Centers for Disease Control and Prevention 

Epidemic/Epizootic West Nile Virus in 

the United States:

Guidelines for Surveillance, Prevention, 

and Control

 

 

U.S. Department of Health and Human Services

Public Health Service 

Centers for Disease Control and Prevention 

National Center for Infectious Diseases 

Division of Vector-Borne Infectious Diseases 

Fort Collins, Colorado 

3rd Revision 

2003

i

 Table of Contents 

Introduction ..................................................................................................................................... 5 

I. SURVEILLANCE....................................................................................................................... 7 

A. Ecologic Surveillance ................................................................................................... 7 

1. Avian ....................................................................................................................... 7 

2. Equine ................................................................................................................... 13 

3. Mosquito................................................................................................................ 15 

B. Surveillance for Human Cases ................................................................................... 18 

1. Recent Experience................................................................................................. 18 

2. Types of Surveillance............................................................................................. 20 

 3. Specimens ............................................................................................................. 21

 4. Surveillance Case Definition.................................................................................. 22 

 5. Minimal Components of a Human Surveillance System ....................................... 22 

C. Geography and Timing ............................................................................................... 22 

1. Northeastern and Midwestern U.S........................................................................ 22 

2. Southern U.S......................................................................................................... 22 

3. Western U.S.......................................................................................................... 22 

4. Other Areas of the Western Hemisphere ............................................................. 22 

II. LABORATORY DIAGNOSIS .................................................................................................. 24 

A. Biocontainment ........................................................................................................... 24 

1. Laboratory Safety Issues ...................................................................................... 24 

2. Shipping of Agents................................................................................................ 25 

B. Serologic Laboratory Diagnosis.................................................................................. 25 

1. Human................................................................................................................... 25 

2. Animal ................................................................................................................... 26 

C. Virologic Laboratory Diagnosis .................................................................................. 26 

1. Virus Isolation........................................................................................................ 26 

2. Virus Detection in Tissues .................................................................................... 27 

D. Training and Infrastructure.......................................................................................... 27 

1. State and Local Arbovirus Laboratories .............................................................. 27 

2. Training Programs ............................................................................................... 27 

III. PREVENTION AND CONTROL ............................................................................................. 28 

A. Surveillance................................................................................................................. 28 

1. Larval Mosquito Surveillance................................................................................ 28 

2. Adult Mosquito Surveillance.................................................................................. 28 

3. Virus Surveillance ................................................................................................. 29 

B. Source Reduction ....................................................................................................... 29 

1. Sanitation .............................................................................................................. 30 

2. Water Management .............................................................................................. 30 

C. Chemical Control......................................................................................................... 31 
ii 

1. Larviciding ............................................................................................................. 31 

2. Adulticiding............................................................................................................ 32 

D. Resistance Management ............................................................................................ 33 

E. Biological Control......................................................................................................... 34 

F. Continuing Education of Mosquito Control Workers ................................................... 34 

G. Vector Management in Public Health Emergencies ................................................... 34 

H. Adult Mosquito Control Recommendations ................................................................ 35 

I. Determining the Scope of Mosquito Adulticiding Operations...................................... 36 

J. Evaluation of Adult Mosquito Control........................................................................... 37 

K. Health Education, Public Information, and Human Behavior Change........................ 37 

 1. Key WNV Prevention Messages .......................................................................... 37 

 2. Selected Best Practices ........................................................................................ 38 

 3. Research and Program Development Priorities ................................................... 39 

 4. Resources ............................................................................................................. 40 

L. Legislation.................................................................................................................... 41 

M. Guidelines for a Phased Response to WN Virus Surveillance Data........................... 41 

IV. HEALTH DEPARTMENT INFRASTRUCTURE ..................................................................... 45 

A. Staffing and Personnel .............................................................................................. 45 

B. Training and Consultation........................................................................................... 45 

C. Laboratory Capacity.................................................................................................... 45 

1. Testing for West Nile Virus Infections .................................................................. 45 

D. Developing Local Public Health Agency Infrastructure .............................................. 46 

V. INTERJURISDICTIONAL DATA SHARING AND NATIONAL REPORTING OF HUMAN 

CASES .................................................................................................................................... 48 

A. Human Epidemiological, Clinical, and Laboratory Data Collection ............................ 48 

B. National Reporting of Human WNV Disease Cases .................................................. 48 

1. WNME................................................................................................................... 48 

2. WNF...................................................................................................................... 48 

C. Ecologic Data.............................................................................................................. 48 

1. Accurate Taxonomic Identification of Specimens ................................................ 49 

2. Unique Identifier (UID) Numbering System for Specimens.................................. 49 

3. Durable Tagging System for Field-Collected Specimens..................................... 49 

VI. RESEARCH PRIORITIES ...................................................................................................... 50

A. Current and Future Geographic Distribution of WNV................................................. 50 

B. Bird Migration as a Mechanism of WNV Dispersal .................................................... 50 

C. Vector and Vertebrate Host Relationships and Range .............................................. 50 

D. Virus Persistence Mechanisms................................................................................... 50 

E. Mosquito Biology, Behavior, Vector Competence, Surveillance, and Control ........... 50 

F. Development and Evaluation of Prevention Strategies.............................................. 50 

G. Laboratory Diagnosis .................................................................................................. 51 

H. Clinical Spectrum of Disease and Long-Term Prognosis in Humans ....................... 51 
iii

I. Risk Factor Studies..................................................................................................... 51 

J. Detailed Clinical Descriptions and Outcome in Human Cases .................................. 51 

K. Viral Pathogenesis ...................................................................................................... 51 

L. Genetic Relationships and Molecular Basis of Virulence........................................... 51 

M. Vaccine Development for Animals and Humans........................................................ 52 

N. Antiviral Therapy for West Nile Virus and Other Flaviviruses .................................... 52 

O. The Economic Cost of WNV Epidemic/Epizootic....................................................... 52 

P. WNV Impact on Wildlife.............................................................................................. 52 

Q. Investigate Alternate Modes of WNV Transmission to Humans ......................... 52 

Appendix A B National West Nile Virus Surveillance System ...................................................... 53 

Appendix B B Surveillance Case Definition for West Nile Virus Infection in Equines.................. 66 

Appendix C B National Surveillance Case Definition for Arboviral Encephalitis/Meningitis......... 68 

Appendix D B CDC-Recommended Surveillance Case Definition for WN Fever........................ 70 

Appendix E B Recommended Framework for Standardized "Extended" Clinical Variables in 

Studies of Human WNV Disease ........................................................................................... 72 

References.................................................................................................................................... 73 

Department of Health and Human Services 

Centers for Disease Control and Prevention 

Julie Louise Gerberding, M.D., M.P.H., Director 

National Center for Infectious Diseases (NCID) 

James M. Hughes, M.D., Director 

Stephen M. Ostroff, M.D., Deputy Director 

The following CDC, Division of Vector-Borne Infectious Diseases staff members prepared this 

report: 

Duane J. Gubler, Sc.D., Director 

Lyle R. Petersen, M.D., M.P.H., Deputy Director 

John T. Roehrig, Ph.D. 

Grant L. Campbell, M.D., Ph.D. 

Nicholas Komar, Sc.D. 

Roger S. Nasci, Ph.D. 

Emily Zielinski-Gutierrez, Ph.D. 

Anthony A. Marfin, M.D., M.P.H. 

Robert S. Lanciotti, Ph.D. 

Michel L. Bunning, DVM 

Daniel R. O’Leary, DVM 

Mel Fernandez, Deputy Director 

Lauren Dieterich 

Barbara B. Tuttle 

Rebecca L. Deavours 
4 

Prepared in consultation with: 

Association of Public Health Laboratories 

Council of State and Territorial Epidemiologists 

Environmental Protection Agency 

Department of Army 

National Association of County and City Health Officials 

National Institutes of Health 

National Parks Service 

State Public Health Veterinarians 

State Public Health Vector Control Conference 

United States Department of Agriculture, Veterinary Services 
5

INTRODUCTION

In late summer 1999, the first domestically acquired human cases of West Nile (WN) 

encephalitis were documented in the U.S.1-6 The discovery of virus-infected, overwintering 

mosquitoes during the winter of 1999-2000 presaged renewed virus activity for the following 

spring and precipitated early season vector control and disease surveillance in New York City 

(NYC) and the surrounding areas.7, 8 These surveillance efforts were focused on identifying and 

documenting WN virus (WNV) infections in birds, mosquitoes and equines as sentinel animals 

that could alert health officials to the occurrence of human disease. Surveillance tracked the 

spread of WNV throughout much of the U.S. between 2000 and 2002. By the end of 2002, 

WNV activity had been identified in 44 states and the District of Columbia. The 2002 WNV 

epidemic and epizootic resulted in reports of 4,156 reported human cases of WN disease 

(including 2,942 meningoencephalitis cases and 284 deaths), 16,741 dead birds, 6,604 infected 

mosquito pools, and 14,571 equine cases. The 2002 WNV epidemic was the largest recognized 

arboviral meningoencephalitis epidemic in the Western Hemisphere and the largest WN 

meningoencephalitis epidemic ever recorded. Significant human disease activity was recorded 

in Canada for the first time, and WNV activity was also documented in the Caribbean basin and 

Mexico. In 2002, 4 novel routes of WNV transmission to humans were documented for the first 

time: 1) blood transfusion, 2) organ transplantation, 3) transplacental transfer, and 4) breastfeeding. 

WNV is a member of the family Flaviviridae (genus Flavivirus). Serologically, it is a member of 

the Japanese encephalitis virus antigenic complex, which includes St. Louis, Japanese, Kunjin, 

and Murray Valley encephalitis viruses.9,10 WNV was first isolated in the WN province of 

Uganda in 1937.11,12 Human and equine outbreaks have been recorded in portions of Africa, 

southern Europe, North America, and Asia.13,14 

Although it is still not known when or how WNV was introduced into North America, international 

travel of infected persons to New York, importation of infected birds or mosquitoes, or migration 

of infected birds are all possibilities. In humans, WNV infection usually produces either 

asymptomatic infection or mild febrile disease, sometimes accompanied by rash, but it can 

cause severe and even fatal diseases in a small percentage of patients. The human casefatality rate in the U.S. has been 7% overall, and among patients with neuroinvasive WNV 

disease, 10%. 

Unlike WNV within its historical geographic range, or St. Louis encephalitis (SLE) virus in the 

Western Hemisphere, mortality in a wide variety of bird species has been a hallmark of WNV 

activity in the U.S. The reasons for this are not known; however, public health officials have been 

able to use bird mortality (particularly birds from the family Corvidae) to effectively track the 

movement of WNV. WNV has now been shown to affect 162 species of birds. Previous earlyseason field studies have determined that areas with bird mortality due to WNV infection were 

experiencing ongoing enzootic transmission. However, most birds survive WNV infection as 

indicated by the high seroprevalence in numerous species of resident birds within the regions of 

most intensive virus transmission. The contribution of migrating birds to natural transmission 

cycles and dispersal of both WN and SLE viruses is poorly understood. 

WNV has been transmitted principally by Culex species mosquitoes, the usual vectors of SLE 

virus. Thirty-six species of mosquitoes have been shown to be infected with WNV. This wide 

variety of WNV-infected mosquito species has widened this virus’ host-range in the U.S.: 27 

mammalian species have been shown to be susceptible to WNV infection and disease has been 
6 

reported in 20 of these (including humans and horses). It must be remembered, however, that 

the detection of WNV in a mosquito species is necessary but not sufficient to implicate that 

species as a competent vector of WNV. 

Since 1999, the Centers for Disease Control and Prevention (CDC) and a variety of other U.S. 

governmental agencies and partners have sponsored yearly national meetings of 

arbovirologists, epidemiologists, laboratorians, ecologists, vector-control specialists, wildlife 

biologists, communication experts, and state and local health and agriculture officials to assess 

the implications of the WNV introduction into the U.S. and to refine the comprehensive national 

response plan. Recommendations from these meetings have been used to develop and to 

update these guidelines.15,16 This document is available electronically from the CDC Web site 

at: http://www.cdc.gov/ncidod/dvbid/westnile/publications.htm. 

To assist guideline implementation in 2000, CDC developed an electronic-based surveillance 

and reporting system (ArboNet) to track WNV activity in humans, horses, other mammals, birds 

and mosquitoes. In 2003, the ArboNet surveillance system has been updated to streamline 

reporting to CDC of WNV activity by the state public health departments. 

Today=s rapid transport of people, animals, and commodities increase the likelihood that other 

introductions of exotic pathogens will occur. CDC continues to implement its plan titled 

APreventing Emerging Infectious Diseases, a Plan for the 21st Century”.17 
7

I. SURVEILLANCE

A universally applicable arbovirus surveillance system does not exist. In any given 

jurisdiction, surveillance systems should be tailored according to the probability of arbovirus 

activity and available resources. In jurisdictions without pre-existing vector-borne disease 

surveillance and control programs, newly developed avian-based and/or mosquito-based 

arbovirus surveillance systems will be required. In some, resurrection of previously 

abandoned systems will be necessary. In others, modification and/or strengthening of 

existing arbovirus surveillance systems (i.e., those intended to monitor eastern equine 

encephalitis [EEE], western equine encephalitis [WEE], and/or St. Louis encephalitis [SLE] 

virus activity) will be the most appropriate response. In yet other jurisdictions in which the 

probability of arbovirus activity is very low and/or resources to support avian-based and/or 

mosquito-based surveillance are unavailable, laboratory-based surveillance for neurologic 

disease in humans and equines should be employed at minimum. 

Seasonality of surveillance activities may vary depending upon geographic region. With the 

anticipated spread of West Nile virus (WNV) to all of the 48 contiguous United States in 

2003, all states should initiate surveillance after mosquitoes become active in the spring. 

Appropriate and timely response to surveillance data is the key to preventing human and 

animal disease associated with WNV and other arboviruses. That response must include 

effective mosquito control and public education without delay, if an increasing intensity of 

virus activity is detected by bird- or mosquito-based surveillance systems (see Section III.M). 

For basic information on arbovirus surveillance, see CDC Guidelines for Arbovirus 

Surveillance Programs in the United States,

18 this document can be obtained from CDC’s 

Division of Vector-Borne Infectious Diseases, Fort Collins, Colorado, and is also available 

from the CDC Web site at: www.cdc.gov/ncidod/dvbid/arbor/arboguid.htm. 

A. Ecologic Surveillance 

 Detection of WNV in bird and mosquito populations helps health officials predict and 

prevent human and domestic animal infections. Surveillance to detect WNV should 

focus on the avian and mosquito components of the enzootic transmission cycle. Nonhuman mammals, particularly equines, may also serve as effective sentinels because a 

high intensity of mosquito exposure makes them more likely to be infected than people. 

Descriptions of the avian-, mosquito-, and non human mammal-based surveillance 

strategies follow. 

1. Avian 

 a) Avian morbidity/mortality surveillance

 Avian morbidity/mortality surveillance appears to be the most sensitive early 

detection system for WNV activity, and should be a component of every state’s 

arbovirus surveillance program. Its utility for monitoring ongoing transmission in a 

standardized fashion is currently being investigated, but should include at least 

two basic elements: the timely reporting and analysis of dead bird sightings and 

the submission of selected individual birds for WNV testing. 
8 

GOAL OF AVIAN MORBIDITY/MORTALITY SURVEILLANCE: Utilize bird 

mortality associated with WNV infection as a means of detecting WNV activity in 

a location. 

1) Protocols and specimens 

The level of effort involved in this surveillance activity will depend on a risk 

assessment in each jurisdiction. Generally, avian surveillance should be 

initiated when local adult mosquito activity begins in the spring. A database 

should be established to record and analyze dead bird sightings with the 

following suggested data: caller identification and call-back number, date 

observed, location geocoded to the highest feasible resolution, species, and 

condition. Samples from birds in good condition (unscavenged and without 

obvious decomposition or maggot infestation) may be submitted for laboratory 

testing. As with all dead animals, carcasses should be handled carefully, 

avoiding direct contact with skin. For greatest sensitivity, a variety of bird 

species should be tested, but corvids should be emphasized.19 The number of 

bird specimens tested will be dependent upon resources and whether WNVinfected birds have been found in the area; triage of specimens may be 

necessary on the basis of sensitive species (such as corvids) and geographic 

location. Many jurisdictions may limit (or even stop) avian mortality 

surveillance once WNV is confirmed in their region. It is suggested that avian 

mortality surveillance be continued in each region as long as it remains 

necessary to know whether local transmission persists, because dead-birdbased surveillance is the most sensitive method for detection of WNV activity 

in most regions. 

A single organ specimen from each bird is sufficient to detect WNV or viral 

RNA. Kidneys, brains, or hearts are preferable.20-22 Oral swabs from corvids 

have been validated as a sensitive alternative to organ samples, and because 

fewer resources are necessary to acquire them, oral swabs are the preferred 

specimen from corvid carcasses.23 Testing involves isolation of infectious 

virus, specific RNA detection by reverse transcription-polymerase chain 

reaction (RT-PCR), 24 or antigen detection,25,26 and will generally be positive 

within 1-2 weeks after specimen submission. 

2) Recent experience

 Analysis of recent avian morbidity and mortality data indicated that 

 (a) The American crow was the most sensitive species for avian morbidity/ 

mortality surveillance in northern regions. However, some areas did not 

have WNV-positive American crows, but only WNV-positive birds of other 

species. In southern regions, blue jays have been more sensitive than 

crows. 

 (b) Almost all of the positive birds were found singly and not as part of a mass 

die-off at a single time and place. 

 (c) Approximately one-third of the WNV-positive birds had signs of trauma on 

necropsy. 
9

 (d) Many WNV-positive birds did not have pathology indicative of WNV 

infection on necropsy. No lesions are pathognomonic for WNV infection. 

 (e) WNV-positive dead birds usually provided the earliest indication of viral 

activity in an area. In 2002, the detection of WNV-infected dead birds was 

the first positive surveillance event in 1,534 (61%) of 2,531 counties 

reporting WNV activity. 

 (f) The detection of WNV-positive dead birds preceded reports of human 

cases (although knowledge of the test result did not necessarily predate 

the onset of human cases). In 2002, 527 (89%) of 589 counties reporting 

human WN meningoencephalitis cases first detected WNV transmission 

in animals. In 327 (72%) of these 527 counties, detection of WNVinfected dead birds was the first positive surveillance event, preceding 

human illness onset by a median of 38.5 days (range, 2-252 days). 

 (g) Many counties with human cases of WNV infection tended to have high 

dead bird surveillance indices, both WNV-positive and sightings. Notable 

exceptions included sparsely populated counties, particularly those in the 

midwestern states.27,28

 (h) Experimental evidence of direct transmission among corvids and gulls 

exists, but whether this occurs in nature is unknown. 29 If it does, then in 

some settings, virus-infected mosquitoes might not be necessary to 

maintain enzootic transmission cycles. 

