitoes in the home, and whether it is likely that people will spend significant amounts of time in outside areas where they are likely to be exposed to mosquitoes.

What action should be taken (see section below) depends on whether and to what degree WN virus is considered to be a problem.

Further information on surveillance in mosquitoes, birds, horses and humans is provided in the pages listed below.

Associated techniques linked from Wildpro

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What Action should be taken against West Nile Virus?

The principle goal of actions taken against West Nile virus is "to minimize the health impact of the WN virus in humans, as well as in domestic and zoo animals." (D67). It is important to recognise that prevention and control measures are not able to prevent all infection of humans by WN virus. This is a natural consequence of the still limited understanding of WN virus ecology and epidemiology in the USA, the fact that arbovirus encephalitides are by their nature sporadic in occurrence, and the inherent limitations of the various control options available. Recommendations for actions against WN virus should be interpreted in the light of practices which are already established for other arboviral encephalitis control programs. (D67)

In the USA it has been suggested that all states should prepare for WN virus activity since data to date suggest that there are no obvious barriers to spread of this virus throughout the USA. (D147)

"Measures of the intensity of WNV epizootic in an area should be considered when determining the level of the public health response. Accumulating data analyses indicate that intensity of epizootic WNV activity as measured by avian mortality and mosquito infection rates are good indicators of subsequently increased human infection risk." (D147)

Actions which may be taken against West Nile virus include surveillance, mosquito management, education of the public and education of health professionals. Vaccination is an additional option available for equines but not at this time for humans [January 2004]. 

Depending on the determined level of risk for a given area at a given time, the actions to be taken may involve any or all of the following:

  • Development of a response plan for a given area.
  • Education of health and other professionals required for surveillance and control. 

Further information is provided in:

  • Surveillance for WN virus, including mosquito surveillance, surveillance of avian mortality, equine encephalitis and human encephalitis/meningitis surveillance. The "definition" given by CDC for each risk category below shows clearly that in order for decisions to be taken regarding an appropriate response level, surveillance data is needed. 

Further information is provided in:

  • Public education and community outreach programs. 

Further information is provided in: 

  • Larval mosquito control (habitat management, use of larvicides as appropriate).

Further information is provided in:

Further information is provided in:

  • Vaccination.

Further information is provided in: 

It is also necessary to make decisions in the light of local and regional characteristics; it is not possible to give an absolute prescription of the level of mosquito infection or the rate of crow deaths at which specific actions must be taken. (D67)

  • It is important to allow a degree of flexibility when implementing the guidelines, in order to ensure that the response is appropriate for the local scenario and takes into account any new information which may become available from ongoing surveillance and research. (D67, D147)
  • No set of recommendations will cover all possible scenarios. (D147)
  • Risk assessment is an important part of decision making for WN virus control. Decision making regarding the actions to be taken in a given area must take into account and balance the risks of WNV infection and the risks which may be associated with control actions such as the use of pesticides for control of mosquitoes.

The table below, taken directly from CDC Epidemic/Epizootic West Nile Virus in the United States: Guidelines for Surveillance, Prevention and Control 3rd Revision (D147), provides suggestions regarding the responses which may be appropriate for different levels of risk. [Text copied directly]

Table 1. Suggested Guidelines for Phased Response to West Nile Virus Surveillance Data


of human


Recommended response*




Off-season; adult vectors inactive; climate unsuitable.

Develop WNV response plan. Secure surveillance and control resources necessary to enable emergency response. Initiate community outreach and public education programs. Conduct audience research to develop/ target education & community involvement. Contact community partners

1 Remote

Spring, summer, or fall; areas anticipating WNV epizootic based on previous WNV activity in the region; no current surveillance findings indicating WNV epizootic activity in the area.

Response as in category 0, plus: conduct entomologic survey (inventory and map mosquito populations, monitor larval and adult mosquito density), Initiate source reduction; use larvicides at specific sources identified by entomologic survey and targeted at likely amplifying and bridge vector species; Maintain avian mortality, vector and virus surveillance; Expand community outreach and public education programs focused on risk potential and personal protection, and emphasizing residential source reduction; Maintain surveillance (avian mortality, mosquito density /IR [MIR], human encephalitis/meningitis and equine illness).


