In Equids in the Northeastern

United States in 2000

August 2001

United States 

Department 

Of Agriculture

Animal and 

Plant Health

Inspection 

Service

Veterinary 

Services
West Nile Virus (WNV) in Equids in the Northeastern

United States in 2000

Report Contributors

Introduction.................................................................................................................................................... 1

Overview of the Case-Control Study and Spatial Data Analysis of WNV in Equids.................................... 1

Materials and Methods.................................................................................................................................. 1

I. Case-Control Study.................................................................................................................................. 1

Objective.................................................................................................................................................. 1

Case Definition ........................................................................................................................................ 1

Selection of Premises............................................................................................................................... 1

Premises Data........................................................................................................................................... 2

Animal Data ............................................................................................................................................. 2

Data Analysis Methods............................................................................................................................ 2

Categories of Equid Infection Status ....................................................................................................... 3

Categories of Premises............................................................................................................................. 3

Method of Reporting Case-Control Study Results .................................................................................. 3

II. Spatial Data Analysis.............................................................................................................................. 4

Spatial Data Sources ................................................................................................................................ 4

Spatial Data Analysis............................................................................................................................... 5

Results............................................................................................................................................................. 6

I. Case-Control Study.................................................................................................................................. 6

Table 1. Premises Results........................................................................................................................ 6

Table 2. Animal Results on Case Premises........................................................................................... 10

II. Spatial Analysis..................................................................................................................................... 12

Discussion...................................................................................................................................................... 29

I. Case-Control Study................................................................................................................................. 29

II. Spatial Analysis..................................................................................................................................... 30

Conclusions................................................................................................................................................... 31

References..................................................................................................................................................... 32

Appendix A: Sample Questionnaire ........................................................................................................... 33

Section 1 – Premises Information.......................................................................................................... 33

Section 2 - VMO Evaluation Sheet........................................................................................................ 36

Section 3 – Blood Collection Record..................................................................................................... 37

Section 4 – Equine Travel Log .............................................................................................................. 38

Report Contributors

Participants in the Study 

l Private equine veterinary practitioners, equine owners and equids in Connecticut, Delaware, Massachusetts, New 

Jersey, New York, Pennsylvania and Rhode Island

Connecticut 

l Agriculture Experiment Station: Theodore Andreadis

l Department of Agriculture: Mary Jane Lis

l Department of Public Health: James Hadler, Randall Nelson

l University of Connecticut, Veterinary Diagnostic Laboratory: Richard French, Sandra Bushmich

Delaware 

l Department of Agriculture, Poultry and Animal Health: H. Wesley Towers, Heather Ockels, Jeremy Henderson, 

JoAnn Davis, Kelly Degano, Robert Moore

New Jersey 

l Department of Agriculture, Division of Animal Health: Ernest W. Zirkle, Nancy E. Halpern, Jennifer J. Fowler, 

Anne L. Pierok, Alan R. Belson, Dale Kravcak, Karen Richardson, Lana Budrewicz

l Department of Agriculture, Division of Animal Health, Animal Health Laboratory: Robert J. Eisner

New York 

l Cornell University: Susan C. Trock, Amy Glaser

l Department of Agriculture and Markets, Division of Animal Industry: John P. Huntley, Annette Gonzalez

Pennsylvania 

l Department of Agriculture, Bureau of Animal Health and Diagnostic Services: John Enck, Nan Hanshaw

Roberts, Fred Rommel

Rhode Island 

l Department of Environmental Management, Division of Agriculture, Animal Health: Susan Littlefield

USDA: APHIS Veterinary Services 

l Centers for Epidemiology and Animal Health (CEAH): Jerry Freier, Chris Kopral, Barbara Corso, Lindsey

Garber, Josie Traub-Dargatz (Colorado State University), Jennifer Duncan, Albert Kane, Priscilla FitzMaurice, 

Janice Garner, Susan Maroney

l Eastern Regional Office: Jose Diez, Thomas Holt

l Emergency Programs: Randall Crom

l Kentucky Area Office: Barry Meade

l Maryland/Delaware Area Office: Kent Holm

l National Veterinary Services Laboratories (NVSL): Eileen Ostlund, Douglas Pedersen, Kevin Lake, Donna

Johnson, Kathryn Moser, Beverly Schmitt

l New England Area Office: Robert Brady, Noreen Roche, Lech Szkudlarek, William Smith

l New Jersey Area Office: Leslie Bulaga, Roxanne Mullaney

l New York Area Office: Kay Wheeler

l Pennsylvania Area Office: Roberta Duhaime, Lynn Siegfried
USDA: APHIS: VS West Nile Virus Report