3) Advantages of avian morbidity/mortality surveillance include the following: 

 

 (a) Certain species of birds, in particular corvids (e.g., crows and jays) 

experience high clinical attack rates. 

 (b) The size and coloration of certain dead birds makes them conspicuous 

(e.g., crows). 

 (c) RT-PCR and antigen-detection assays can be used to rapidly detect WN 

viral RNA and protein, respectively, in tissues, even if the tissue is partly 

decomposed. Both assays have now been adapted for field applications. 

 (d) Due to public involvement in reporting dead bird sightings, dead wild birds 

are readily available over a much wider region than can be sampled by 

other surveillance methods. 

 (e) Detection of WNV in dead birds likely signifies local transmission.30

 (f) This type of surveillance provides a temporally and spatially sensitive 

method for the detection of WNV activity. 

 (g) It can be used for early detection and possibly also for ongoing monitoring 

of WNV transmission. 

 (h) It may be used to estimate risk of human infection with WNV. 27,31,32 

4) Disadvantages of avian morbidity/mortality surveillance include the following: 

 (a) Dead bird surveillance data from different jurisdictions are difficult to 

compare. 

 (b) Birds are highly mobile and often have extensive home ranges, so that the 

site of death may be distant from the site of infection (especially after the 

breeding season, when birds are generally less territorial). 

 (c) Collection, handling, shipping, and processing of birds or their clinical 

specimens is cumbersome. 
10 

 (d) Systems for handling, processing, and testing have at times been 

overwhelmed by high public response and public expectations. 

 (e) The long-term usefulness of this system is uncertain because natural 

selection for disease-resistant birds may occur, populations of susceptible 

species may become very low, or the virus may evolve, resulting in low or 

no avian mortality. In areas where WNV annually recurs, intense 

environmental sampling might not be as useful. 

 (f) Success is influenced by public participation, which is highly variable, and 

depends on the number of public outreach programs, level of public 

concern, etc. 

 (g) The system may be less sensitive in rural areas, where there are fewer 

persons to observe dead birds over a wider geographic area. In the 

western U.S., low observer density is coupled with the presence of a 

vector (Culex tarsalis) that is less ornithophilic, resulting in fewer reports of 

dead birds relative to other non-avian surveillance indicators. 

 b) Live bird surveillance

Live-bird surveillance has been used traditionally both to detect and monitor 

arbovirus transmission (e.g., for SLE, EEE and WEE viruses). Two 

approaches are captive sentinel surveillance (typically using chickens, but 

other species have been used as well), and free-ranging bird surveillance.33 

Both depend on serological testing, which generally requires at least 3 weeks 

to detect and confirm an infection. Successful application of these 

approaches requires extensive knowledge of local transmission dynamics. It 

is recommended that further research be done before relying on sentinel birds 

as a primary means of WNV surveillance. Use of sentinel birds may require 

institutional animal use and care protocols, and other authorization permits. 

GOAL OF LIVE-BIRD SURVEILLANCE: Utilize seroconversions in captive or 

free-ranging bird species as indicators of local WNV activity. 

1) Captive sentinel surveillance 

Although an ideal captive avian sentinel for WNV -- or any other arbovirus 

– may not exist, such a species would meet the following criteria: 1) is 

universally susceptible to infection, 2) has a 100% survival rate from 

infection and universally develops easily detectable antibodies, 3) poses 

no risk of infection to handlers, and 4) never develops viremia sufficient 

to infect vector mosquitoes.18 Captive sentinels have been effectively 

used to monitor transmission of arboviruses in a standardized fashion, 

including SLE virus in California and Florida, especially in historical 

enzootic transmission foci. Captive sentinel flocks should be placed in 

likely transmission foci (e.g., near vector breeding sites or adult mosquito 

congregation sites), and presented appropriately to allow feeding by 

enzootic WNV vectors. Alternatively, pre-existing captive birds (e.g.,

domestic poultry or pigeons, or zoo birds) may be used as sentinels. 
11

 (a) Protocols and specimens 

Whole blood can be collected and centrifuged for serum. Serum is 

screened by either hemagglutination inhibition (HI), enzyme-linked 

immunosorbent assay (ELISA) or plaque-reduction neutralization test 

(PRNT).34 It is important to note that the extraction of avian serum 

samples to remove non specific inhibitors of hemagglutination for use 

in the HI test follows procedures different from those used in tests of 

human serum samples.35 Positive tests must be confirmed by 

neutralization to rule out false positives and cross-reactions due to 

infection with related flaviviruses (e.g., SLE virus). 

 (b) Recent experience 

(i) In 2000, sentinel chickens were used in selected counties in New 

York State, New York City (NYC), New Jersey, Pennsylvania, 

Maryland, and Delaware. Small numbers of seroconversions were 

detected late in the season in New Jersey and New York. As used 

in 2000, chickens were ineffective sentinels. In NYC in 2001, 

sentinel chickens were placed in known transmission foci and 

seroconverted earlier in the season, but not earlier than the first 

human cases. In 2002, hundreds of sentinel chickens in the 

Southeast seroconverted, but these were rarely the earliest 

indicators of WNV activity at the county level. 

(ii) IgM capture enzyme-linked immunosorbent assay (MAC-ELISA) 

testing of experimentally infected chickens points to the need for 

biweekly sampling of sentinels.36 

(iii) Experimental studies have shown that chickens, pigeons, and 

pheasants (CDC, unpublished data) are candidate sentinels due to 

their susceptibility to infection, low mortality, and relative 

incompetence as amplifying hosts. However, small amounts of 

WNV were detected in cloacal swabs from infected chickens and 

pigeons.29,37 

(iv) Field studies of avian seroprevalence in Queens in 1999 indicated 

that captive chickens frequently were infected.38 In Staten Island 

in 2000, captive pigeons frequently were infected.39

 (v) Some mortality in chickens was attributed to WNV at various 

locations in New York State.40

 (c) Advantages of sentinel captive bird surveillance include the following: 

(i) Chickens have been successfully used in flavivirus surveillance for 

over 6 decades. 

 (ii) Birds are readily fed upon by Culex mosquitoes. 

(iii) Captive birds can be serially bled, making the geographic location 

of infection definite. 

(iv) The system is flexible and therefore can be expanded and 

contracted as appropriate. 

(v) Mosquito-abatement districts can maintain and bleed flocks and 

submit specimens for testing. 

(vi) Collection of specimens is inexpensive compared with the costs of 

free-ranging bird surveillance. 
12 

 (d) Disadvantages of captive sentinel surveillance include the following: 

(i) Sentinel flocks detect only focal transmission, requiring multiple 

flocks be positioned in representative geographic areas. This is 

particularly true when vector mosquitoes have short flight ranges 

(e.g., Culex pipiens). 

 (ii) Flocks are subject to vandalism and theft. 

 (iii) Flocks must be protected from predators. 

(iv) Flock set-up and maintenance (i.e., birds, cages, feed, 

transportation) are expensive. Training is required for proper 

maintenance and sampling. 

(v) Pre-existing flocks may already have been exposed due to 

previous local WNV transmission. 

2) Free-ranging bird surveillance 

Free-ranging birds provide the opportunity for sampling important 

reservoir host species and may be used both for early detection and for 

monitoring virus activity. This type of surveillance has been used 

effectively for SLE, EEE and WEE virus surveillance in several states. In 

each geographic area, the optimal free-ranging bird species to be 

monitored should be determined by serosurveys. The best species for 

serologic surveillance are those in which infection is rarely, if ever, fatal, 

and population replacement rates are high, ensuring a high proportion of 

uninfected individuals. 

 (a) Protocols and specimens 

The use of free-ranging birds requires differentiation of recent 

infection from infections acquired in previous years. For most species, 

assays for detection of IgM antibody will not be available and other 

tests such as IgG (IgY)-detection ELISAs41,42 and the PRNT34 must be 

used to detect WNV-specific antibody. Antibody-positive birds less 

than 1 year old may be presumed to have been infected recently 

(during current transmission season). Weak seropositivity in very 

young birds (less than 1 month old) may be due to maternal transfer of 

antibody. Seroconversion in older birds is also evidence of recent 

transmission, but requires frequent recapture for acquisition of multiple 

specimens from uniquely banded individuals during the course of the 

transmission season. WNV seropositivity among after-hatch-year 

birds, when determined from a single serum specimen, should not be 

interpreted or reported as evidence of recent infection. State and 

federal permits are required for capture and banding of federallyprotected migratory birds. 

 (b) Recent experience 

(i) In urban epizootic transmission foci in NYC, several common 

species (i.e., house sparrows, cardinals, catbirds, mourning doves, 

rock doves) developed high seroprevalence, making them strong 

candidate sentinels, although other species may be important in 

other locations.38,39
13

(ii) A comparison of free-ranging bird surveillance in NYC in 2001 

found that much greater effort was required for this surveillance 

system compared with other surveillance systems (Green Street 

Scientific, LLC, unpublished data). Similar observations have 

been made in Indiana, Louisiana, New Jersey, Ohio, and Texas. 

 (c) Advantages of free-ranging bird surveillance include the following: 

 (i) It has a long history of successful use in flavivirus surveillance. 

 (ii) Local movement of resident wild birds may increase contact with 

enzootic transmission foci, thus increasing sensitivity (relative to 

captive sentinels). 

 (iii) Set-up or maintenance costs may be minimal. 

 (iv) Its sampling capability is highly flexible. 

 (v) It permits evaluation of herd immunity among important amplifying 

hosts. 

 (vi) Owner confidentiality may be less of an issue. 

 (d) Disadvantages of free-ranging bird surveillance include the following: 

 (i) Interpretation of serologic results is complex. 

(ii) Handling and venipuncture of birds increases the risk of exposure 

to pathogens in blood and feces. 

(iii) Movement of free-ranging wild birds makes it impossible to know 

where an infection was acquired. 

(iv) Most birds are protected by federal law, and their collection and 

sampling requires state and federal permits. Banding permits 

require complex data reporting. 

(v) Training is required for live-trapping, blood-sampling, handling, 

and accurate determination of the species and age of wild birds. 

(vi) It is generally not feasible to serially bleed individual free-ranging 

birds because of low recapture rates (although banding can be 

useful). 

2. Equine 

Equines appear to be important sentinels of WNV epizootic activity and human 

risk, at least in some geographic regions. In addition, equine health is an 

important economic issue. Therefore, surveillance for equine WNV disease 

should be conducted in jurisdictions where equines are present. Veterinarians, 

veterinary service societies/agencies, and state agriculture departments are 

essential partners in any surveillance activities involving equine WNV disease. A 

working surveillance case definition of clinical WNV infection in equines is 

presented in Appendix B. 

GOALS OF EQUINE DISEASE SURVEILLANCE: To use data on equine WNV 

disease cases to assess the threat of human disease, identify geographic areas 

of high risk, and assess the need for and timing of interventions. 

 

a) Protocols and Specimens
14 

1) Serum and cerebrospinal fluid (CSF) for antibody testing. Because an 

equine WNV vaccine is now in widespread use, a complete vaccination 

history should accompany all specimens submitted for antibody testing. 

2) Necropsy tissues (especially brain and spinal cord) for gross pathology, 

histopathology, RT-PCR, virus isolation, and immunohistochemistry. The 

differential diagnosis of equine encephalitis includes, but is not limited to, 

the other arboviral encephalitides and rabies. 

b) Recent experience

1) In 2002, equine WNV disease cases were the first indication of WNV 

activity in 95 (16%) of the 589 counties where human disease was 

reported. The majority of these 95 counties were located in the central 

and western U.S. 

2) In general, equine WNV disease cases have been scattered. Few case 

clusters have been documented. 

3) In fatal equine WNV disease cases, pathological findings have been nonspecific. Pathognomonic lesions have not been described. 

4) A licensed equine WNV vaccine has been available in the U.S. since 

2001. No studies of efficacy have been published.

c) Advantages of equine disease surveillance include the following:

1) Equines are highly conspicuous, numerous, and widely distributed in 

some areas. They may be particularly useful sentinels in rural areas, 

where dead birds may be less likely to be detected. 

2) Some equines are routinely bled and tested for other pathogens. 

3) Ill equines have been one of the earliest, if not the earliest, sentinels of 

WNV activity in some geographic areas. 

d) Disadvantages of equine disease surveillance include the following:

1) In some geographic areas, equines may not be an early sentinel (i.e., 

human WNV disease cases may occur simultaneously with or soon after 

equine cases). 

2) Necropsies are expensive and logistically difficult. 

 3) Equines are not present or abundant in many areas of the U.S. (e.g., 

densely populated metropolitan areas), and proximity of equines to human 

populations varies. 

4) Widespread use of equine WNV vaccines may decrease the incidence of 

equine WNV disease and therefore the usefulness of equines as 

sentinels. 
15

 5) Because the costs of clinical equine specimen collection and testing are 

usually borne directly by the owner, economic factors work against the 

submission and testing of equine specimens for arboviral infections. 

e) Minimal components of an equine surveillance program

1) All equine neurologic disease cases should be promptly reported; the 

equines should be tested for infection with WNV and other arboviruses as 

geographically appropriate, and for rabies. 

2) Clusters of equine neurologic disease cases should be promptly 

investigated. 

3. Mosquito 

While dead-bird-based surveillance has proven to be the most sensitive method 

of detecting WNV presence in an area, mosquito-based surveillance remains the 

primary tool for quantifying the intensity of virus transmission in an area, and 

should be a mainstay in most surveillance programs for WNV and other 

arboviruses. 

GOALS OF MOSQUITO-BASED SURVEILLANCE: To 1) use data on mosquito 

populations and virus infection rates to assess the threat of human disease; 2) 

identify geographic areas of high risk; 3) assess the need for and timing of 

interventions; 4) identify larval habitats for targeted control; 5) monitor the 

effectiveness of this type of surveillance and improve prevention and control 

measures; and 6) develop a better understanding of transmission cycles and 

potential vector species. 

 

 a) Protocols and specimens 

1) Adult mosquitoes are collected using a variety of trapping techniques and 

are used to identify the mosquito species and primary vector species 

present in an area and the relative density of those species. When 

coupled with virus detection protocols, mosquito collections can be 

screened for the presence of virus and provide a quantifiable index of 

WNV activity. Adequate sampling requires trapping regularly at 

representative sites throughout a community, and rapid testing of 

collections of sufficient size to detect low infection rates in the vector 

population. Minimally, adult mosquito density (number collected per trap 

night) and infection rate (number of individual mosquitoes estimated 

containing WNV per 1,000 specimens tested) should be recorded for 

each area to provide a basis for tracking mosquito density and virus 

incidence. 

2) Larval mosquitoes are collected by taking dip samples from a variety of 

habitats to identify species present in the area and to identify mosquito 

sources. Thorough mapping of larval habitats will facilitate larval control 

or source reduction activities. In addition, where larval management is not 

feasible, quantitative estimates of larval densities will permit anticipation of 

new adult emergences. Minimally, the number of larvae collected per dip 
16 

and location where collected should be recorded to provide a basis for 

tracking larval production and association of larval density with resulting 

adult mosquito population density. 

b) Recent experience

1) If mosquito trapping effort is intensive, detection of WNV in mosquitoes 

might precede detection of virus activity by other surveillance tools. If 

mosquito trapping effort is inadequate, WNV-positive mosquitoes may not 

be detected prior to the identification of a virus in dead bird, sentinel 

animal, or human WNV disease cases. 

2) Moderate to high infection rates sustained for several weeks in Cx. 

pipiens or Cx. quinquefasciatus have been associated with subsequent 

human outbreaks. Sustained high infection rates early in the year are 

associated with a higher risk for subsequent outbreaks. 

3) Several intense, focal outbreaks during 2002 were associated with 

relatively low vector densities, but with high infection rates in key vector 

species (i.e., infection rates in Cx. pipiens or Cx. quinquefasciatus of 

approximately 10 per 1,000 or greater). 

4) Large numbers of WNV-positive Cx. tarsalis pools have been found in 

association with WNV activity in areas where this species is common. 

Meaningful infection rates have not yet been determined. 

5) Avian epizootics may occur without demonstrable human WNV infection. 

The epizootics are demonstrated, in part, by detection of WNV-positive 

mosquito pools containing only species that feed predominantly on birds 

(e.g., Cx. restuans). 

6) During 1999-2002, WNV was detected in 36 mosquito species in the U.S. 

(see www.cdc.gov/ncidod/dvbid/westnile/mosquitoSpecies.htm). The vast 

majority of isolates came from Cx. pipiens, Cx. quinquefasciatus and Cx. 

restuans. Numerous isolates have also come from several potential 

accessory vectors (i.e., Cx. tarsalis, Cx. salinarius, Oc. Ae. albopictus, Oc. 

triseriatus, Ae. vexans, Cx. nigripalpus). While detection of WNV in these 

species demonstrates intensified virus transmission (i.e., virus in primarily 

mammal-feeding or opportunistic mosquitoes), the contribution of these 

species to human risk is poorly understood. 

 c) Advantages of mosquito-based surveillance include the following:

1) It may provide the earliest evidence of transmission in an area. 

2) It helps establish information on potential mosquito vector species. 

3) It provides an estimate of vector species abundance. 

4) It gives quantifiable information on virus infection rates in different 

mosquito species. 
17

5) It provides quantifiable information on potential risk to humans and 

animals. 

6) It provides baseline data that can be used to guide emergency control 

operations. 

7) It allows evaluation of control methods. 

 d) Disadvantages of mosquito-based surveillance include the following:

1) It is labor-intensive and expensive. 

2) Substantial expertise is required for collecting, handling, sorting, species 

identification, processing, and testing. 

3) Collectors may be at risk from mosquito bites, especially if day biting 

species are important bridge vectors, and should wear topical repellents 

and/or repellent-treated clothing when working in areas where a risk of 

WNV transmission exists. 

 e) Minimal components of an entomological surveillance program

A comprehensive mosquito surveillance program must include larval and 

adult sampling components, a mapping/record keeping component, a virustesting component, and a data analysis component. To provide useful data, 

the surveillance program must be sustained and maintain a consistent effort 

over several seasons. The exact design of mosquito-based surveillance 

programs will vary by geography and availability of financial and personnel 

resources. Not every community will be able to support a comprehensive 

mosquito-based surveillance program. Minimally, a mosquito-based WNV 

surveillance program must include the following: 

1) Collection of adult mosquitoes using gravid traps and/or light traps, 

providing representative geographic coverage and with sufficient trap sites 

and trapping frequency to obtain sample sizes required to detect WNV at 

relatively low infection rates. Use both fixed and flexible trap positions if 

possible. 