2 Low

Spring, summer, or fall; areas with limited or sporadic WNV epizootic activity in birds and/or mosquitoes. No positives prior to August.

Response as in category 1, plus: increase larval control, source reduction, and public education emphasizing personal protection measures, particularly among the elderly. Enhance human surveillance and activities to further quantify epizootic activity (e.g., mosquito trapping and testing). Implement adulticide applications if vector populations exceed locally established threshold levels, emphasizing areas where surveillance indicates potential for human risk to increase.

3 Moderate

Spring, summer, or fall; areas with initial confirmation of epizootic WNV in birds before August; a horse and/or a human case, or sustained WNV activity in birds and/or mosquitoes.

Response as in category 2, plus: intensify adult mosquito control in areas where surveillance indicates human risk, Initiate adult mosquito control if not already in progress, Initiate visible activities in community to increase attention to WNV transmission risk (speaker, social marketing efforts, community mobilization for source reduction, etc.), Work with collaborators to reduce risks to elderly (e.g., screen repair).

4 High

Spring, summer, or fall; quantitative measures indicating WNV epizootic activity at a level suggesting high risk of human infection (e.g., high dead bird densities in early summer, sustained high mosquito infection rates, multiple positive mosquito species, horse or mammal cases indicating escalating epizootic transmission, or a human case and high levels of epizootic activity). Areas with early season positive surveillance indicators where WN epidemic activity has occurred in the past.

Response as in category 3, plus: Expand public information program to include TV, radio, and newspapers (use of repellents, personal protection, continued source reduction, risk communication about adult mosquito control), Increase visibility of public messages, engage key local partners (e.g., government officials, religious leaders) to speak about WNV; intensify and expand active surveillance for human cases; intensify adult mosquito control program, repeating applications in areas of high risk or human cases. 

Outbreak in progress

Multiple confirmed cases in humans; conditions favoring continued transmission to humans (e.g., persistent high infection rate in mosquitoes, continued avian mortality due to WNV)

Response as in category 4, plus: Intensify emergency adult mosquito control program repeating applications as necessary to achieve adequate control. Enhance risk communication about adult mosquito control. Monitor efficacy of spraying on target mosquito populations. If outbreak is widespread and covers multiple jurisdictions, consider a coordinated widespread aerial adulticide application; emphasize urgency of personal protection through community leaders and media, and emphasize use of repellent at visible public events.

* Local and regional characteristics may alter the risk level at which specific actions must be taken.

(D147 - CDC Epidemic/Epizootic West Nile Virus in the United States: Guidelines for Surveillance, Prevention and Control 3rd Revision)

Additional factors may need to be taken into consideration in determining actions to be taken (see section below).

Associated techniques linked from Wildpro

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Additional Factors which may affect the actions taken against West Nile Virus

Additional factors which may need to be considered when deciding on actions to be taken include:
  • Current weather and predicted climatic anomalies.
  • The quality and availability of surveillance data, including its timeliness;
    • Development of an ideal surveillance program may take several years.
  • The feasibility of the planned actions, acknowledging restraints imposed by available budgets and infrastructure;
    • Actions of all types require appropriate personnel, laboratories, equipment etc.
  • Public concerns regarding proposed actions for prevention and control.
  • Expected future duration of WNV transmission (surveillance events occurring earlier in the transmission season will generally have greater significance)
  • Mosquito control activities which are already ongoing for control of nuisance mosquitoes and/or vectors of other viruses causing arboviral encephalitides.

(D67, D73, D147)

Associated techniques linked from Wildpro

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Authors & Referees

Authors Debra Bourne (V.w5)
Referee Suzanne I. Boardman (V.w6); Becki Lawson (V.w26); Dr Robert G. McLean (V.w42)

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