August 2001 - 1 -

Introduction

Overview of the Case-Control Study and Spatial Data Analysis of WNV in Equids

The first new world outbreak of West Nile virus (WNV) in equids occurred in the Northeastern United States in 

1999. Following investigations in 1999, it was recognized there were still some important questions regarding 

exposure and occurrence of disease among equids in this region that remained unanswered. Thus, a case-control 

study of premises was designed so that if cases of WNV in equids were identified in 2000, it could be

implemented in a timely manner. States with confirmed WNV equine cases were requested to participate in the 

case-control study coordinated by USDA: APHIS: VS. This study was designed to gather information from

premises with equids that developed clinical signs of WNV infection in 2000, as well as from premises that did 

not have confirmed disease due to WNV infection in their equids. In addition, a spatial analysis was conducted 

to describe the geographic and ecological aspects of case premises. Factors looked at included precipitation, 

temperature, and locations of case premises relative to equid inventories, WNV-positive mosquito pools, WNVinfected wild birds, elevation and ecoregions.

Materials and Methods

I. Case-Control Study

Objective

The objective of the case-control study was to identify potential environmental and management factors

associated with equine exposure to WNV in the year 2000, and generate hypotheses that may direct future

studies.

Case Definition

A case for this study was defined as any equid identified with exposure to WNV. A case premises was any 

premises with one or more cases. 

Selection of Premises

During 2000, a USDA: APHIS: VS confirmed case was defined as any equid with clinical signs1plus one or 

more of the following (Ostlund, et. al, 2001):

♦ Isolation of WNV from tissue, blood, or cerebrospinal fluid (CSF)

♦ An associated 4-fold or greater change in plaque reduction neutralization test (PRNT) antibody (Ab) titer to 

WNV in appropriately-timed, paired sera

♦ Detection of both IgM Ab to WNV by IgM-capture enzyme linked immunosorbent assay (ELISA) and an 

elevated titer (positive at 1:10 or greater) to WNV Ab by PRNT in a single serum sample.

 

1 Clinical signs included one or more of the following: ataxia (including stumbling, staggering, wobbly gait or 

incoordination), inability to stand, multiple limb paralysis, or acute death.
West Nile Virus Report USDA: APHIS: VS

- 2 - August 2001

A goal of this case-control study was to include all premises with equine WNV cases confirmed by USDA:

APHIS: VS. For each premises with a confirmed clinical case of equine WNV infection, data were collected 

from up to five premises, which were the nearest neighbors (that owned equids and were willing to participate) to 

the premises with confirmed cases. Participation of each premises was voluntary. Each participant signed an 

owner/operator agreement form and data collection was in compliance with Office of Management and Budget 

regulations. Questionnaire data regarding equine management and environmental conditions, as well as

information on individual equids and serum samples on those equids were collected. Confidentiality regarding

origin of data was assured with the exception of diagnosis or suspicion of dangerously infectious or exotic

diseases such as African Horse Fever. States were alerted of the desire to gather such data when the first case in 

a state was identified. Premises in the following states were included in this study: Connecticut (n=40),

Delaware (n=28), Massachusetts (n=9), New Jersey (n=59), New York (n=3), Pennsylvania (n=7), and Rhode 

Island (n=4).

Premises Data

A questionnaire was administered on each of the premises included in the study, and geographic coordinates

were determined by global positioning system (GPS) methods. Questions were asked regarding the number and 

characteristics of the equids, whether they had any equids with signs consistent with WNV infection, and the 

management of equids with an emphasis on housing methods and insect control methods. Data were also 

collected regarding the bird and mosquito habitat, and the observation of dead birds on each operation. Lastly, 

there was an assessment by the data collector (a VS or State Veterinary Medical Officer, VMO) regarding the 

mosquito and bird habitat and the cleanliness of the premises based on described criteria. For more detail on the 

questionnaire and VMO assessment see Appendix A.

Animal Data

Data collectors were requested to collect serum from equids on the operation. The number of equids that were to 

be sampled was on a sliding scale based on the number of equids on the premises (up to 25 equids per operation). 