(a) Fixed positions allow for the development of a database that would let 

public health officials compare population data to previous years and 

spatially map changes in mosquito abundance. 

(b) Flexible sites allow for response to epidemiological and natural events 

(e.g., a suspected human case, dead crow, or a flood). 

(c) A variety of trapping methods should be used, including the following: 

 (i) CDC light traps baited with CO2 for sampling potential accessory

vectors. 

 (ii) Gravid traps for Cx. pipiens and Cx. quinquefasciatus to sample 

primary WNV vectors. 

 (d) Trap distribution will be influenced by the following species factors: 

 (i) Habitat diversity, size, and abundance; 

 (ii) Resource availability; 
18 

 (iii) Proximity to human population centers and/or recreational areas; 

 and 

 (iv) Flight range of vector species in the area. 

2) Laboratory support to identify the mosquitoes’ species, and to test the 

specimens for the presence of WNV. Determine infection rates by 

species. 

 (a) Make arrangements with a lab for testing. Rapid turnaround is 

essential. 

 (b) Focus initially on Culex mosquitoes to provide first indication of WNV 

presence. 

 (c) Once virus is detected in Culex mosquitoes, pool and test all potential 

vector species with emphasis on incriminated or suspected species. 

(3) Data management and analysis capabilities to allow tracking of adult 

mosquito densities and infection rates over time and space. Patterns of 

virus activity are more likely to be useful than predetermined threshold 

levels. 

(4) Development of a plan with descriptions of actions that will be taken in 

response to indicators of WNV activity. 

 

B. Surveillance for Human Cases 

Because the primary public health objective of surveillance systems for neurotropic 

arboviruses is prevention of human infections and disease, human case surveillance 

alone should not be used for the detection of arbovirus activity, except in jurisdictions 

where arbovirus activity is rare, or resources to support avian-based and/or mosquitobased arbovirus surveillance are unavailable. 

GOALS OF SURVEILLANCE FOR HUMAN CASES: To 1) assess the local, state and 

national public health impact of WNV disease and monitor national trends; 2) 

demonstrate the need for public health intervention programs; 3) allocate resources; 4) 

identify risk factors for infection and determine high-risk populations; 5) identify 

geographic areas in need of targeted interventions; and 6) identify geographic areas in 

which it may be appropriate to conduct analytic studies of important public health issues. 

1. Recent Experience 

a) In the U.S. during 1999-2002, the peak human risk for WN viral infection occurred 

in August and September, although in 2002 human illness onset was reported as 

early as mid-May and as late as mid-December. In many regions, the peak 

minimum infection rates in mosquitoes and a rapid increase in the number of 

reported avian and equine WN viral infections occurred just prior to the period of 

maximal human risk. 

b) In 1999-2002, the majority of reported, confirmed, or probable cases of human 

WN viral disease were among persons with meningoencephalitis. Testing of 

patients with aseptic meningitis or unexplained febrile illnesses for evidence of 
19

WN viral infection may be beneficial, but can also overwhelm laboratory testing 

capacity and appears to be of relatively low yield for surveillance purposes since 

the majority of these cases will not be due to WNV infection. 

c) Most patients with WN encephalitis or meningitis (WNME) are older adults, 

generally over 50 years old. In the U.S. in 1999-2001, the median age among the 

142 reported WNME cases was 68 years. In 2002, among 2,942 reported cases 

of WN meningoencephalitis, the median age was 59 years. Although 21% of 

reported cases were in persons younger than 40, only 4% of reported cases were 

in persons younger than 18. 

d) When WN viral infections were first identified in the U.S., WN encephalitis was 

associated with a Guillain-Barrè-like syndrome with generalized muscle 

weakness. In 1999-2000, generalized muscle weakness was reported in 29% of 

WN encephalitis cases. In 2002, at least 2 new neurologic syndromes associated 

with WN viral infection were identified: acute flaccid paralysis (“WN poliomyelitislike syndrome”) and brachial plexopathy.

e) Using CDC-recommended test methods in public health laboratories, WNVspecific IgM antibody was detected in acute-phase (i.e., those collected 8 or less 

days after illness onset) serum or CSF specimens, or both, in the large majority of 

confirmed cases. In contrast, only a small minority of suspected cases were 

subsequently confirmed in which specific IgM antibody reactivity in acute-phase 

serum or CSF was in the equivocal or low-positive range. 

f) Longitudinal studies of WNME cases have shown that WNV-specific IgM 

antibody can persist in serum for 12 months or longer.43 Thus, the presence of 

WNV-specific IgM antibody in a single serum sample is not necessarily diagnostic 

of acute WN viral infection. For this reason, especially in areas where WNV is 

known to have circulated previously, suspected, acute WN viral disease cases 

should be confirmed by observing a fourfold or more change in titer of WNVspecific antibody in serum and the presence of WNV-specific IgM antibody in 

CSF, when available. 

g) In 1999 in the U.S., the sensitivity of polymerase chain reaction (PCR) tests of 

CSF for the diagnosis of human WN encephalitis cases was only 57%; more 

recent statistics are currently unavailable. Thus, PCR for the diagnosis of WN 

viral infections of the human central nervous system (CNS) continues to be 

experimental and should not replace tests for the detection of WNV-specific 

antibody in CSF and serum, tests that are far more sensitive. 

h) During 1999-2001, 7 cases of uncomplicated WN fever (WNF) were reported in 

the U.S., which represents 5% of the total number of WNV disease cases 

reported. In 2002, over 1,100 WNF cases were reported (30% of total). 

Contributing factors likely include the intensive media attention paid to the 2002 

epidemic that may have led to increased consumer demand for WNV diagnostic 

testing by patients and physicians, and the greater availability of commercial 

testing. Nevertheless, during 1999-2002, WNF was probably significantly 

underdiagnosed in the U.S. It has been estimated that approximately 20 WNF 

cases occur for every WNME case.44
20 

i) For suspected WNV disease cases in immunocompromised patients, WNVspecific antibody may not be present. Since longer viremias may be observed in 

these patients, testing serum and CSF samples for the presence of virus or viral 

RNA may be useful. 

2. Types of Surveillance 

 a) Clinical syndromes to monitor

Monitoring of encephalitis cases is the highest priority. Monitoring milder illnesses 

(e.g., aseptic meningitis, Guillain-Barré syndrome, acute flaccid paralysis, and 

brachial plexopathy, and fever or rash illnesses) is resource-dependent and 

should be of lower priority. 

 b) Types of human surveillance

1) Enhanced passive surveillance 

In the absence of known WNV activity in an area, enhanced passive 

surveillance* for hospitalized cases of encephalitis (and milder clinical 

syndromes as resources allow**), and for patients who have IgM antibodies to 

either WN or SLE virus in tests conducted in diagnostic or reference 

laboratories, should be employed. A high clinical suspicion for arboviral 

encephalitis should be encouraged among health care providers. When the 

diagnosis is in doubt, appropriate clinical specimens should be submitted to 

CDC or another laboratory capable of performing reliable serologic testing for 

antibodies to domestic arboviruses. Testing of CSF and paired acute- and 

convalescent-phase serum samples should be strongly encouraged to 

maximize the accuracy of serologic results. 

 2) Active surveillance 

Active surveillance should be strongly considered in areas with known WNV 

activity. In general, one or both of the following approaches should be taken: 

(a) Contact physicians in appropriate specialties (i.e., infectious diseases, 

neurology, and critical care) and hospital infection control personnel on a 

regular basis to inquire about patients with potential arboviral infections; 

(b) Implement laboratory-based surveillance to identify CSF specimens 

meeting sensitive but nonspecific criteria for arboviral infections (e.g., mild to 

moderate pleocytosis and negative tests for the presence of nonarboviral 

 *

Passive surveillance enhanced by general alerts to key health care personnel such as primary care 

providers, infectious disease physicians, neurologists, hospital infection control personnel, and diagnostic 

laboratories.

** While human infections with neurotropic arboviruses are usually clinically inapparent, most clinically 

apparent infections are associated with fever, with or without neurologic manifestations, which can range from mild 

aseptic meningitis to fulminant and fatal encephalitis. Signs and symptoms may include fever, headache, stiff neck, 

confusion or other mental status changes, nausea, vomiting, meningismus, cranial nerve abnormalities, paresis or 

paralysis, sensory deficits, altered reflexes, abnormal movements, convulsions, and coma of varying severity. 

Arboviral meningitis or encephalitis cannot reliably be clinically distinguished from other central nervous system 

infections. 
21

agents such as bacteria, fungi, herpesviruses, and enteroviruses) and test 

them for evidence of WNV infection. 

 3) Special surveillance projects 

Special projects may be used to enhance arboviral disease surveillance. Such 

projects include the Emerging Infections Network of the Infectious Diseases 

Society of America (IDSA EIN), Emergency Department Sentinel Network for 

Emerging Infections (EMERGEncy ID NET), Unexplained Deaths and Critical 

Illnesses Surveillance of the Emerging Infections Programs (EIP), and the 

Global Emerging Infections Sentinel Network of the International Society of 

Travel Medicine (GeoSentinel). In some areas, syndromic surveillance 

systems may be considered. “Piggy-backing” surveillance for WNME and 

milder clinical forms of WN viral infection, such as fever with rash or 

lymphadenopathy, onto existing syndromic surveillance systems, especially 

those involving large health maintenance organizations, may be considered. 

Real-time computerized syndromic surveillance in emergency departments, 

and special surveillance projects to identify WNV disease in pediatric 

populations, may be useful. 

 3. Specimens 

 a) Cerebrospinal fluid (CSF)

In WNME cases, WNV-specific IgM antibody commonly can be found in CSF on 

the day of illness onset using antibody-capture ELISA. Virus also may be isolated 

(rarely) or detected by RT-PCR (in up to 60% of cases) in acute-phase CSF 

samples. 

b) Serum

Paired acute-phase (collected 0-8 days after onset of illness) and convalescentphase (collected 14-21 days after the acute specimen) serum specimens are 

useful for demonstration of seroconversion to WNV and other arboviruses by 

ELISA or neutralization tests. Although tests of a single acute-phase serum 

specimen may provide evidence of a recent WNV infection, a negative acutephase specimen is inadequate for ruling out such an infection, underscoring the 

importance of collecting paired samples. As mentioned previously, antibody 

synthesis in immunocompromised individuals might be delayed or absent 

altogether. 

 c) Tissues

When arboviral encephalitis is suspected in a patient who undergoes a brain 

biopsy or who dies, tissues (especially brain samples, including samples of 

cortex, midbrain, and brainstem), heart/venous blood, and buffy coat samples 

should be submitted to CDC or other specialized laboratories for arbovirus and 

other testing. Tissue specimens should be divided; half should be frozen at -70°C 

and the other half fixed in formalin. Available studies include gross pathology, 

histopathology, RT-PCR tests, virus isolation, and immunohistochemistry. 
22 

 4. Surveillance Case Definition 

The national case definition for arboviral encephalitis (available at 

www.cdc.gov/epo/dphsi/casedef/encephalitiscurrent.htm) should be used to classify 

cases as confirmed or probable, once appropriate laboratory results are available 

(also see Section II). In CDC publications of national arbovirus surveillance data, no 

distinction is usually made between confirmed and probable human cases for the 

purposes of case counting. 

5. Minimal Components of a Human Surveillance System 

Enhanced passive surveillance for hospitalized encephalitis cases of unknown 

etiology, and for patients who have IgM antibodies to either WN or SLE virus in tests 

conducted in diagnostic or reference laboratories. 

C. Geography and Timing 

In general, the WNV transmission season in the U.S. is longer than that for other 

domestic arboviruses and requires longer periods of ecologic and human 

surveillance. 

1. Northeastern and Midwestern U.S. 

In the northeastern states in 2001-2002, human illness onset occurred as early as 

early July and as late as mid-November. During these same years, avian cases 

occurred as early as the first week of April and as late as the second week of 

December. Active ecological surveillance and enhanced passive surveillance for 

human cases should begin in early spring and continue through the fall until mosquito 

activity ceases because of cold weather. Surveillance in urban and suburban areas 

should be emphasized. 

2. Southern U.S. 

In 2001-2002, WNV circulated throughout the year, especially in the Gulf states. 

Although, in 2001-2002, human illness onset was reported as early as mid-May and 

June and as late as mid-December, equine and avian infections were reported in all 

months of the year. Active ecologic surveillance and enhanced passive surveillance 

for human cases should be conducted year round in these areas. 

3. Western U.S. 

In 2002, WNV activity was first reported among humans and animals in Rocky 

Mountain states and among animals in Pacific coast states. These events occurred 

relatively late in the year (mid-August). Predicting the temporal characteristics of 

future WNV transmission seasons based on these limited reports is not possible. 

Despite this limitation, active ecological surveillance and enhanced passive 

surveillance for human cases beginning in early spring and continuing through the fall 

until mosquito activity ceases because of cold weather should be encouraged. 

4. Other Areas of the Western Hemisphere 

In 2002, Canada experienced a WNV epidemic in Ontario and Quebec provinces and 

an equine/avian epizootic that extended from the maritime provinces to 

Saskatchewan. 
23

Recent serologic evidence supports the conclusion that WNV has now reached 

Central America. Further spread to South America by migratory birds seems 

inevitable, if this has not already occurred. Development of surveillance systems 

capable of detecting WNV activity should be encouraged in the Caribbean and 

Central and South America. WNV surveillance should be integrated with dengue 

surveillance in these areas, and with yellow fever surveillance in areas where urban 

or peri-urban transmission of this virus occurs. 
24 

II. LABORATORY DIAGNOSIS

The clinical presentation of most patients with viral encephalitis is similar regardless of the 

cause. Also, infection by many of the arboviruses that cause encephalitis, including West 

Nile and St. Louis encephalitis viruses, usually is clinically inapparent, or causes a 

nonspecific viral syndrome in most patients. Definitive diagnosis, therefore, can only be 

made by laboratory testing using specific reagents. To be successful, active surveillance 

must have adequate laboratory support. 

The basic laboratory diagnostic tests—and how they should be used at the national, state, 

and local level—are outlined below. The initial designation of reference and regional 

laboratories that can do all testing will be based on the availability of biosafety level 3 (BSL3) 

containment facilities. Details of the surveillance case definition for human West Nile virus 

(WNV) disease and of how the laboratory diagnostic tests are used to support surveillance 

are presented in Appendix B. 

A. Biocontainment 

1. Laboratory Safety Issues 

Laboratory-associated infections with WNV have been reported in the literature. The 

Subcommittee on Arbovirus Laboratory Safety (SALS) in 1980, reported 15 human 

infections from laboratory accidents. One of these infections was attributed to 

aerosol exposure. Recently, two parenteral inoculations have been reported during 

work with animals. 

a) WNV may be present in blood, serum, tissues and CSF of infected humans, 

birds, mammals and reptiles. The virus has been found in the oral fluids and 

feces of birds. Parenteral inoculation with contaminated materials poses the 

greatest hazard; contact exposure of broken skin is a possible risk. Sharps 

precautions should be strictly adhered to when handling potentially infectious 

materials. Workers performing necropsies on infected animals may be at high 

risk of infection. 

b) Biosafety Level 2 practices and facilities are recommended for activities for 

human diagnostic specimens. In some cases it may be advisable to perform 

initial processing of clinical samples in a biosafety cabinet, particularly if high 

levels of virus is suspected (such as tissues from fatal human cases). Biosafety 

Level 2 is recommended for processing field collected mosquito pools. Biosafety 

Level 3 and Animal Biosafety Level 3 practices, containment equipment, and 

facilities are recommended, respectively, for all manipulations of West Nile 

cultures and for experimental animal and vector studies. Containment 

specifications are available in the Centers for Disease Control and 

Prevention/National Institutes of Health publication Biosafety in Microbiological 

and Biomedical Laboratories (BMBL).45 This document can be found online at 

both http://bmbl.od.nih.gov/ and 

http://www.cdc.gov/od/ohs/biosfty/bmbl4/bmbl4toc.htm. 

c) All bird necropsies should be done in a Class 2 biological safety cabinet. 
25

2. Shipping of Agents 

Shipping and transport of WNV and clinical specimens should follow current 

International Air Transport Association (IATA) and Department of Commerce 

recommendations. Because of the threat to the domestic animal population, a U.S.

Department of Agriculture (USDA) shipping permit is required for transport of known 

WNV isolates. For more information, visit the IATA dangerous goods Web site at 

http://www.iata.org/cargo/dg/, and the USDA Animal and Plant Health Inspection 

Service (APHIS), National Center for Import /Export’s Web site at 

http://www.aphis.usda.gov/ncie/.

B. Serologic Laboratory Diagnosis 

Accurate interpretation of serologic findings requires knowledge of the specimen. For 

human specimens the following data must accompany specimens submitted for serology 

before testing can proceed or results can be properly interpreted and reported: 1) 

symptom onset date (when known); 2) date of sample collection; 3) unusual 

immunological status of patient (e.g., immunosuppression); 4) state and county of 

residence; 5) travel history in flavivirus-endemic areas; 6) history of prior vaccination 

against flavivirus disease (e.g., yellow fever, Japanese encephalitis, or Central European 

encephalitis); and 7) brief clinical summary including clinical diagnosis (e.g., 

encephalitis, aseptic meningitis). 

1. Human 

a) Commercial kits for human serologic diagnosis of WNV infection are currently in 

development. Until these kits are available, the CDC-defined IgM and IgG ELISA 

should be the front-line tests for serum and CSF.46-48 These ELISA tests are the 

most sensitive screening assays available. The HI and indirect 

immunofluorescent antibody (IFA) test may also be used to screen samples for 

flavivirus antibodies. Laboratories performing HI assays need be aware that the 

recombinant WNV antigens produced to date are not useful in the HI test; mouse 

brain source antigen (available from CDC) must be used in HI tests. The 

recombinant WNV antigen is available from commercial sources. 

b) To date, the prototype WNV strains Eg101 or NY99 strains have performed 

equally well as antigens in diagnostic tests for WNV in North America. 

c) To maintain Clinical Laboratory Improvements Amendments (CLIA) certification, 

CLIA recommendations for positive and negative ranges should be followed, and 

laboratories doing WNV testing should participate in a proficiency testing program 

through experienced reference laboratories; CDC’s Division of Vector-Borne 

Infectious Diseases in Fort Collins, Colorado and the National Veterinary Services 

Laboratories in Ames, Iowa both offer this type of program. 

d) Because the ELISA can cross-react between flaviviruses (e.g., SLE, dengue, 

yellow fever, WN), it should be viewed as a screening test only. Initial 

serologically positive samples should be confirmed by neutralization test. 