Data collectors were instructed to sample horses that represented the premises’ equine population based on use, 

age and gender. Information gathered on each of the equids that was sampled included: housing method, insect 

control methods, overall health (hair/coat), color (light colored yes/no), age, sex, primary use, number of days out 

of the county in the past 45 days, and if it showed signs consistent with WNV infection since June 1, 2000. The 

serology test performed on all equids was a PRNT. Equids were considered seropositive if they had a titer of 

1:10 or greater on the PRNT. 

 Sliding Scale for Sampling of Equids: 

Number of Equids on the Operation Number of Equids to be Sampled

Less than 20 equids Sample all

20 to 49 equids Sample 20

50 or more equids Sample 25

Data Analysis Methods

Data from the questionnaires were entered into Lotus Approach databases and then exported into Statistical

Analysis Systems (SAS) datasets. Using SAS, descriptive tabulations were run on all characteristics for which 

data were collected. Data were screened for associations of factors with WNV exposure. Chi-Square tests, or in 

some cases a Fisher’s exact test, were run for selected characteristics to determine if the observed differences 

between groups were statistically significant. Odds ratios and 95 percent confidence intervals were calculated in 

EpiInfo. A p-value of <0.10 was considered an indication of potential association. Analysis of horse-level data 

was limited to equids on case premises.
USDA: APHIS: VS West Nile Virus Report

August 2001 - 3 -

Categories of Equid Infection Status

It was anticipated that there would be several different groups or categories of equids: those that had clinical

signs of WNV and were confirmed to have been infected through virus isolation or serology (see confirmed 

USDA: APHIS: VS case definition, page 1), those that did not have signs of WNV infection but had evidence of 

exposure based on serologic testing, those that had clinical signs consistent with WNV but did not have evidence 

of exposure to this virus, and those that were clinically normal and had no evidence of exposure to WNV based 

on testing. For the purposes of the case-control study, a “case” equid was defined as any animal that had 

evidence of exposure to WNV through serologic testing performed at the National Veterinary Services

Laboratories (NVSL) virology laboratory, or that was confirmed by USDA: APHIS: VS as a WNV case (see 

criteria, page 1). A “non-case” equid was defined as one that did not have serologic evidence of virologic 

evidence of exposure to WNV.

Categories of Premises

The premises were defined based on the status of the equids tested on that premises regarding their exposure to 

WNV. A “case” premises was a premises that had one or more equids that were identified to be exposed to 

WNV (serologic or virologic evidence of exposure to WNV). A “control” premises was a premises that had no 

evidence that the equids were exposed to WNV based on serologic testing of at least a portion of the equids on 

that premises. 

Method of Reporting Case-Control Study Results

The results are reported in two tables (Table 1 and Table 2). 

Table 1 shows the premises-level results. The goal was to determine if there were any premises-level factors that 

were different between those premises that had evidence of exposure of their equids to WNV, and those that did 

not have evidence of exposure. There are two columns in Table 1. The first column is the percentage of premises 

with one or more equids that were confirmed WNV cases or with one or more equids with serologic evidence of 

WNV infection, i.e. case premises. The second column is the percentage of premises that had all tested equids 

with a negative serologic test, i.e. control premises. Each is broken out by various premises characteristics. 

Table 2 shows equid-level results on premises with evidence of one or more equids with exposure to WNV, i.e. 

case premises. The goal of this analysis was to evaluate factors associated with individual animals. Using equids 

from only case premises avoids attributing to individual animals some of the variability that may be more likely 

due to differences between case and control premises. There are two columns for equids on case premises. The 

first column is the percentage of case equids on case premises broken out by equid characteristics or individual

management factors. The second column is the percentage of non-case equids on case premises.
West Nile Virus Report USDA: APHIS: VS

- 4 - August 2001

II. Spatial Data Analysis

Spatial Data Sources

Each data collector obtained the geographic coordinates of case and control premises, using standardized global

positioning system (GPS) methods. On the premises information form (see Appendix A), coordinate data were 

recorded in decimal degrees, using the 1983 North American Datum (NAD-83). Premises coordinates and the 

survey database were imported into a geographic information system and this spatial information was displayed 

as point data, using ArcView, version 3.2a, (ESRI, Redlands, CA). In summarizing state and county data, point 

information was aggregated, using a point-in-polygon process, and then attributes were assigned to polygons

representing administrative boundaries.

Premises with one or more infected equids in which questionnaire data and geographic coordinates were not

obtained were geocoded using geographic coordinates of the nearest city. City coordinate data were obtained 

from an on-line gazetteer, the Getty Thesaurus of Geographic Names (www.getty.edu/research/tools/vocabulary) 

and added to the spatial database. Nearest city coordinates were used in analyses when comparisons involved all 

case equid sites, including those without a completed questionnaire. However, in analyses that included

questionnaire-completed case premises, only the more accurate GPS coordinates were used.