Specimens submitted for arboviral serology should also be tested against other 

arboviruses known to be active or be present in the given area (e.g., test against 

SLE, WN and EEE viruses in Florida). 
26 

2. Animal 

a) In general, the procedures for animal serology should follow those used with 

humans cited above. 

b) Plaque-reduction neutralization test (PRNT) and HI assays, although technically 

more demanding, may be useful because they are species independent. 

C. Virologic Laboratory Diagnosis 

Experience gained in WNV diagnostic testing over the past 4 years has led to the 

following recommendations: 

1. Virus Isolation 

a) Virus isolation attempts should be performed in known susceptible mammalian or 

mosquito cell lines. Mosquito origin cells may not show cytopathic effect and 

should be screened by immunofluorescence. 

b) Appropriate samples for virus isolation are prioritized as follows: 

1) Clinically ill humans - CSF (serum samples may be useful early in infection) 

2) Human (biopsy or postmortem) - brain tissue 

3) Horses (postmortem) - brain tissue (including brainstem), spinal cord tissue 

4) Birds - kidney, brain, heart 

5) Other mammals - multiple tissues, especially kidney and brain 

c) Confirmation of virus isolate identity can by accomplished by indirect 

immunofluorescence assay (IFA) using virus-specific monoclonal antibodies, 

nucleic acid detection, or virus neutralization. 

d) The IFA using well-defined murine monoclonal antibodies (MAbs) is the most 

efficient, economical, and rapid method to identify flaviviruses. MAbs are 

available that can differentiate WNV and SLE virus from each other and from 

other flaviviruses. Flavivirus-grouping MAbs are available for use as positive 

controls, and MAbs specific for other arboviruses can be used as negative 

controls. In addition, incorporating MAbs specific for other arboviruses known to 

circulate in various regions will increase the rapid diagnostic capacities of state 

and local laboratories. These reagents are available and should be used. 

e) Nucleic acid detection methods including RT-PCR, TaqMan and nucleic acid 

sequence based amplification (NASBA) methods may be used to confirm virus 

isolates as WNV. 

f) Virus neutralization assays also may be used to differentiate viruses, by using 

fourfold or greater titer differences as the diagnostic criterion in paired specimens 

(acute- and convalescent-phase). 
27

2. Virus Detection in Tissues 

a) Antigenic analysis

1) Immunohistochemistry (IHC) using virus-specific MAbs on brain tissue has 

been very useful in identifying both human and avian cases of WNV 

infection. In suspected fatal cases, IHC should be performed on formalinfixed autopsy, biopsy, and necropsy material, ideally collected from 

multiple anatomic regions of the brain, including the brainstem, midbrain, 

and cortex.24, 49

2) Well-characterized antigen-capture ELISAs are now available for 

detection of SLE50,51 and WNV antigen in mosquito pools and avian 

tissues.25 

b) Nucleic acid analysis

A number of nucleic acid detection methods have recently been employed for 

WNV diagnostic and surveillance purposes. An independent antigen or nucleic 

acid test is required to confirm detection of WNV nucleic acid with any of these 

methods. 

1) RT-PCR of tissues, mosquito pools, and CSF has proven to be a useful 

surveillance tool. RT-nested PCR has detected WNV nucleic acid in 

equine brain and spinal cord tissues. Standardized protocols developed 

by reference laboratories should be disseminated, and primer design 

information should be included so that other laboratories can prepare 

primers. A proficiency testing program should be developed by the 

reference laboratories so that these tests can be CLIA-certified in local 

laboratories. 

2) Fluorogenic 5' nuclease techniques (real-time PCR) and nucleic acid 

sequence-based amplification (NASBA) methods have been developed 

and have undergone initial validation in specific diagnostic 

applications.24,52-54

D. Training and Infrastructure

1. State and Local Arbovirus Laboratories 

Greater numbers of capable state and local laboratories performing screening assays 

(such as ELISA) should be developed to reduce time demands on reference 

laboratories. Reference laboratories should be utilized to confirm results of state and 

local laboratories, particularly for the initial identification of WNV in new locations and 

in new hosts. 

2. Training Programs 

Laboratory training programs have been developed and implemented at the federal 

level. Additional regional training programs may be beneficial. 
28 

III. PREVENTION AND CONTROL

Prevention and control of arboviral diseases is accomplished most effectively through a 

comprehensive, integrated mosquito management program using sound integrated pest 

management (IPM) principles.55 IPM is based on an understanding of the underlying biology 

of the transmission system, and utilizes regular monitoring to determine if and when 

interventions are needed to keep pest numbers below levels at which intolerable levels of 

damage, annoyance, or disease occur. IPM-based systems employ a variety of physical, 

mechanical, cultural, biological and educational measures, singly or in appropriate 

combination, to attain the desired pest population control. 

Programs consistent with best practices and community needs should be established at the 

local level and, at a minimum, should be capable of performing surveillance sensitive enough 

to detect West Nile Virus (WNV) enzootic/epizootic transmission that has been associated 

with increased risk of disease in humans or domestic animals. Integrated mosquito 

management programs designed to minimize risk of WNV transmission and prevent 

infections of humans and domestic animals should optimally include the following 

components (modified from information provided by the American Mosquito Control 

Association, the New Jersey Mosquito Control Association, and the Florida Coordinating 

Council on Mosquito Control)56-58

A. Surveillance 

 Effective mosquito control begins with a sustained, consistent surveillance program that 

targets pest and vector species, identifies and maps their immature habitats by season, 

and documents the need for control. Records should be kept on the species composition 

of mosquito populations prior to enacting control of any kind and to allow programs to 

determine the effectiveness of control operations. All components of the integrated 

management program must be monitored for efficacy using best practices and standard 

indices of effectiveness. The following is a list of surveillance methodologies used by 

mosquito control agencies. 

1. Larval Mosquito Surveillance

Larval surveillance involves sampling a wide range of aquatic habitats for the 

presence of pest and vector species during their developmental stages. Most 

established programs have a team of trained inspectors to collect larval specimens 

on a regular basis from known larval habitats, and to perform systematic surveillance 

for new sources. A mosquito identification specialist normally identifies the larvae’s 

species. Properly trained mosquito identification specialists can separate nuisance 

and vector mosquito species. Responsible control programs target vector and 

nuisance populations for control and avoid managing habitats that support benign 

species. 

2. Adult Mosquito Surveillance 

Adult mosquito surveillance is used to monitor species presence and relative 

abundance of adult mosquitoes in an area. Information derived from adult mosquito 

surveillance programs using standardized and consistent surveillance efforts provide 

information essential to monitoring potential vector activity, setting action thresholds, 

and evaluating control efforts. Various methods are available for this purpose and 
29

have been demonstrated to be effective in collecting a variety of mosquito species.59 

The New Jersey light trap, CDC’s miniature light trap, and other modifications of this 

design, with or without carbon dioxide bait, have been used extensively for collecting 

host-seeking adult mosquitoes.60 Gravid traps frequently are used to monitor the 

ovipositing segment of Cx. pipiens and Cx. restuans populations. These species 

have been incriminated as the primary enzootic vectors of WNV in the northeastern 

states.61,62 Host-seeking Cx. tarsalis, a species that has been strongly associated 

with WNV transmission in areas where this species is common, are readily collected 

in CO2-baited CDC miniature light traps. Resting boxes frequently are used to 

measure populations of Culiseta melanura, a bird-feeding mosquito that is important 

in the amplification of eastern equine encephalitis (EEE) virus. Pigeon-baited traps 

may be employed to measure host-seeking Culex mosquitoes that amplify St. Louis 

encephalitis (SLE) and West Nile viruses. Day-active mosquitoes like Ae. albopictus

are difficult to collect, and obtaining a sample representative of the local populations 

requires extra effort. Where these species are important, sample sizes may be 

enhanced by using CO2-baited CDC miniature light traps during daylight hours or by 

using alternative trap configurations that may be more effective in collecting these 

species (e.g., Fay trap or traps using a counterflow geometry). Trap deployment 

should carefully address species habitat requirements on several spatial scales. 

3. Virus Surveillance 

The purpose of this component of the vector management program is to determine 

the prevalence of WNV in the mosquito population. This is often expressed simply 

as the number of WNV-positive mosquito pools of a given species collected at a 

defined location and time period. While the number of positive pools provides 

valuable information, it does not provide an index of virus prevalence in the vector 

population. Preferably, the proportion of the mosquito population carrying the virus 

should be expressed as the infection rate (IR, expressed as the estimated number of 

infected individual mosquitoes per 1,000 specimens tested). This is a more useful 

index of virus prevalence. The IR can be calculated by dividing the number of positive 

pools by the total number of specimens tested for that species and collection period, 

and multiplying the proportion by 1,000. This assumes that a positive pool contains 

only one infected mosquito, which is a valid assumption in most circumstances. 

When infection rates are high or sample sizes are low, a more accurate estimate of 

IR may be obtained by using a maximum likelihood estimate of the infection rate – 

see www.cdc.gov/ncidod/dvbid/westnile/software.htm. Elevated infection rates, 

particularly if sustained over several weeks or in populations of opportunistic bloodfeeders that may act as bridge vectors, are indicators of increased WNV transmission 

risk. Specimens collected in the routine adult mosquito surveillance program plus 

special collections from key areas identified by other surveillance indicators (e.g., 

dead birds, sentinel flocks) can be used for this purpose. Mosquito collections made 

at permanent study sites in a sustained program provide important baseline data to 

which new surveillance data are compared and decisions about human risk and need 

for emergency interventions are made. 

B. Source Reduction 

Source reduction is the alteration or elimination of mosquito larval habitat breeding. This 

remains the most effective and economical method of providing long-term mosquito 

control in many habitats. Source reduction can include activities as simple as the proper 
30 

disposal of used tires and the cleaning of rain gutters, bird baths and unused swimming 

pools by individual property owners, to extensive regional water management projects 

conducted by mosquito control agencies on state and/or federal lands. All of these 

activities eliminate or substantially reduce mosquito breeding habitats and the need for 

repeated applications of insecticides in the affected habitat. Source reduction activities 

can be separated into the following two general categories: 

1. Sanitation 

The by-products of human’s activities have been a major contributor to the creation of 

mosquito breeding habitats. An item as small as a bottle cap or as large as the 

foundation of a demolished building can serve as a mosquito breeding area. 

Sanitation, such as tire removal, stream restoration, catch-basin cleaning and 

container removal, is a major part of all integrated vector management programs. 

Mosquito control agencies in many jurisdictions have statutory powers that allow for 

due process and summary abatement of mosquito-related public health nuisances 

created on both public and private property. The sanitation problems most often 

resolved by agency inspectors are problems of neglect, oversight, or lack of 

information on the part of property owners. Educational information about the 

importance of sanitation in the form of videos, slide shows, and fact sheets 

distributed at press briefings, fairs, schools and other public areas are effective. 

2. Water Management 

Water management for mosquito control is a form of source reduction that is 

conducted in fresh and saltwater breeding habitats. Water management programs for 

vector control generally take two forms, described below. Water management 

through impoundment and open water management have been very effective in the 

past. Recently, restrictions on modification of aquatic habitats have limited the 

implementation of these practices, and in many areas, water management for vector 

control is no longer routinely employed and may be impractical in many settings. In 

these situations, alternative methods of mosquito management must be employed. 

a) Impoundment Management

Impoundments are mosquito-producing marshes around which dikes are 

constructed, thereby allowing water to stand or to be pumped onto the marsh 

surface from the adjacent estuary. This eliminates mosquito oviposition sites on 

the impounded marsh and effectively reduces their populations. Rotational 

Impoundment Management (RIM) is the technique developed to minimally flood 

the marsh during the summer months and then use flapgated culverts to 

reintegrate impoundments to the estuary for the remainder of the year, thereby 

allowing the marsh to provide many of its natural functions. Although 

impoundments usually achieve adequate control of salt-marsh mosquitoes, there 

are situations in which impoundments can collect stormwater or rainwater and 

create freshwater mosquito problems that must be addressed using other 

techniques. 

b) Open Marsh Water Management (OMWM)
31

 Ditching as a source-reduction mosquito control technique has been used for 

many years. Open marsh water management is a technique whereby mosquitoproducing locations on the marsh surface are connected to deep-water habitat 

(e.g., tidal creeks, deep ditches) with shallow ditches. Mosquito broods are 

controlled without pesticide use by allowing larvivorous fish access to 

mosquito-producing depressions. Conversely, the draining of these locations 

occurs before adult mosquitoes can emerge. OMWM can also include 

establishing or improving a hydrological connection between the marsh and 

estuary, providing natural resource enhancement as well as mosquito control 

benefits. The use of shallow ditching (ditches approximately 3 feet or less in 

depth rather than the deep ditching used in years past) is considered more 

environmentally acceptable because than deep ditching because fewer unnatural 

hydrological impacts occur to the marsh. 

c) Management in Stormwater Retention Structures

Source reduction and water management practices may also be applied to 

stormwater retention structures designed to hold runoff before it is discharged 

into groundwater or surface water. Mosquito control should be considered in the 

design, construction, and maintenance of these structures, as appropriate. 

Stormwater retention structures should be designed in consultation with experts 

in mosquito biology and control to prevent as much mosquito production as 

possible, and to facilitate proper functioning and maintenance in the future. 

Regulations associated with stormwater retention and flood control structures 

should incorporate appropriate operations and maintenance provisions including 

considerations for routine monitoring and control of mosquito populations. 

C. Chemical Control 

Insecticides can be directed against either the immature or adult stage of the mosquito 

life cycle when source reduction and water management are not feasible or have failed 

because of unavoidable or unanticipated problems, or when surveillance indicates the 

presence of infected adult mosquitoes that pose a health risk. 63 Chemicals used by 

mosquito control agencies must comply with state and federal requirements. Public 

health pesticide applicators and operators in most states are required to be licensed or 

certified by the appropriate state agencies. 

1. Larviciding 

Larviciding, the application of chemicals to kill mosquito larvae or pupae by ground or 

aerial treatments, is typically more effective and target-specific than adulticiding, but 

less permanent than source reduction. An effective larviciding program is an 

important part of an integrated mosquito control operation. The objective of larviciding 

is to control the immature stages at the breeding habitat before adult populations 

have had a chance to disperse and to maintain populations at levels at which the risk 

of arbovirus transmission is minimal. Larvicides can be applied from the ground or by 

aerial application if large or inaccessible areas must be treated. Several materials in 

various formulations are labeled for mosquito larviciding including the 

organophosphate temephos (Abate); several biological larvicides such as Bacillus 

thuringiensis israelensis (Bti, a bacterial larvicide), Bacillus sphaericus; methoprene, 

an insect growth regulator (e.g., Altosid,); several larvicidal oils (e.g., petroleum-
32 

based Golden Bear and mineral-based Bonide) and monomolecular surface films 

(e.g., Agnique, Arosurf); and in some limited habitats diflubenzuron (e.g., Dimilin, a 

chitin synthesis inhibitor). Applications of larvicides often encompass fewer acres 

than adulticides because treatments are made to relatively small areas where larvae 

are concentrated, as opposed to larger regions where adults have dispersed. When 

applying larvicides, it is important that the material be specific for mosquitoes, 

minimize impacts on non-target organisms, and, where appropriate, be capable of 

penetrating dense vegetation canopies. Larvicide formulations (i.e., liquid, granular, 

solid) must be appropriate to the habitat being treated, accurately applied, and based 

on surveillance data. Accuracy of application is important because missing even a 

relatively small area can cause the emergence of a large mosquito brood resulting in 

the need for broad-scale adulticiding. 

2. Adulticiding 

Adulticiding is the application of pesticides to kill adult mosquitoes. The ability to 

control adult mosquitoes is an important component of any integrated mosquito 

management program, and like the other components of the program, its use should 

be based on surveillance data. Mosquito adulticiding may be the only practical control 

technique available in situations where surveillance data indicate that is necessary to 

reduce the density of adult mosquito populations quickly to lower the risk of WNV 

transmission to humans. In some situations, source reduction and larvicide 

applications are not practical, and adulticide application is the only available control 

strategy. Mosquito adulticides typically are applied as an Ultra-Low-Volume (ULV) 

spray where small amounts of insecticide are dispersed either by truck-mounted 

equipment or from fixed-wing or rotary aircraft.64-68 Thermal fog applications of 

adulticides by ground or air are also used in some areas, but to a much lesser 

degree. Barrier treatments, typically applied as high volume liquids with hand-held 

spray equipment using compounds with residual characteristics, are common in 

some U.S. locations. This technique is especially attractive to individual homeowners 

living near mosquito-producing habitats where residual chemicals applied along 

property boundaries can provide some control benefits. Mosquito adulticiding differs 

fundamentally from techniques used to control many other adult insects. For adult 

mosquito control, insecticide must drift through the habitat in which mosquitoes are 

flying in order to provide optimal control benefits. The EPA has determined that the 

insecticides labeled nationally for this type of application do not pose unreasonable 

health risks to humans, wildlife, or the environment when used according to the 

label.56 Adulticides labeled for mosquito control include several organophosphates 

such as malathion and naled. Some natural pyrethrins and synthetic pyrethroids 

(permethrin, resmethrin and sumithrin) also hold adulticide labels. Insecticide 

selection and timing of application should be based on the distribution and behavior 

of the target mosquito species. Application of adulticides should be timed to coincide 

with the activity period of the target mosquito species. Many Culex species are 

nocturnal and are active in the tree canopy level. This should be taken into 

consideration when planning adulticide applications. Operational experience 

indicates that Cx. pipiens and Cx. quinquefasciatus may require more frequent 

adulticide application to achieve desired levels of population reduction during an 

outbreak. Control of adult day-active species poses additional problems because 

ULV adulticide effectiveness is greatly reduced during daylight hours. Early-morning 

use of adulticides, applied before temperatures rise, may provide a measure of 

control for these species. 
33

D. Resistance Management 

In order to delay or prevent the development of insecticide resistance in vector 

populations, integrated vector management programs should include a resistance 

management component (modified from Florida Coordinating Council on Mosquito 

Control, 1998).57 Ideally, this should include annual monitoring of the status of resistance 

in the target populations to: 

1. Provide baseline data for program planning and pesticide selection before the start of 

control operations. 

2. Detect resistance at an early stage so that timely management can be implemented 

(even detection of resistance at a late stage can be important in elucidating why 

disease control may fail); however, in such cases, management options other than 

replacement of the pesticide may not be possible). 

3. Continuously monitor the effect of control strategies on resistance. In addition to 

monitoring resistance in the vector population, the integrated program should include 

options for managing resistance that are appropriate for the local conditions.69-70 The 

techniques regularly used include the following: 

a) Management by Moderation - preventing onset of resistance by 

1) Using dosages no lower than the lowest label rate to avoid genetic selection. 

2) Using less frequent applications. 

3) Using chemicals of short environmental persistence. 

4) Avoiding slow-release formulations. 

5) Avoiding the use of the same class of insecticide to control both adults and 

immature stages. 

6) Applying locally. Currently, most districts treat only hot spots. Area-wide 

treatments are used only during public health alerts or outbreaks. 

7) Leaving certain generations, population segments, or areas untreated. 

8) Establishing high pest mosquito densities or action thresholds prior to 

insecticide application. 

9) Alternation of biorational larvicides and insect growth regulators annually or at 

longer intervals. 

b) Management by continued suppression - a strategy used in areas of high-value 

(e.g., heavily touristed areas) or where arthropod vectors of disease must be kept 

at very low densities.

This does not mean saturation of the environment by pesticides, but rather the 

saturation of the defense mechanisms of the insect by insecticide dosages that 

can overcome resistance. This is achieved by the application of dosages within 

label rates but sufficiently high to be lethal to susceptible as well as to 

heterozygous-resistant individuals. If the heterozygous individuals are killed, 

resistance (which is a homozygous trait) will be slow to emerge. This method 
34 

should not be used if any significant portion of the population in question is 

resistant. Another approach more commonly used is the addition of synergists 

that inhibit existing detoxification enzymes and thus eliminate the competitive 

advantage of these individuals. Commonly, the synergist of choice in mosquito 

control is piperonyl butoxide (PBO). 

c) Management by multiple attack - achieving control through the action of several 

different and independent pressures such that selection for any one of them 

would be below that required for the development of resistance. 

This strategy involves the use of insecticides with different modes of action in 

mixtures or in rotations. There are economic problems (e.g., costs of switching 

chemicals or having storage space for them) associated with this approach, and 

critical variables in addition to mode of action must be taken into consideration 

(i.e., mode of resistance inheritance, frequency of mutations, population dynamics 

of the target species, availability of refuges, and migration). General 

recommendations are to evaluate resistance patterns at least annually and the 

need for rotating insecticides at annual or longer intervals. 

E. Biological Control 

Biological control is the use of biological organisms, or their by-products, to control pests. 

Biocontrol is popular in theory, because of its potential to be host-specific and virtually 

without non-target effects. Overall, larvivorous fish are the most extensively used 

biocontrol agent for mosquitoes. Predaceous fish, typically Gambusia or other species 

which occur naturally in many aquatic habitats, can be placed in permanent or 

semipermanent water bodies where mosquito larvae occur, providing some measure of 

control. Other biocontrol agents that have been tested for mosquito control, but that to 

date generally are not widely used, include the predaceous mosquito Toxorhynchites, 

predacious copepods, the parasitic nematode Romanomermis, and the fungus 

Lagenidium giganteum. Biocontrol certainly holds the possibility of becoming a more 

important tool and playing a larger role in mosquito control in the future, but will likely be 

effective only as part of an integrated approach. 

F. Continuing Education of Mosquito Control Workers 

Continuing education is directed toward operational workers to instill or refresh 

knowledge related to practical mosquito control. Training is primarily in safety, applied 

technology, and requirements for the regulated certification program mandated by most 

states. 

G. Vector Management in Public Health Emergencies 

A surveillance program adequate to monitor WNV activity levels associated with human 

risk must be in place. Detection of epizootic transmission of enzootic arboviruses 

typically precedes detection of human cases by several days to 2 weeks or longer (e.g., 

as found in SLE epidemics).71,72 If adequate surveillance is in place, the lead time 

between detecting significant levels of epizootic transmission and occurrence of human 

cases can be increased, which will allow for more effective intervention practices.19,27,31 

Early-season detection of enzootic or epizootic WNV activity appears to be correlated 

with increased risk of human cases later in the season. Control activity should be 

intensified in response to evidence of virus transmission, as deemed necessary by the 

local health departments. Such programs should consist of public education 
35

emphasizing personal protection and residential source reduction; municipal larval 

control to prevent repopulation of the area with competent vectors; adult mosquito control 

to decrease the density of infected, adult mosquitoes in the area; and continued 

surveillance to monitor virus activity and efficacy of control measures. 

As evidence of sustained or intensified virus transmission in an area increases, 

emergency response should be implemented. This is particularly important in areas 

where vector surveillance indicates that infection rates in Culex mosquitoes are 

increasing, or that potential accessory vectors (e.g., mammalophilic species) are infected 

with WNV. Delaying adulticide applications in such areas until human cases occur is 

illogical and negates the value and purpose of the surveillance system. 

H. Adult Mosquito Control Recommendations 

Ground-based (truck-mounted) application of adult mosquito control agents has several 

positive attributes. Where road access is adequate, such as in urban and suburban 

residential areas, good coverage may be achieved. In addition, ground-based 

application can be done throughout the night, thereby targeting night-active mosquito 

species. Such applications are prone to skips and patchy coverage in areas where road 

coverage is not adequate or in which the habitat contains significant barriers to spray 

dispersal and penetration. 

Aerial application is capable of covering larger areas in shorter time periods than a 

ground-based application. This is a critical positive attribute when large residential areas 

must be treated quickly. In addition, aerial application is less prone to patchy coverage 

than ground-based application in areas where road coverage is not adequate. One 

limitation of aerial application is that many applicators will not fly at night, potentially 

reducing the effectiveness of the applications in Culex species control efforts. Cost 

benefits of aerial application over ground application may not be realized unless relatively 

large areas are treated. 

Several formulations of a variety of active ingredients are available for adulticide 

applications. Material choice for ground-based or aerially applied mosquito control in 

public health emergency situations is limited by EPA restrictions on the pesticide label 

and applicable state and local regulations. 

Multiple applications will likely be required to appreciably reduce Culex populations and 

interrupt arbovirus transmission. An emergency SLE virus response plan developed for 

New Orleans, Louisiana63 indicates the need for repeated applications to control Cx. 

quinquefasciatus, and the need to repeatedly apply adulticides in high-risk areas (areas 

with human cases or positive surveillance events). Two to three adulticide applications 

spaced 3-4 days apart may be required to significantly reduce Cx. pipiens populations. 

Effective surveillance must be maintained to determine if and when re-treatment is 

required to maintain suppression of the vector populations. 

Urban/suburban population centers with multiple positive surveillance events as 

described above should be treated first to most efficiently protect the largest number of 

people from exposure to WNV. Applications should be timed to coincide with the peak 

activity periods of the target species. For example, applications should be made at night 

to maximize control of night-active Culex species. Other species such as Oc. sollicitans

or Ae. vexans are active shortly after sunset and are effectively controlled with 
36 

appropriately timed applications. Day-active potential accessory vectors (e.g., Oc. 

japonicus, Oc. triseriatus, Ae. albopictus) must be addressed separately and are most 

effectively controlled by residential source reduction efforts, though there is preliminary 

evidence that early morning ULV applications may be used to control these species. 

I. Determining the Scope of Mosquito Adulticiding Operations

Once arbovirus activity is detected in a jurisdiction and a decision is made to implement 

or intensify mosquito control by using adulticides, the size of the area to be treated must 

be determined. In the broadest context, the underlying program objective (i.e., 

interruption of the enzootic transmission cycle vs. prevention of transmission to humans 

and domestic animals) should determine the amount of adulticide coverage that is 

required. For most jurisdictions the objective is the prevention of transmission to humans 

and domestic animals. There is no simple formula for determining how large an area to 

treat around a positive surveillance indicator or a suspected or confirmed human case of 

WNV. Nor is there adequate information to guide decisions about the degree of vector 

population suppression that must be attained, or for how long this suppression must be 

maintained to reduce human disease risk. At a minimum, the following factors must be 

considered when deciding the scope of the adulticiding effort: 

1. The general ecology of the area, e.g., key habitat types and the presence of natural 

barriers such as large rivers; 

2. The population density, distribution, flight range, and age structure (proportion of 

parous females) of the target mosquito species; 

3. The flight range of the avian amplifying host(s); 

4. The length of time since birds started dying or became infected in the affected area 

(typically, there may be a lag of several weeks between recovery of dead birds and 

confirmation of WNV infection) or since virus-positive mosquito pools were collected; 

5. The human population characteristics – spatial distribution and density relative to the 

positive locality (e.g., urban vs. rural), age demographics; 

6. Evidence of persistent WNV activity detected by the surveillance program; and 

7. Season of the year and how long WNV activity can be expected to persist until the 

epizootic/epidemic vector(s) enter diapause. 

Several of these factors will be unknown or poorly understood. Technical assistance 

from a mosquito control professional, particularly one experienced in mosquito 

control in the region, is crucial in this process. Practical experience in conducting 

mosquito control is required to refine control recommendations. For example, the 

size of an area selected for control applications may be reduced in response to 

structures like open areas, bodies of water, major highways, or other barriers that 

may restrict the distribution of targeted species. Alternatively, adulticide coverage 

may be expanded to cover large urban or suburban residential neighborhoods with 

dense human populations. 

Hypothetically, in some settings where focal early season enzootic WNV activity has 

been detected, early season adulticiding may be useful in interrupting virus 

transmission and lead to lower transmission rates later in the season. However, 

effective larval control of the principal enzootic mosquito vector is probably a more 

cost-effective way to interrupt early-season virus amplification. 
37

J. Evaluation of Adult Mosquito Control 

The following parameters should be periodically monitored during control operations: 

1. Minimum requirements: 

a) Pre- and post spray vector mosquito densities inside and outside control area 

using CO2-baited traps and gravid traps; 

b) Vector mosquito infection rates pre- and post-spray inside and outside the control 

area; and 

c) Weather conditions during application (temperature, wind speed, direction). 

2. Desirable additions if capacity exists: population age structure of key mosquito 

species (Cx. pipiens) 

3. In addition, both droplet size and flow rate should be documented for each piece of 

ULV application equipment: 

4. During aerial application, GPS monitoring of spray track should be conducted if 

equipment is available on aircraft. 

K. Health Education, Public Information, and Human Behavior Change 

The goals of health education, public information, and behavior change programs are to 

inform the public about WNV, promote the adoption of preventive behaviors that reduce 

disease risk, and gain public support for control measures. Health education/public 

information includes use of print materials (posters, brochures, fact sheets), electronic 

information (Web sites), presentations (health experts or peers speaking to community 

groups), and the media. 

Information alone is seldom sufficient to encourage people to adopt new behaviors or to 

change old practices. Programs should include strategies to facilitate protective actions 

and to address barriers that hinder preventive actions. Examples of programs that go 

beyond information include developing a community task force, interventions to improve 

access to window screening materials or repellents, and social marketing to reinforce 

preventive behaviors. 

The following section covers key prevention messages, selected best practices, and 

research/program development priorities for promotion of personal and community 

measures to decrease risk of WNV infection. Public education and risk communication 

activities must be ramped up to respond to the degree of WNV risk in a community, as 

noted in Table 1. 

 1. Key WNV Prevention Messages 

 a) Address the multiple levels at which prevention can occur: personal protection 

(use of repellent on skin and clothing, use of protective clothing, awareness of 

prime mosquito-biting hours); household protection (eliminating mosquito 

breeding sites, repairing/installing screens); and community protection (reporting 

dead birds, advocating for organized mosquito abatement, participating in 

community mobilization). 
38 

b) Use of DEET-based repellents on skin and clothing is the backbone of personal 

protection. (For current recommendations, see 

www.cdc.gov/ncidod/dvbid/westnile/qa/insect_repellent.htm.) Permethrin-based 

repellents should be promoted for use on clothing. 

c) Emphasize the feasibility of actions that can lower an individual’s WNV risk 

through personal protection measures. Messages should acknowledge the 

seriousness of the disease but should not be fear-driven. Fear-driven messages 

may heighten the powerlessness many people express in dealing with emerging 

diseases. 

d) Recommendations to avoid being outdoors from dusk to dawn may conflict with 

neighborhood social patterns or practices of persons without air-conditioning or 

without other health programs seeking to increase physical activity. An 

alternative is to emphasize that the hours from dusk until dawn are prime 

mosquito-biting hours, and that protecting oneself through repellent use during 

these hours is important, with the option of remaining indoors. 

e) Communication about adulticiding: Public acceptance of emergency adult 

mosquito control is critical to its success, especially where mosquito control is 

unfamiliar or unpopular. Questions about the products being used, their safety, 

and their effects on the environment are common. Improved communication 

about surveillance and how decisions to adulticide are made may help residents 

weigh the risks and benefits of control. When possible, provide detailed 

information regarding the schedule for adulticiding through newspapers, radio, 

the Internet, or a recorded phone message 

f) Keep messages clear and consistent with the recommendations of coordinating 

agencies. Use plain language whenever possible, and adapt materials for lowerliteracy and non-English speaking audiences. 

 2. Selected Best Practices 

a) Targeted prevention

Audience members have different disease-related concerns and motivations for 

action. Proper message targeting permits better use of limited communication 

and prevention resources. The following are some audience groups that require 

specific targeting: 

1) Persons over age 50: While persons of any age can be infected with WNV, 

US surveillance data indicate that persons over age 50 are at higher risk for 

severe disease and death due to WNV infection.

Collaborate with organizations that have an established relationship with 

mature adults, such as the AARP, senior centers, or programs for adult 

learners. Include images of older adults in your promotional material. Identify 

activities in your area where older adults may be exposed to mosquito bites 

(e.g. jogging, golf, gardening). 

2) Persons with outdoor exposure: While conclusive data are lacking, it is 

reasonable to infer that persons engaged in extensive outdoor work or 

recreational activities are at greater risk of being bitten by WNV-infected 

mosquitoes. Develop opportunities to inform people engaged in outdoor 

activities about WNV. Encourage use of repellent and protective clothing, 
39

particularly if outdoors during evening, night, or early morning hours. Local 

spokespersons (e.g., union officials, job-site supervisors, golf pros, gardening 

experts) may be useful collaborators. 

3) Homeless persons: Extensive outdoor exposure and limited financial 

resources in this group present special challenges. Application of repellents 

with DEET or permethrin to clothing may be most appropriate for this 

population. Work with social service groups in your area to reach this 

population segment. 

4) Persons who live in residences lacking window screens: The absence of 

intact window/door screens is a likely risk factor for exposure to mosquito 

bites. Focus attention on the need to repair screens and resources to do so. 

Partner with community organizations that can assist elderly persons or 

others with financial or physical barriers to screen installation or repair. 

b) Partnerships with media and the community

Cultivate relationships with the media. Obtain media training for at least one 

member of your staff, and designate that individual as the organization’s 

spokesperson. Develop clear press releases and an efficient system to answer 

press inquiries. 

Develop partnerships with agencies/organizations that have relationships with 

populations at higher risk (such as persons over 50) or are otherwise recognized 

as community leaders (e.g., churches, service groups). Working through sources 

trusted by the target audience can heighten the credibility of and attention to 

messages. Partnerships with businesses that sell materials to fix or install 

window screens or that sell insect repellent may be useful in some settings. 

c) Community mobilization and community outreach

Community mobilization can further education and behavior change goals. To 

counter any idea that health departments/mosquito control programs are able to 

control WNV alone, develop community ownership for prevention activities. A 

community task force that includes civic, business, health, and environmental 

concerns can be valuable in achieving buy-in from various segments of society 

and in developing a common message. Community mobilization activities can 

include clean-up days to get rid of mosquito breeding sites. 

Community outreach involves presenting messages in person, in addition to 

media and educational materials, and incorporating citizens in prevention 

activities. Hearing the message of personal prevention from community leaders 

can validate the importance of the disease. Health promotion events reinforce 

the importance of prevention in a community setting. 

3. Research and Program Development Priorities 

a) Audience research

Attitudes toward arboviral disease prevention vary considerably by region. 

Previous experience with nuisance mosquitoes and mosquito control will affect 
40 

the acceptability of prevention efforts. Audience research can identify local 

attitudes, motivations, barriers to prevention, and opportunities to promote 

desired behaviors. 

Audience research should ideally combine qualitative and quantitative efforts. 

Surveys assessing knowledge, attitude, and practice levels in the target 

population can be very helpful, especially in evaluation, though they are a 

substantial undertaking. Qualitative research techniques, such as interviews and 

focus groups, can yield valuable data, and are more adaptable to resource levels. 

Expertise to undertake such efforts may be available from other divisions within a 

health department (e.g., chronic disease programs, maternal and child health). 

Pretesting of educational materials is an important step to ensure the usability of 

materials by the intended audience. Pretesting does not always have to involve 

considerable time or expense; simply having representatives of the intended 

audience review materials before printing will be useful. 

b) Evaluation

Outcome evaluation should be conducted whenever possible to measure the 

efficacy of the intervention in achieving protective behaviors (e.g., frequency of 

repellent use, presence of household mosquito breeding sites). Outcome 

measurement requires extensive effort and must be planned from the outset of a 

program. 

c) Social marketing and risk communication

The goal of social marketing is to achieve specific behaviors, using the concepts 

of product, price, place, and promotion. Use of social marketing approaches can 

help programs plan to achieve specific behavior change goals. 

Risk communication is already used by many health departments, and can be 

useful in refining communication messages for WNV, especially as the disease 

becomes endemic in new areas, and in discussing community control. Risk 

communication can help people analyze the choices that are available to them 

and to their community. 

4. Resources 

The CDC Web site (www.cdc.gov/westnile) is updated frequently to reflect new 

findings and recommendations. Materials on the CDC Web site are generally in the 

public domain, and serve as a resource for state and local health departments and 

other organizations. 

CDC staff can provide technical assistance in the development of audience research 

and strategies for public education and community outreach. Contact CDC/Division 

of Vector-Borne Infectious Diseases’ health communication staff at 970-221-6400. 

CDC can provide other communication planning resources, including CDCynergy 

(www.cdc.gov/cdcynergy/), an interactive CD-ROM designed to help systematically 

plan health communication programs. 
41

Other organizations that can provide useful information are the American Mosquito 

Control Association (www.mosquito.org/) and the National Pesticide Information 

Center (NPIC) (npic.orst.edu), a program of EPA and Oregon State University 

concerning pesticides and repellents. They can be contacted at 1-800-858-7378. 

L. Legislation 

In addition to statutes permitting legal action to abate mosquito-related public health 

nuisances, legislation must be in place to allow creation of and provide funding for 

municipally-based integrated mosquito management programs. Local jurisdictions can 

contact state mosquito control associations to provide examples of enabling legislation. 

M. Guidelines for a Phased Response to WNV Surveillance Data 

The principal goal is to minimize the health impact of the WNV in humans, as well as in 

domestic and zoo animals. Given the limited understanding of the ecology and 

epidemiology of WNV in the U.S., the low incidence of arboviral encephalitis, and the 

limitations of prevention methods, prevention and control measures, regardless of 

intensity, may not prevent all WNV infections in humans. 