Information on WNV infection in equids on premises not included in the case-control survey was obtained from 

the USDA: APHIS West Nile Virus Statistics web site (http://www.aphis.usda.gov/oa/wnv/wnvstats.html). This 

site also provided data on the approximate location of human cases, test results from the dead bird surveys, and 

virus detection results from field collected mosquitoes. For comparative purposes, a summary of information on 

the 1999 WNV outbreak was obtained from the USDA: APHIS: West Nile virus Summary web site

(http://www.aphis.usda.gov/vs/ep/WNV/summary.html).

Temperature and precipitation data for August 1 through October 31 were obtained from the National Oceanic 

and Atmospheric Administration’s (NOAA) National Climate Data Center. Records from 93 weather stations

were used to calculate daily mean minimum temperature and data from 141 stations were used to determine the 

daily mean precipitation for the 32,900 mi2, affected area.

Thirty-year average total precipitation data for 1961-1990 were obtained from The Climate Source, LLC

(Corvallis, OR), in a grid format with a spatial resolution of 1.25 arc-minutes (about a 2-km2 area). Average 

precipitation values, measured in millimeters, were based on reports from 7,700 National Weather Service

stations. Precipitation data were interpolated between weather stations and variation due to topography was 

minimized, using the Parameter-elevation Regressions on Independent Slopes Model (PRISM) developed by

Daly and Neilson (1992; Daly et al., 1994). In this regression-based interpolation process, precipitation values 

from weather reporting stations were weighted according to distance, elevation, cluster, vertical layer,

topographic facet, coastal proximity, and effective terrain.

Administrative boundary data were obtained from ESRI Data and Maps, 1999 edition. County-level equid 

population data were obtained from the 1997 U.S. Census of Agriculture, Geographic Area Series (USDA:

National Agricultural Statistics Service). Information on ecological regions was obtained from Bailey (1995). 

Terrain data were obtained from the Digital Terrain Elevation Data (DTED), Level 0, 30 arc second data blocks 

(DoD: National Imagery and Mapping Agency, formerly the Defense Mapping Agency).

Variation in vegetation conditions was assessed with image data from NOAA polar-orbiting satellites obtained 

from the U.S. Geological Survey’s EROS Data Center (EDC), Sioux Falls, SD. Daily observations with the 

Advanced Very High Resolution Radiometer (AVHRR) sensor provided 1-km resolution data. Sensor data were 

calibrated to wavelength standards and processed into biweekly composites designed to exclude clouds and other 

forms of atmospheric interference. From these data, the maximum normalized difference vegetation index

(NDVI) was calculated, using the method of Eidenshink (1992). The NDVI is the difference of near-infrared 

(channel 2) and visible (channel 1) reflectance values normalized over the sum of channels 1 and 2. Although 
USDA: APHIS: VS West Nile Virus Report

August 2001 - 5 -

the NDVI equation produces values ranging from –1.0 to 1.0, we used scaled values which were expanded over a 

zero to 200 range. Values are proportional to increasing amounts of vegetative biomass as measured by the 

property of light reflectance from chlorophyll pigment contained in plant chloroplasts.

Spatial Data Analysis

Thematic data layers were assembled using ArcView GIS, version 3.2a and analyzed with Spatial Analyst (ver. 

1.1), Grid Analyst, X-Tools, and other ArcView extensions. Spatial Stats for S-Plus, version 2000 (Insightful, 

Inc., Seattle, WA) was used to create spatial regression models, using nearest neighbor methods. S-Plus Spatial 

Stats was also used in the analysis of spatial clusters, using Geary’s C spatial autocorrelation methods (Geary, 

1954; Cressie, 1993), and Ripley’s K-function statistics (Venables and Ripley, 1999), and an intensity analysis 

(Diggle, 1983; Bailey and Gatrell, 1995). In addition, space-time clusters were evaluated using SatScan

software, version 2.1.3 (Kulldorff, et. al, 1998, National Cancer Institute). The geographical center of the

outbreak was determined using minimum polygon fitting methods (Harvey and Barbour, 1965). With this

technique, line segments were drawn to connect the outermost case sites and then the resulting polygon’s

geometric properties were determined. Fourier polygon models were generated in TNTmips, version 6.5

(MicroImages, Inc., Lincoln, NE), using a point data fitting method described by Anderson (1982).