The recommended response levels for the prevention and control of WNV should 

augment, but not replace, long-standing mosquito control efforts by established 

programs. These programs often have two objectives: 1) to control nuisance 

mosquitoes, and 2) to control vector mosquitoes that can transmit pathogenics. 

Nuisance mosquito control often has different objectives than vector control, and the 

target mosquito species may also differ. Established mosquito control programs often 

have long-standing experience with the surveillance and control of indigenous 

neurotropic arboviruses such as SLE virus. These programs have established 

thresholds for response based on historical data. Long-standing experience with WNV 

does not exist in the U.S. 

These guidelines for the prevention and control of WNV should be interpreted according 

to the following considerations: 

1. All states should prepare for WNV activity. Given the extensive geographic spread of 

WNV since 1999, its occurrence in many different habitats and ecosystems in the Old 

World, its expansion into numerous habitat types in the Western Hemisphere, and 

the fact that SLE virus, a related flavivirus, is widespread in the U.S., there appear to 

be no barriers to the spread of WNV throughout the U. S. At a minimum, a plan for 

the surveillance, prevention, and control of WNV should be developed at the state 

and local levels. 

2. 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. Data from NYC indicate that human WNV disease cases were more 

likely to occur in counties that had experienced more than 0.1 dead crow reports per 

square mile per week. In the Staten Island outbreak of 2000, the density exceeded 

1.5 dead crow reports per square mile per week. Also, analysis of 2001 and 2002 

surveillance data indicate that counties reporting WNV-infected dead birds early in 
42 

the transmission season are more likely to report subsequent WNV disease cases in 

humans than are counties that do not report early WNV-infected dead birds. These 

observations should be interpreted as a guide rather than an absolute. Levels of 

epizootic activity that correlate with increased human risk will vary by region. 

3. Flexibility is required when implementing the guidelines. Knowledge gained from 

ongoing surveillance and research could change the phased response 

recommendations. Specific and detailed recommendations that will fit all possible 

scenarios are not possible, particularly at a local level. Therefore, public health 

action should depend on interpreting the best available surveillance data in an area, 

in light of these general guidelines. In addition, the following factors should be 

considered when translating these guidelines into a plan of action: 

a) Current weather and predicted climate anomalies; 

b) Quality, availability, and timeliness of surveillance data; 

c) Feasibility of the planned prevention and control activities, given existing budgets 

and infrastructure; 

d) Public acceptance of the planned prevention and control strategies; 

e) Expected future duration of WNV transmission (surveillance events earlier in the 

transmission season will generally have greater significance); and 

f) Other ongoing mosquito control activities, such as nuisance mosquito control or 

vector mosquito control for the established arboviral encephalitis viruses. 

The recommended phased response to WNV surveillance data is shown in Table 1. 

Local and regional characteristics may alter the risk level at which specific actions must 

be taken. 
43

Table 1. Suggested Guidelines for Phased Response to WNV Surveillance Data 

 Risk 

category 

Probability 

of human 

outbreak 

 

Definition 

 

Recommended response* 

 0 None 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, 

human encephalitis/meningitis and equine illness). 

 

2 Low 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. 
44 

5 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. 
45

IV. HEALTH DEPARTMENT INFRASTRUCTURE

State and Local Health Departments 

In the 48 contiguous United States, state and local health departments should have a 

functional arbovirus surveillance and response unit, staffed by well-trained personnel who 

have adequate data-processing resources, appropriate laboratory facilities, and an adequate 

operating budget. The size and complexity of these units will vary by jurisdiction, depending 

on both the risk of arboviral transmission in the area and available resources. A functional 

arbovirus surveillance unit at the state level should be considered an essential component of 

any emerging infectious diseases program. Local health department expertise and 

capabilities should be supported in a manner that complements statewide programmatic 

goals. 

A. Staffing and Personnel 

Ideally, arboviral surveillance involves epidemiologists, virologists, medical 

entomologists, vertebrate biologists, veterinarians, laboratory staff, environmental 

toxicologists, public affairs personnel, and data managers. In a particular jurisdiction, the 

combination of personnel needed to conduct arboviral surveillance will depend on the 

importance of arboviral diseases in the area and on resources. Many health 

departments experience a chronic shortage or complete absence of medical 

entomologists and expertise in wildlife pathobiology. Addressing these deficiencies 

should be a high priority. In the event of an arboviral disease outbreak, local health 

departments will likely require significant surge capacity to ensure an adequate public 

health response. Contingency planning to identify resources to assist with the enhanced 

surveillance, laboratory, environmental, and public health needs should be identified 

ahead of time. 

B. Training and Consultation 

Opportunities exist at federal and state agencies for appropriate training of and 

consultation with laboratorians, medical entomologists, epidemiologists, vertebrate 

biologists, and others involved in arbovirus surveillance. 

C. Laboratory Capacity 

The infrastructure of arbovirus laboratories in the U.S. has deteriorated significantly in 

recent decades, not only in terms of the total number of functional laboratories and 

overall capacity, but also in terms of the staffing, physical plant, and financial support of 

many remaining laboratories. This is a problem of national scope and significance, the 

solution for which will require leadership at all levels of government. 

1. Testing for West Nile Virus (WNV) Infections 

In the wake of the introduction of WNV into the Western Hemisphere, it is important 

to distinguish between increasing short-term and long-term laboratory capacity. The 

latter is preferred and should be emphasized over the former. Laboratories with an 

existing capability for arbovirus serology should consider adding serologic screening 

tests for WNV to their repertoire. For serologic screening of patients and mosquito 

pools, arrangements can be made with CDC to transfer existing technology and 

reagents, and to obtain appropriate training. Samples giving positive or equivocal 
46 

screening results should be confirmed by CDC or another laboratory capable of 

definitive testing. For selected laboratories, similar technology transfer arrangements 

can be made with regard to RT-PCR primers for use in the testing of tissues and 

mosquito pools. In the wake of the recent epidemic of WN encephalitis in the 

Northeast, it is important that programs continue to routinely test for other arboviruses 

historically active in their area, such as St. Louis encephalitis, eastern equine 

encephalitis, western encephalitis, and La Crosse viruses, as well as for other causes 

of acute encephalitis. 

D. Developing Local Public Health Agency Infrastructure 

The function of local public health agencies is assessment, assurance, and policy 

development to promote and protect the health of the public. As part of this function, 

local public health agencies are responsible for preventive activities to reduce the risk of 

WNV infection to individuals in their jurisdictions. This responsibility includes educating 

communities about reducing mosquito breeding sites and taking personal protective 

measures. Local public health agencies also must have the capacity to assess human 

risk by gathering surveillance data or having access to surveillance data gathered on a 

district, regional, or statewide basis. These local public health agencies are important to 

formulating local recommendations on the indications and decisions concerning 

mosquito adulticiding. Education of and communication with the public, and 

maintenance of local media contacts are generally primary functions of the local public 

health agency. Included in this responsibility is communicating risk regarding the use of 

pesticides. 

The following infrastructure and functional capacities fall within the province of local 

public health agencies. Where these are not directly provided, access to these 

capacities is to be ensured). 

1. Risk assessment based on surveillance data (including mosquito, bird, and human 

data). Surveillance data may also include reports from individuals or healthcare 

providers indicating possible adverse health effects from pesticide use. 

2. Health education regarding personal protection, reduction of mosquito breeding sites 

and minimum health risks posed by approved pesticides applied according to the 

label.73,74

3. Communication with the media. 

4. Development of a preventive plan including education, mosquito source reduction, 

and larviciding. 

5. Public response capability, particularly when surges of public inquiries arise. This 

may include the use of telephone hotlines and Internet Web sites. 

6. Training of staff. 

7. Coordination with state and federal agencies. 

8. Local coordination by formulation of a task force with organizations such as 

departments of public works, offices of public affairs, city/county building 

management, departments of parks and recreation, departments of planning and 

zoning, property or building inspection services, police, public schools, colleges and 
47

universities, nonprofit and grassroots organizations, businesses, zoos, animal/vector 

control, local mosquito control districts, emergency medical services, hospitals, 

poison control centers, departments of game and inland fisheries, departments of 

environmental quality, emergency, management agencies, etc. 
48 

V. INTERJURISDICTIONAL DATA SHARING AND NATIONAL REPORTING OF 

HUMAN CASES

The public and animal health response to West Nile virus (WNV) epidemics/epizootics 

involves all levels of government, including the federal governments of the U.S. and 

neighboring countries, and the Pan American Health Organization. In addition, multiple 

government agencies at each level are often involved. Rapid, efficient, secure, and 

coordinated systems are needed to allow the sharing of human and ecologic data between 

these multiple agencies to support long-term surveillance activities, and to support activities 

that are part of the rapid outbreak response. 

During an epidemic involving multiple jurisdictions, CDC staff and other authorized persons 

will use Epi-X, a CDC-sponsored, Web-based system for secured electronic communication, 

or similar integrated communication systems, for rapid dissemination of information on public 

health events of public health significance. 

 A. Human Epidemiological, Clinical, and Laboratory Data Collection 

Patient confidentiality statutes vary among jurisdictions. Data can be shared between 

jurisdictions if recipients agree to adhere to the confidentiality statutes of the jurisdiction 

providing the data. Electronic databases should be appropriately secured by passwords 

to limit access and minimize opportunities for breaches in confidentiality or security. 

 B. National Reporting of Human WNV Disease Cases 

 

1. National Reporting of Human Cases of West Nile Meningoencephalitis (WNME) 

WNME is included in the list of nationally notifiable diseases maintained by the 

Council of State and Territorial Epidemiologists (CSTE) in consultation with CDC. 

CDC has designated 10056 as a specific disease code (“EVENT” code) for use in 

reporting WNME cases via the National Electronic Telecommunications System for 

Surveillance (NETSS). For national reporting purposes, states should use the 

national surveillance case definition of arboviral encephalitis/meningitis for classifying 

cases as either confirmed or probable (see Appendix C). Until such time as ArboNET 

and NETSS are consolidated under the National Electronic Disease Surveillance 

System (NEDSS) standards, duplicate reporting of human cases of WNME to both 

ArboNET and NETSS will be encouraged. 

 2. West Nile Fever (WNF) 

Although WNF is not included in the list of nationally notifiable diseases, states are 

encouraged to report WNF cases to CDC via ArboNET, using a CDC recommended 

case definition (see Appendix D). States may also choose to report WNF cases to 

NETSS using EVENT code 10049. 

C. Ecologic Data 

Many of the issues that apply to the interjurisdictional sharing of human data apply to the 

sharing of ecologic data as well, although key differences exist. For example, 

confidentiality is generally not an issue with nonhuman cases, particularly wild animals 

identified as part of a surveillance program. Maintaining confidentiality may be important 

for certain owned animals. Data standardization is a far more challenging issue because 

of the relatively large number of species being studied. Specific needs include the 
49

following: 

 1. Accurate Taxonomic Identification of Specimens 

Fully understanding the epidemiology and developing effective prevention and control 

strategies for WNV requires accurate identification of all animal species involved in 

the virus transmission and maintenance cycles. This is especially true for birds and 

mosquitoes. 

 2. Unique Identifier (UID) Numbering System for Specimens 

A UID numbering system should be used in each jurisdiction (e.g., state, county, city, 

surveillance area). Such a system should distinguish readily between each major 

animal group reported (i.e., humans, birds, and mosquitoes), and encode the location 

of collection (county or town), date of collection (day/month/year), and a specimenspecific number. 

 3. Durable Tagging System for Field-Collected Specimens 

Use appropriate labels containing complete specimen information on all samples 

(blood, tissues, or whole animals) so field specimen identification will not be lost 

during shipment to testing facilities. 
50 

VI. RESEARCH PRIORITIES

The human and animal health implications of the introduction of West Nile virus (WNV) to 

the U.S. and to the Western Hemisphere continue to emerge. Many questions remain, the 

answers to which will require considerable research. A research agenda should be 

supported, with priority given to research questions whose answers can be directly applied to 

prevention and control. 

A. Current and Future Geographic Distribution of WNV 

To determine the geographic distribution of WNV in the Western Hemisphere, existing 

laboratory-based surveillance systems for WNV in human, birds, other selected animals, 

and mosquitoes should be enhanced, or new, active systems should be developed and 

implemented (see Section I). 

B. Bird Migration as a Mechanism of WNV Dispersal 

Experience in Europe and the Middle East suggests that WNV regularly is introduced to 

new geographic areas along bird migration routes. A better understanding of this 

potential is required for the Western Hemisphere. Studies should include the frequency 

and duration of chronic infections that will allow the long-range transport and 

recrudescence of viremias necessary to infect mosquitoes. 

C. Vector and Vertebrate Host Relationships and Range 

Relatively little is known about the vertebrate host and mosquito vector relationships of 

WNV in the U.S. and the Western Hemisphere. Effective prevention and control 

strategies will require targeting selected species involved in maintenance, epidemic/ 

epizootic transmission cycles, or both. It is critical that the principal species and the 

range of these species be determined. 

D. Virus Persistence Mechanisms 

It is not known whether or how WNV will be maintained in the U.S. over the long term. 

Overwintering mechanisms in Culex and Aedes species should be investigated, as well 

as persistence and maintenance of the virus in ticks. Other possibilities that should be 

investigated include the duration of chronic infection and reactivation in birds or other 

animals, and the introduction of the virus by migratory birds. 

E. Mosquito Biology, Behavior, Vector Competence, Surveillance, and Control 

It is critical that a better understanding is gained of the principal mosquito vectors 

involved in maintenance, bridge (from enzootic to peridomestic), and epidemic/epizootic 

transmission. Different vector species may be important in different geographic or 

ecologic regions. Understanding their biology and behavior will allow for more effective 

surveillance and development of targeted control methods. 

F. Development and Evaluation of Prevention Strategies 

Effective prevention and control of WNV transmission will require evaluation of the 

efficacy of current control methods and research on new and innovative control 

strategies for the principal mosquito vectors. Ultimately, prevention strategies must be 

integrated and use a variety of approaches to control mosquitoes and reduce the risk of 

transmission. Research should also be conducted to better define target areas for 

mosquito control in response to documented WNV activity in an area. 
51

A very long-term goal is the identification and implementation of new, natural compounds 

to repel and control mosquito vectors of disease. With efforts to decertify current 

pesticides, new compounds will be needed in the fight against vector-borne diseases. 

Much effort has been expended to increase public awareness of the WNV threat and of 

the actions needed to reduce exposure to infected mosquitoes. These actions include 

using mosquito repellents, reducing periresidential mosquito breeding sites, and wearing 

protective clothing when entering mosquito-infested areas. The success of these public 

information campaigns has not been formally evaluated using scientific instruments such 

as knowledge and behavior surveys. The cost of such campaigns is high, so formal 

attempts to assess their success are needed. 

G. Laboratory Diagnosis 

Surveillance for WNV will continue to require accurate laboratory diagnostic tests. 

Ideally, these tests will be simple and inexpensive, and will distinguish between WNV 

and other flaviviruses such as the SLE, dengue, and yellow fever viruses. Virus-specific 

tests for IgM or IgG antibody will be required for humans, various species of birds, 

horses, and other mammals. Sensitive viral detection methods will be required for both 

human and animal tissues as well as for mosquito pools. 

H. Clinical Spectrum of Disease and Long-Term Prognosis in Humans 

A better understanding of the spectrum of illness caused by WNV infection in humans is 

needed, including the long-term consequences of acute infection of the central nervous 

system. In addition to the severe end of the clinical spectrum (viral encephalitis), it is 

important to know the degree to which mild viral syndromes occur and whether these 

patients have any unique clinical presentations that may be characteristic or even 

pathognomonic. It is also important to know whether they have viremia and, if so, its 

magnitude and duration. Effective clinical management of severe disease will require 

detailed clinical studies of confirmed human cases of WNV infection. 

I. Risk Factor Studies 

Data on the risk factors associated with human and animal infection with WNV are 

required to develop more effective prevention strategies, particularly when educating the 

public to take specific prevention measures to reduce exposure to infection. 

J. Detailed Clinical Descriptions and Outcome in Human Cases 

Larger and more detailed case series, as well as studies of short- and long-term 

outcomes, are needed to better understand the clinical features, clinical course, and 

public health impact of WNV disease in humans. A suggested framework for collecting 

standardized “extended” clinical variables is included in Appendix E. 

K. Viral Pathogenesis 

Little is known of the pathogenesis of WNV in humans or other animals. Research is 

needed to better understand the organ systems affected, the mechanism of central 

nervous system (CNS) infection, and the role of virus strain in pathogenesis. 

L. Genetic Relationships and Molecular Basis of Virulence 

Only since 1996 has WNV been associated with significant numbers of severe disease 

cases and fatalities in humans. It is important to better understand whether genetic 

changes in WN viruses influence their phenotypic expression (i.e., host and vector range, 

clinical expression in various hosts, and epidemic potential). This will require detailed 
52 

studies of the genome of WN virus strains isolated from different epidemics in various 

geographic areas. 

M. Vaccine Development for Animals and Humans 

Ultimately, the most effective prevention strategy may be vaccination. It is important to 

support research on the development of both human and equine vaccines. 

N. Antiviral Therapy for West Nile Virus and Other Flaviviruses 

To date, none of the available antiviral agents are effective against flaviviruses, including 

WNV. Research in this area is critical to effective management of severe disease in 

humans. 

O. The Economic Cost of the WNV Epidemic/Epizootic 

It is important to estimate the total economic cost of the epidemic/epizootic. These data 

will help set priorities for capacity building and prevention programs. 

P. WNV Impact on Wildlife 

WNV has the potential to greatly impact the wildlife populations in the Western 

Hemisphere. This is especially true for birds, in many of which the infection appears to 

have high mortality rates (i.e., Corvidae). Research is needed to analyze and define this 

impact to determine if the development of new epizootic intervention strategies is 

needed. Research is also needed to determine what long-term effects WNV infection 

may have on its animal hosts. 

Q. Investigate Alternate Modes of WNV Transmission to Humans 

Four new modes of WNV transmission to humans were identified in 2002: blood 

transfusion, tissue transplantation, transplacental transfer, and breast-feeding. New 

modes of transmission should be investigated to determine the impact they have on 

human infection and to develop effective approaches for prevention and control of WNV 

infection by these routes. 
53

Appendix A - National WNV Surveillance System

Objectives:

The objectives of the national West Nile virus (WNV) surveillance system are to: 

• Monitor the geographic and temporal spread of WNV in the U.S. 

• Develop national public health strategies for WNV surveillance, prevention, and control. 

• Develop a more complete regional picture of the geographic distribution and incidence of the 

other clinically important arboviruses in the U.S. 

• Provide national and regional information to public health officials, elected government 

officials, and the public. 

• Evaluate the use of cooperative agreement funds and the need for additional resources. 

Scope: 

Coordinated, multi-state surveillance of WNV infections in humans and animals has been 

repeatedly identified as a high priority by states affected by WNV in 1999-2002. All states 

conducting surveillance for WNV and other arboviruses are encouraged to participate in 

ArboNET, a CDC-coordinated program to collect these surveillance data. While the 

components of WNV surveillance systems employed in individual jurisdictions will vary, national 

WNV surveillance should, at a minimum, focus on collection of data from: 

• Mosquito surveillance 

• Avian (dead bird) surveillance 

• Equine surveillance 

• Human surveillance 

In addition to data from states, data from commercial laboratories will be sought. CDC will 1) 

formally notify all such laboratories of the need to report any positive laboratory results to the 

appropriate state or local health department who, in turn, will notify CDC; 2) provide them with a 

list of state health department contact persons; 3) periodically contact them to encourage 

reporting; and 4) remind them of the need to have all positive screening tests for arboviral 

infections confirmed by state public health laboratories. In addition, CDC will provide a list of 

these commercial laboratories to its cooperative agreement partners, to facilitate their efforts to 

conduct active laboratory-based surveillance for arboviral infections. 

Categories of Data to be Collected:

National surveillance will focus on the collection of two general categories of data: 
54 

• “Denominator” data 

Definition: Weekly totals of dead birds (classified as either corvids or ‘others’) and mosquito 

pools (classified by species) collected and/or tested by a jurisdiction’s WNV surveillance 

system, stratified by county within a state. Because recent experience has demonstrated that 

the following categories of denominator data are of limited use in meeting national 

surveillance goals, as of 2003, CDC will discontinue the collection of totals of sentinel and 

free-ranging wild birds, horses, or other non-human mammals tested. 

• “Numerator” data 

Definition: Detailed information on individual mosquito pools, sentinel species, dead birds, 

and ill humans, horses, or other species with confirmed or suspected WNV infections, as 

determined by laboratory-confirmed or -probable test results. 

General Procedures:

Reporting “denominator” data:

CDC will collect aggregate denominator data via a secure file upload system using a state-based 

database provided by CDC, continuous data entry into a database stored on a secured CDC 

web site, or importation of delimited records in a specified format. Denominator data variables 

are specified in Table 1. An appropriate submission schedule will be arranged by CDC with the 

jurisdictions submitting surveillance data via file uploading. In addition, 

• CDC will distribute the necessary software and provide the adequate licenses that will allow 

regular secured file upload or continuous web-based data entry. 

• CDC will accommodate state health departments with existing integrated data collection 

systems, e.g., by arranging for uploads of XML-formatted data. 

• The data entry screens will be designed as a series of simple forms or tables. 

• The system will accommodate updates and corrections of previously transmitted data by 

jurisdictions. 

• Following the entry of a week’s data into the database at the state level, transmission of the 

data file to CDC will involve a minimal number of keystrokes. Security will be insured by use 

of the sender’s digital certificate. CDC will arrange for those who will be transmitting 

surveillance data to CDC to obtain digital certificates. 

• Upon arrival at CDC, records from the specific reporting week of interest will automatically be 

captured and imported into a master database on the CDC fileserver and also transmitted to 

USGS in Reston, Virginia. 

• Using these data, reports will be generated automatically each week. Maps will be generated 

by CDC and USGS and made available on the USGS web site. A basic set of dynamic maps 

and corresponding graphs and tables will be made available weekly. The CDC web site and 

Epi-X (or a similar secured communication network) will contain links to the relevant USGS 

web pages. 
55

Reporting “numerator” data:

CDC strongly encourages prompt (“real-time”) reporting of numerator data. CDC will collect 

such reports in a standardized manner to allow monitoring of regional and national trends, and 

facilitate prompt confirmatory testing when necessary. As the arbovirus transmission season 

progresses, the need for immediate reporting of certain data to CDC may diminish. For 

example, once numerous WNV-positive mosquito pools have been previously documented in a 

given geographic area, there may not be a compelling need to immediately report further 

findings. In addition, if at any time the volume of reporting becomes overwhelming, adoption of 

an alternative system may be necessary. 

Numerator data variables that will be collected are specified in Table 2. WNV laboratory and 

surveillance case criteria are specified in Table 3. 

Specified, line-listed numerator data may be submitted using one of three methods: 

• Web-based data entry to a CDC server; 

• Use of state-based, CDC-distributed, Microsoft Access-based data entry/management 

software (ArboNET) with continuous file upload to a CDC server; or 

• Data messaging from a unique data collection system to a CDC server (e.g., in XML format). 

All data entry will be done by the reporting jurisdiction and data is transmitted to a CDC server. 

After data entry and submission, numerator data will be available on the CDC Secure Data 

Network (SDN) so that authorized personnel from the reporting jurisdiction may “verify” 

(proofread, correct, and clear for publication) individual numerator data records in selected 

surveillance categories. 

It is essential that each numerator data record include a unique identifier (UID) assigned by the 

reporting state agency. UIDs will be used by CDC staff to track and update individual numerator 

data records, and by states to verify records via the CDC SDN. The UID will not appear in 

output products for public release. Most jurisdictions already have systems in place for 

generating UIDs, and they should continue to use them. CDC’s databases will accommodate 

numeric or alphanumeric UIDs up to 25 characters long. Jurisdictions are encouraged to begin 

their UIDs with their state’s 2-letter postal code (or “NYC” for New York City). 

The issue of numerator data records associated with laboratory-probable results deserves 

special mention. Although CDC encourages confirmation of all laboratory-probable results, it is 

realized that under some circumstances some states may choose not to do so, depending on 

the epidemiologic situation, laboratory capacity, and volume. For example, during a known WN 

viral epizootic, a state may decide that a crow brain associated with a single positive result for 

WN viral RNA by RT-PCR will undergo no further testing. Although this bird is a laboratoryprobable case (see table below), the jurisdiction may decide to upload that bird’s numerator data 

record to CDC and subsequently authorize CDC to release it publicly. In contrast, a jurisdiction 

may opt to delay the release of such results to the public until they have been laboratoryconfirmed. CDC will rely on individual jurisdictions to decide when to authorize the public 

release of numerator data records based on laboratory-probable results. 
56 

CDC will not publicize numerator data records associated with laboratory-equivocal results. 

In terms of human surveillance, the national surveillance case definition of arboviral 

encephalitis/meningitis includes two official case-status categories: confirmed and probable 

(Table 3). For national arboviral encephalitis surveillance, CDC has traditionally combined 

records in these two categories for its annual summary reports, and will continue this practice 

within the WNV surveillance system. States are encouraged to promptly report both laboratoryconfirmed and laboratory-probable human WN encephalitis cases as numerator data records.

CDC encourages the reporting of human WN viral illnesses other than WNME (e.g., WNF, acute 

flaccid paralysis, other clinical syndrome, or unspecified). To determine case status (confirmed 

or probable) for reporting purposes, refer to the national surveillance case definition of arboviral 

encephalitis/meningitis (Appendix C) and the CDC-recommended surveillance case definition for 

WNF (Appendix D). A working case definition for WNME in equines is shown in Appendix B. 

Arboviruses other than WNV:

It is anticipated that enhanced WNV surveillance will result in increased recognition of other 

domestic arboviral activity, including eastern equine encephalitis (EEE), western equine 

encephalitis (WEE), SLE, La Crosse (LAC), and Powassan (POW) virus activity. Surveillance 

numerator (laboratory-positive) data regarding these viruses may be reported to CDC/DVBID via 

ArboNET, telephone, FAX, or e-mail. 

Data Security Issues:

General principles: 

• State and local health authorities will retain control of the timing of data release. 

• As of 2003, reporting agencies will electronically report to ArboNET all categories of 

surveillance data, including human numerator data. For non-human data, agencies will 

verify accuracy and readiness for public release prior to submission. Upon the electronic 

submission of non-human data to CDC, these reports will be considered verified and 

publishable. With the 2003 version of ArboNET, human data will be automatically verified 

upon entry, and the reporting agency has the option to unverify the data via electronic 

checkbox. CDC will not publicly release unverified human case reports. 

• Personal identifying or localizing (more specific than county) information will not be released. 

Specific issues: 

• To report data via secure file upload to the CDC fileserver or to enter data directly onto a 

secured web site, states will utilize the CDC SDN, which provides data encryption for 

transmission via the Internet. To use the SDN, users must obtain and install a digital 

certificate from the CDC certificate server. This allows for unique identification of the 

computer/browser that is accessing a secure web site. 
57

• To obtain a digital certificate and be approved to use the SDN, the digital certificate authority 

at CDC/DVBID must approve the request and forward it to CDC/Atlanta. CDC requests that 

a maximum of 3 persons from each state be designated to receive digital certification. 

These should include those who will transmit data to CDC, as well as those who will verify 

data on the SDN. 

Summary Reports to be Produced by CDC and USGS: 

A working list of basic summary reports is shown in Table 4. The exact list and formats of these 

reports remain to be determined, and this should be viewed as a dynamic process. 

Modifications, additions, and deletions may take place over time, as dictated by feedback, 

experience, technical issues, and events. 

Using state-approved numerator and denominator data, reports will be generated weekly. Maps 

and tables will be generated by DVBID and by USGS. Maps and corresponding graphs and 

tables will be updated at least weekly on the USGS web site (www.USGS.gov). 

Communication Issues:

• A dedicated telephone line (970-266-3592), electronic mailbox (dvbid2@cdc.gov), and fax 

machine (970-266-3599) will be available at CDC/DVBID (in Fort Collins, Colorado) 24 

hours/day for reporting numerator data or other urgent WNV-related business. During nights 

and weekends, calls to the dedicated phone line will be forwarded to the cellular phone of an 

on-call CDC/DVBID staff scientist. Because of potential delays in the receipt and reading of 

email and fax messages, in general please use the telephone for time-sensitive business.

• In addition to periodic conference calls between CDC, cooperating states, and other federal 

agencies, Epi-X and the WNV Information Exchange (WNVIX, part of the Epi-X Forum) will 

be available to participating jurisdictions and agencies using the CDC SDN. For further 

information, contact the CDC/DVBID ArboNET staff at 970.221.6400 or send electronic mail 

to dvbid2@cdc.gov.

Submission of Laboratory Specimens to CDC for WNV Testing: 

 See Table 5. 

 
58 

Table 1. Denominator Data Variable List 

(Note: As of 2003, denominator data will no longer be collected in the following categories: 

sentinel animals, seroprevalence in free-ranging birds, and ill equines or humans.) 

I. Avian mortality: (Includes ill or dead birds, except for sentinels.) 

 Year 

 MMWR week that bird collected (“MMWR week collected”) 

(Note: “MMWR week collected” corresponds to the earliest date associated with a 

specimen. Preferably, this should be MMWR week that corresponds to the date that 

the bird was reported by the public. But, if a date of report is not available, use the 

MMWR week that corresponds to the date that the specimen was collected in the 

field. This “MMWR week collected” should remain associated with this specimen 

throughout testing.) 

 County 

 State 

 Number of reported corvids by “MMWR week collected” and by county (Data source: 

State, county or township WNV surveillance coordinators through the state to CDC) 

 Number of corvids tested by “MMWR week collected” and by county (Data source: 

Testing laboratories through state) 

 Number of other reported birds by “MMWR week collected” and by county (Data source: 

Jurisdictional WNV surveillance coordinators to CDC via state or municipal health 

departments) 

 Number of other birds tested by “MMWR week collected” and by county (Data source: 

Testing laboratories through state) 

Note: Laboratory-positive” results are reported through the numerator system by the testing 

facility/agency. In this report, the date of reporting/sighting or field collection is routinely obtained. 

 By definition, each numerator data record of a WNV-positive dead bird should also be included 

within an aggregated denominator data record.) 

II. Mosquito collections: 

 Year 

 MMWR week of collection 

(Note: This is the MMWR week that corresponds to the date of field collection. This date 

should remain associated with this specimen throughout testing.) 

 County 

 State 

 Species of mosquito 

 Number of mosquitoes collected by MMWR week of collection, by county, and by species 

(Data source: Jurisdictional WNV surveillance coordinators to CDC via state or municipal 

health departments) 

 Number of mosquitoes tested by MMWR week of collection, by county, and by species 

(Data source: Testing laboratories through state). 

(Note: Laboratory-positive results are reported through the numerator system by the testing 

facility/agency. In this report, the date of field collection is routinely obtained. By definition, each 

numerator data record of a WNV-positive mosquito pool should also be included within an 

aggregated denominator data record.) 
59

Table 2. Numerator data variables 

Mosquito surveillance – state, county, pool UID, date of mosquito collection, week of collection, species, 

arbovirus, case status 

Sentinel species surveillance - State, county, group UID, date of serum collection, week of serum 

collection, species, arbovirus, case status 

Avian mortality surveillance – state, county, bird UID, week bird found collected, date bird collected, 

species (including “captive species”), arbovirus, case status 

Avian seroprevalence surveillance – state, county, bird UID, week bird trapped & bled, date bird 

trapped & bled, species, arbovirus, case status 

Veterinary (non-avian) surveillance – state, county, animal UID, week of illness onset, date of illness 

onset, species (canine, equine, feline, bat, squirrel, rabbit, raccoon, or other species), arbovirus, case 

status. 

Human surveillance – state, county, patient UID, week of illness onset, date of illness onset, imported 

from, arbovirus, case status, age, age unit, birthdate, sex, race, ethnicity, clinical syndrome, fatality, 

date of death, lab acquired, non-lab acquired, blood donor, blood recipient. organ donor, organ 

transplant recipient, breast fed infant at time of illness, potential in-utero infection, pregnant at time of 

illness 
60 

Table 3. WNV Laboratory and Surveillance Case Criteria Laboratory case definitions:

Surveillance Type Laboratory-confirmed WNV infection Laboratory-probable WNV infection* 

Mosquito • WNV isolation (identity of virus established 

by at least two of the following techniques: 

• Positive RT-PCR test for WN viral RNA with 

validation by 1) repeated positive test using different primers, 2) positive PCR result using another system (e.g., TaqMan), or 3) 

virus isolation. 

• Detection of WN viral antigen (e.g., IFA, 

EIA, VecTestTM) validated by inhibition test 

(for ELISA), RT-PCR, or virus isolation 

• Positive RT-PCR test for WN 

viral RNA in a single test 

• Antigen detection not 

validated by another procedure 

Sentinel species • WNV isolation, RNA detection, or antigen 

detection as described for mosquitoes, 

• Seroconversion to WNV in serially collected 

serum specimens, by plaque-reduction neutralization** 

• Detection of IgM antibody to WNV, validated 

by demonstration of neutralizing antibody to WNV** 

• Detection of IgM antibody to 

WNV 

• Seroconversion to WNV in 

serially collected serum specimens, strongly reactive by EIA or IFA 

Avian mortality • WNV isolation, RNA detection, or antigen 

detection as described for mosquitoes, , 

• Positive RT-PCR test for WN 

viral RNA in a single test 

• Antigen detection not 

validated by another procedure 

Veterinary (non-avian) • As for humans (see below) • As for humans (see below) 
61

Surveillance Type Laboratory-confirmed WNV infection Laboratory-probable WNV infection* Human • See national surveillance case definitions 

(Appendices C and D) 

• See national surveillance case 

definitions (Appendices C and D) 

* CDC strongly encourages attempts to confirm all laboratory-probable and -equivocal results. Further testing of laboratory-probable human specimens 

confirmatory testing. 

** SLE virus infection should be ruled-out by cross-neutralization; criterion for PRNT positive is a 90% neutralization titer of at least 1:10, and 4-fold great

flaviviruses such as SLE.
62 

Table 4. Working List of Basic Weekly Summary Reports to be Produced by CDC 

NOTE: The exact list and formats of these reports remain to be determined, and this should be 

viewed as a dynamic process. Modifications, additions, and deletions may occur over time, as 

dictated by feedback, experience, technical issues, and events.

A. National map: U.S. map with state boundaries reflecting cumulative data.

1. Mosquito surveillance: 

 a. Map showing each state’s counties as WNV-positive, WNV-negative, or blank 

(no data) 

2. Sentinel chicken surveillance: 

 a. Map showing each state’s counties as WNV-positive or blank (no data) 

3. Avian morbidity/mortality surveillance: 

 a. Map showing each state’s counties as WNV-positive, WNV-negative, or blank 

(no data) 

 4. Veterinary (non-avian) surveillance: 

 a. Map showing each state’s counties as WNV-positive (# cases) or blank (no 

data) 

 5. Human surveillance: 

 a. Map showing each state’s counties as WNV-positive (# cases) or blank (no 

data) 

B. State Maps: Selecting an individual state from the national map will produce a map of that 

state with its county boundaries indicating the positive specimens reported for that county 

and an accompanying table of cumulative positive specimens reported by county. 

1. Mosquito surveillance: 

 a. Map showing each county as WNV-positive with a count of positive specimens 

reported, WNV-negative, or blank (no data) 

2. Sentinel species surveillance: 

 a. Map showing each county as WNV-positive with a count of positive specimens 

reported or blank (no data) by sentinel species (e.g., horse, chicken) 

3. Avian mortality surveillance: 

 a. Map showing each county as WNV-positive with a count of positive specimens 

reported, WNV-negative, or blank (no data) 

4. Veterinary (non-avian) surveillance: 

 a. Map showing each county as WNV-positive with a count of positive specimens 

reported or blank (no data) 

5. Human surveillance: 

 a. Map showing each county as WNV-positive with a count of positive specimens 

reported or blank (no data) 
63

Table 5. Instructions for Submitting Laboratory Specimens to CDC for WNV Testing 

Arrangements for Testing:

Mosquito specimens: Specimens will be accepted for confirmatory testing at CDC when 

requested by a state health department vector surveillance coordinator. For specimens 

considered by a state health department vector surveillance coordinator to be of high priority and 

beyond the capacity of the state public health laboratory or collaborating laboratory, initial and 

confirmatory testing can be obtained at CDC by special arrangement, depending on CDC 

laboratory capacity. For further information, please contact Dr. Roger Nasci, tel. 970-221-6432, 

RNasci@cdc.gov; if Dr. Nasci cannot be reached, please phone 970-266-3592. 

Sentinel chicken specimens: Serum specimens will be accepted for confirmatory testing at CDC 

when requested by a state health department vector or vertebrate surveillance coordinator. For 

specimens considered by a state health department vector or vertebrate surveillance coordinator 

to be of high priority and beyond the capacity of the state public health laboratory or 

collaborating laboratory, initial and confirmatory testing can be obtained at CDC by special 

arrangement, depending on CDC laboratory capacity. For further information, please contact Dr. 

Rob Lanciotti, tel. 970-221-6440, RSLanciotti@cdc.gov; if Dr. Lanciotti cannot be reached, 

please call 970-266-3592. 

Avian morbidity/mortality specimens: On a case-by-case basis, special arrangements can by 

made for CDC to conduct initial and/or confirmatory tests of tissue specimens (especially brain, 

heart, kidney, and spleen) from dead birds that cannot otherwise be tested in state health 

department laboratories or by the National Wildlife Health Center, USGS. For further 

information, please contact Dr. Nick Komar, tel. 970-221-6496, NKomar@cdc.gov; if Dr. Komar 

cannot be reached, please call 970-266-3592. 

Veterinary (non-avian) specimens: Specimens will be accepted for confirmatory testing at CDC 

when requested by a state health department laboratory director. For routine testing of 

veterinary specimens, contact the state health department laboratory or the National Veterinary 

Services Laboratory, USDA, in Ames, IA (Tel. 515-663-7751), or another collaborating 

laboratory. For specimens considered by a state health department laboratory director to be of 

high priority and beyond the capacity of that state’s public health laboratory, initial and 

confirmatory testing can be obtained at CDC by special arrangement. For further information, 

please contact Dr. Rob Lanciotti, tel. 970-221-6440, RSLanciotti@cdc.gov; if Dr. Lanciotti cannot 

be reached, please call 970-266-3592. 

Human specimens: Specimens will be accepted for confirmatory testing at CDC when requested 

by a state health department laboratory director. For specimens considered by a state health 

department laboratory director to be of high priority and beyond the capacity of the state public 

health laboratory or collaborating laboratory, initial and confirmatory testing can be obtained at 

CDC by special arrangement. For further information, please contact Dr. Rob Lanciotti, tel. 970-

221-6440, RSLanciotti@cdc.gov; if Dr. Lanciotti cannot be reached, please call 970-266-3592. 

General Shipping Instructions:

All shippers should adhere to International Air Transport Association regulations 

(http://www.iata.org). 

Specimens should be shipped by overnight courier to arrive at CDC on Tuesday-Friday. 
64 

Always notify CDC staff in advance of an impending shipment (tel. 970-221-6445; if no 

answer, phone 970-266-3592). Do not ship specimens on Friday unless special 

arrangements have been made. 

Shipping address: CDC/DVBID 

CSU Foothills Campus/Rampart Road 

Fort Collins, CO 80521 

ATTENTION: Arbovirus Diagnostic Laboratory (tel. 970-221-6445) 

Shipping containers: Use only durable containers. Seal specimen containers tightly. Wrap 

specimen containers in absorbent material and pack them into two different plastic containers 

to insure that any leakage is contained. Specimens for virus isolation must be sent on 

enough dry ice to insure that they remain frozen until receipt. Specimens for serologic testing 

can be shipped on gel-ice and need not remain frozen. Hand-carrying specimens is not 

recommended but if specimens are hand-carried, the above packing instructions are 

applicable. 

Minimal Information to Accompany Specimens Shipped to CDC: 

See information in columns 2, 3, and 4 in Table 2. Please read carefully and supply all available 

information. Use CDC Form 5034 (the ADASH@ form) Form 5034 is available electronically at: 

http://www.cdc.gov/ncidod/dvbid/CDC_form5034.pdf

Tubes, cryovials, and other specimen containers should be clearly labeled with – at minimum –

the specimen’s UID, patient’s name (human), state, date of onset, date of collection, and 

specimen type.

Special Collection, Shipping, and Handling Instructions:

Mosquitoes: Ship on dry ice. 

Serum: Store in externally threaded plastic tubes. Ship at least 0.5 mL per specimen. 

Whenever possible, acute and convalescent specimens should be shipped together. Ship 

fresh-frozen on dry ice (required for virus isolation) or refrigerated on wet ice (acceptable). 

CSF: Store in externally threaded plastic tubes. Ship at least 1.0 mL per specimen. Ship freshfrozen on dry ice (required for virus isolation) or refrigerated on wet ice (acceptable). 

Whole blood: In general, send only if requested for virus isolation attempts in fatal cases (heart 

blood). 

Pregnancy-related specimens: In possible cases of intrauterine arboviral infection, tissues 

collected at the time of delivery can be tested for evidence of infection. The following tissues 

should be shipped fresh-frozen on dry ice: cross-sections of umbilical cord, placental tissue 

(approximately 1 cm3

 per sample), cord serum and maternal serum (0.5 ml each), and colostrum 

or breast milk. For more information, please contact Dr. Dan O’Leary at (970) 266-3525 or 

DOLeary@cdc.gov. 

Autopsy specimens: In suspected cases of arboviral encephalitis in which an autopsy is 

performed, fresh-frozen tissues can be tested, including brain (multiple areas of cortex, 

midbrain, brainstem, and spinal cord), other solid organs (liver, spleen, pancreas, heart, kidney, 
65

etc.), CSF (collected from ventricles), and heart blood (for virus isolation attempts). 

After consulting with Dr. Sherif Zaki or other CDC/Atlanta pathology staff members (tel. 404-639-

3133), tissue samples suspended in formalin should be sent to: 

Infectious Disease Pathology Activity 

DVRD/NCID/CDC 

Building 1, Room 2301 

1600 Clifton Road, N. E. 

Atlanta, GA 30333 

Veterinary (non-avian) tissues: As for human specimens. 

Avian tissues: Submit fresh-frozen brain, heart, kidney, and spleen samples. 
66 

Appendix B B Surveillance Case Definition for WNV Infection in Equines 

Laboratory criteria for diagnosis 

Compatible clinical signs[1] plus one or more of the following: 

! Isolation of West Nile (WN) virus from or demonstration of specific viral antigen or 

genomic sequences in tissue, blood, cerebrospinal fluid (CSF) or other body fluid;[2] or 

! Detection of IgM antibody against WN virus by IgM-capture ELISA in serum (at 1:400 

or greater dilution) or cerebrospinal fluid (CSF) (at dilution 1:2 or greater dilution); or 

! An associated 4-fold or greater change in IgG-capture ELISA or plaque-reduction 

neutralization test (PRNT) antibody titer to WN virus in appropriately timed,[3] paired 

serum specimens from an equid that is unvaccinated against WN virus; or 

! Positive immunohistochemistry (IHC) for WN virus antigen in tissue. 

Case classification 

Probable: compatible clinical signs occurring during a period when arboviral transmission is 

likely, and with the following supportive serology: 1) a single or stable (less than or equal to twofold change) but elevated titer of WN virus-specific IgM-capture ELISA or neutralizing serum 

antibodies without knowledge of prior WN virus vaccination. 

 

Confirmed: compatible clinical signs with laboratory-confirmed evidence of WN virus infection. 

Notes: 

[1] Clinical signs are associated with central and/or peripheral nervous system dysfunction. 

Most horses exhibit secondary CNS-derived neurological manifestations such as ataxia 

(including stumbling, staggering, wobbly gait, or incoordination) or at least two of the 

following: circling, hind limb weakness, inability to stand, multiple limb paralysis, muscle 

fasciculation, proprioceptive deficits, altered mental status, blindness, lip droop/paralysis, 

teeth grinding. (Ostlund et al, Equine West Nile Encephalitis, United States, Emerging 

Infectious Diseases, Vol 7, No 4. Jul – Aug 2001) Fever is not a consistent finding. 

[2] Preferred diagnostic tissues from equids are brain or spinal cord; isolation of WN virus or 

detection of WN viral nucleic acid sequences in equine blood or CSF are infrequent. 

(Bunning et al, Experimental Infection of Horses with West Nile virus, Vol 8, No. 4. April 

2002) 

[3] The first serum should be drawn as soon as possible after onset of clinical signs and the 

second drawn at least 14 days post-onset. 

_______________________________________________ 

Assumptions on which case definitions are based: 

• IgM-capture ELISA testing may give nonspecific results; cross-reactions to closely 

related flaviviruses (e.g., St. Louis encephalitis virus) may occur. Because closely 

related arboviruses exhibit serologic cross-reactivity, positive results of serologic tests 

using antigens from a single arbovirus can be misleading. In some circumstances (e.g., 

in areas where two or more closely related arboviruses occur, or in imported arboviral 

disease cases), it may be epidemiologically important to attempt to pinpoint the infecting 
67

virus by conduction cross-neutralization tests using an appropriate battery of closely 

related viruses. 

• Vaccination refers to one or more doses of the current USDA-licensed inactivated WN 

virus vaccine. 

• IgM antibody in equine serum is relatively short-lived (how long);(ref?) a positive IgMcapture ELISA means infection with WN virus or a closely related flavivirus has occurred, 

probably within the last three months. (personal communication Eileen N. Ostlund, 

USDA)

• Neutralizing antibody, as detected by PRNT, may not be present in equine serum until 

two weeks or more after exposure to WN virus; it is possible that clinical signs may be 

present in an equine before a serum PRNT is positive. (ref) 

• Neutralizing antibody detected in serum by PRNT indicates past infection with WN virus 

or vaccination with WN virus vaccine; equines exposed to WN virus in prior years may 

test positive by PRNT. 
68 

Appendix C - National Surveillance Case Definition for Arboviral 

 Encephalitis/Meningitis, 2001 

 (available at http://www.cdc.gov/epo/dphsi/casedef/encephalitiscurrent.htm) 

Encephalitis or Meningitis, Arboviral (includes California serogroup, Eastern equine, St. 

Louis, Western equine, West Nile, Powassan)

2001 Case Definition 

Clinical description 

Arboviral infections may be asymptomatic or may result in illnesses of variable severity 

sometimes associated with central nervous system (CNS) involvement. When the CNS is 

affected, clinical syndromes ranging from febrile headache to aseptic meningitis to encephalitis 

may occur, and these are usually indistinguishable from similar syndromes caused by other 

viruses. Arboviral meningitis is characterized by fever, headache, stiff neck, and pleocytosis. 

Arboviral encephalitis is characterized by fever, headache, and altered mental status ranging 

from confusion to coma with or without additional signs of brain dysfunction (e.g., paresis or 

paralysis, cranial nerve palsies, sensory deficits, abnormal reflexes, generalized convulsions, 

and abnormal movements). 

Laboratory criteria for diagnosis 

• Fourfold or greater change in virus-specific serum antibody titer, or 

• Isolation of virus from or demonstration of specific viral antigen or genomic sequences in 

tissue, blood, cerebrospinal fluid (CSF), or other body fluid, or 

• Virus-specific immunoglobulin M (IgM) antibodies demonstrated in CSF by antibodycapture enzyme immunoassay (EIA), or 

• Virus-specific IgM antibodies demonstrated in serum by antibody-capture EIA and 

confirmed by demonstration of virus-specific serum immunoglobulin G (IgG) antibodies in 

the same or a later specimen by another serologic assay (e.g., neutralization or 

hemagglutination inhibition). 

Case classification 

Probable: an encephalitis or meningitis case occurring during a period when arboviral 

transmission is likely, and with the following supportive serology: 1) a single or stable (less than 

or equal to twofold change) but elevated titer of virus-specific serum antibodies; or 2) serum IgM 

antibodies detected by antibody-capture EIA but with no available results of a confirmatory test 

for virus-specific serum IgG antibodies in the same or a later specimen. 

Confirmed: an encephalitis or meningitis case that is laboratory confirmed. 

Comment 

Because closely related arboviruses exhibit serologic cross-reactivity, positive results of 

serologic tests using antigens from a single arbovirus can be misleading. In some circumstances 

(e.g., in areas where two or more closely related arboviruses occur, or in imported arboviral 

disease cases), it may be epidemiologically important to attempt to pinpoint the infecting virus by 

conducting cross-neutralization tests using an appropriate battery of closely related viruses. This 

is essential, for example, in determining that antibodies detected against St. Louis encephalitis 
69

virus are not the result of an infection with WN (or dengue) virus, or vice versa, in areas where 

both of these viruses occur. 

The seasonality of arboviral transmission is variable and depends on the geographic location of 

exposure, the specific cycles of viral transmission, and local climatic conditions. Reporting 

should be etiology-specific (see below; the six encephalitides/meningitides printed in bold are 

nationally reportable to CDC): 

St. Louis encephalitis/meningitis (NETSS Event Code: 10051)

West Nile encephalitis/meningitis (NETSS Event Code: 10056) 

Powassan encephalitis/meningitis (NETSS Event Code: 10057) 

Eastern equine encephalitis/meningitis (NETSS Event Code: 10053) 

Western equine encephalitis/meningitis (NETSS Event Code: 10052)

California serogroup viral encephalitis/meningitis (includes infections with the following 

viruses: La Crosse, Jamestown Canyon, snowshoe hare, trivittatus, Keystone, and California 

encephalitis viruses) (NETSS Event Code: 10054) 

Other viral CNS infections transmitted by mosquitoes, ticks, or midges (e.g., Venezuelan equine 

encephalitis/meningitis [NETSS Event Code: 10055] and Cache Valley encephalitis/meningitis 

[NETSS Event Code: 10058]) 
70 

Appendix D - CDC-Recommended Surveillance Case Definition for WN Fever 

What is a CDC-Recommended Case Definition?

CDC-recommended surveillance case definitions are prepared for use by U.S. States and 

Territories interested in conducting public health surveillance for diseases or conditions that 

have not been designated nationally notifiable and have not been officially approved and 

sanctioned by the Council of State and Territorial Epidemiologists (CSTE). A CDCrecommended case definition may not be approved by CSTE in the future, unless CSTE and the 

CDC program with responsibility for prevention and control of the selected disease or condition 

both wish to seek broader and more formalized approval from both organizations. 

CASE DEFINITION 

Case Description

A non-specific, self-limited, febrile illness caused by infection with WNV, a mosquito-borne 

flavivirus. Clinical disease generally occurs 2-6 days (range, 2-15 days) following the bite of an 

infected mosquito. Typical cases are characterized by the acute onset of fever, headache, 

arthralgias, myalgias, and fatigue. Maculopapular rash and lymphadenopathy generally are 

observed in less than 20% of cases. Illness typically lasts 2-7 days. 

Case Classification

A clinically compatible illness, plus: 

Confirmed: 

1) Fourfold or greater change in WNV-specific serum antibody titer; 

2) Isolation of WNV from or demonstration of specific WN viral antigen or genomic 

sequences in tissue, blood, CSF, or other bodily fluid; or 

3) WNV-specific IgM antibodies demonstrated in serum by antibody-capture enzyme 

immunoassay and confirmed by demonstration of WNV-specific serum neutralizing 

antibodies in the same or a later specimen. 

Probable:

1) WNV-specific serum IgM antibodies detected by antibody-capture enzyme immunoassay 

but with no available results of a confirmatory test for WNV-specific serum neutralizing 

antibodies in the same or a later specimen. 

(Note: Some WN fever cases progress to WN meningitis or encephalitis. Cases meeting the 

more restrictive case definition of WN encephalitis/meningitis should be reported as such and 

only once, using event code 10056 for “WN Encephalitis or Meningitis”.)
71

Comment 

The seasonality of arboviral transmission is variable and depends on the geographic location of 

exposure, the specific cycles of viral transmission, and local climatic conditions. Because closely 

related arboviruses exhibit serologic cross-reactivity, positive results of serologic tests using 

antigens from a single arbovirus can be misleading. In some circumstances (e.g., in areas where 

two or more closely related arboviruses occur, or in imported arboviral disease cases), it may be 

epidemiologically important to attempt to identify the infecting virus by conducting crossneutralization tests using an appropriate battery of closely related viruses. This is essential, for 

example, in determining that antibodies detected against WNV are not the result of an infection 

with St. Louis encephalitis or dengue virus, or vice versa. Because dengue fever and WN fever 

can be clinically indistinguishable, the importance of a recent travel history and appropriate 

serologic testing cannot be overemphasized. In some persons, WNV-specific serum IgM 

antibody can wane slowly and be detectable for more than one year following infection. 

Therefore, in areas where WNV has circulated in the recent past, the co-existence of WNVspecific IgM antibody and illness in a given case may be coincidental and unrelated. In those 

areas, the testing of serially collected serum specimens assumes added importance. 

Date case definition was developed: October 2002 

Event Code: 10049

Source of the case definition: National Center for Infectious Diseases, Division of VectorBorne Infectious Diseases, Arbovirus Diseases Branch. 

Questions and comments about the case definition should be directed to the following 

CDC/ADB staff: 

Roy Campbell Phone: (970) 221-6459 E-mail: glc5@cdc.gov 

Dan O’Leary Phone: (970) 266-3525 E-mail: dbo7@cdc.gov 

Tony Marfin Phone: (970) 266-3521 E-mail: aam0@cdc.gov 
72 

Appendix E – Recommended Framework for Standardized “Extended” Clinical 

 Variables in Studies of Human WNV Disease 

Larger and more detailed case series, as well as studies of short- and long-term outcome, are 

needed to better understand the clinical features, clinical course, and public health impact of 

WNV disease in humans. A suggested framework for collecting standardized “extended” clinical 

variables is shown below. During 2003, CDC will work with its partners to populate this 

framework with specific questions in each category. The use of standardized questions will allow 

public health officials and other researchers to compare results more readily. 

1. Epi core data (e.g., age, gender, residence location, race/ethnicity, type of West Nile 

virus illness, etc.) [Note: These are already standard ArboNET variables.] 

2. Past medical history 

3. Previous arboviral infections or vaccinations 

4. Immunosuppressed conditions 

5. New modes of transmission 

6. Clinical presentation – neurology and initial symptoms 

7. Clinical presentation – Standardized scale of neuro/physiologic function (e.g., APACHE, 

Glasgow Coma Scale, PRISM) 

8. Clinical presentation – laboratory 

9. Clinical presentation – WNV diagnostic studies 

10. Clinical presentation – Special diagnostic studies (e.g., MRI, EEG, EMG, lumbar 

puncture) 

11. Treatment (e.g., antivirals, steroids, anti-seizure medications, hyperventilation, interferon, 

intravenous immunoglobulin, plasmapharesis) 

12. Clinical course (e.g., renal function, electrolyte balance, neurologic complications) 

13. Morbidity (e.g., number of hospital-, ICU-, and ventilator-days, number and type of 

nosocomial infections, etc.) [Note: Many of the morbidity parameters can be used in 

determining the costs of WNV disease.] 

14. Nosocomial infections 

15. Discharge disposition 

16. Neurologic and functional status at disposition, at 90 days post-discharge, and 180 days 

post-discharge 

17. Mortality (e.g., cause of death, pathology findings, etc.) 
73

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