Vegetation characteristics were analyzed using biweekly composites of NDVI values. Three time periods that

spanned the midpoint of the epizootic were used and these included: September 8 - 21, September 21 - October 

5, and October 6 - 19. The ISODATA unsupervised classification algorithm (Tou and Gonzales, 1974) was

applied to organize NDVI values into 10 statistically related classes. A correlation matrix, produced from a class 

co-occurrence analysis and a separability dendogram were constructed to assess class relationships. 
West Nile Virus Report USDA: APHIS: VS

- 6 - August 2001

Results

I. Case-Control Study

A total of 60 equine cases of clinical disease due to WNV infection was confirmed by USDA: APHIS: VS in the 

northeastern US in 2000. Of the 60 equids, 37 survived and 23 died or were euthanatized (38% case-fatality 

rate). Equids ranged in age from five months to 38 years with an average age of 14 years; 36 confirmed cases of 

WNV infection occurred in male horses and 24 in females. These cases occurred in seven states including

Connecticut (seven confirmed cases of WNV infection in five counties), Delaware (four confirmed cases in two 

counties), Massachusetts (one confirmed case in one county), New Jersey (27 confirmed cases from 11 counties), 

New York (19 confirmed cases in four counties), Pennsylvania (one confirmed case from one county), and

Rhode Island (one confirmed case from one county).

Forty-one (41) of the 60 premises with USDA: APHIS: VS confirmed cases referred to above participated in this 

case-control study. An additional eight premises were identified through serologic testing conducted as part of 

this study to have evidence of equids exposed to WNV. Of the 49 premises in this study with equids that had 

evidence of WNV exposure (case premises, see definition page 2 and 3), nine had more than one case equid: five 

premises had two case equids, three premises had three case equids, and one premises had five case equids. 

Eight of the 49 case premises had one equid that was seropositive but had no equids with clinical signs of WNV 

infection.

The eight premises which had equids with no clinical signs but serologic evidence of exposure (case premises, 

see definition page 2 and 3) had a mean of 22.0 equids bled per premises and a mean of one seropositive equid 

per premises. The 41 case premises with a clinical case had a mean of 10.3 equids bled per premises and a mean 

of 1.4 seropositive equids per premises (this includes 10 equids that were not available to be bled as part of the 

case-control study but were USDA: APHIS: VS confirmed as WNV infected). The 101 control premises had a 

mean of 9.0 equids bled per premises.

Table 1. Premises Results

Percent case premises and percent control premises by the following characteristics:

 

 

4Note: The observed association between premises’ infection status and 

available acreage was not statistically significant (p=0.35).

Table 1

Number of Premises 49 101

Characteristics of Premises:

Percent Case 

Premises 

(confirmed clinical case 

or seropositive equid)

Percent 

Control 

Premises

1-1. Types of Equids Present on Premises:

Horse 98.0 98.0

Mule 2.0 1.0

Donkey or Burro 12.2 2.0

Pony 28.6 37.6

Miniature horse 12.2 7.9

1-2. Acreage Available for Equine Turn-out:

Less than one acre per equid 12.2 6.9

One plus acre per equid 87.8 93.1

100.0 100.0
USDA: APHIS: VS West Nile Virus Report

August 2001 - 7 -

4Note: The observed association between premises’ infection status and 

stall housing only was not statistically significant (p=0.16).

4Note: The observed association between premises’ infection status and 

any birds present in the barn was not statistically significant (p=0.15).

1-3. Other Types of Domestic Animals Present on Premises:

Cattle 12.2 5.9

Pigs 6.1 4.9

Sheep 14.3 2.0

Goats 24.5 16.8

Chickens 26.5 26.7

Turkeys 4.1 2.0

Domestic waterfowl 8.2 14.9

Other fowl (Pheasants, Guinea Fowl, Peacocks) 14.3 6.9

1-4a. Housing Type - Percent Premises that Housed 1 or More Equids by 

Day in:

Stalls only 34.7 23.8

Outdoor but some shelter

(either stalls with runs, dry lot with shelter or 

pasture with shelter)

67.3 67.3

All outdoors (dry lot or pasture only) 32.7 38.6

1-4b. Categories of Housing Type - Percent Premises Housing by Night in:

Stalls only 44.9 39.6

Outdoor but some shelter 

(either stalls with runs, dry lot with shelter, or 

pasture with shelter)

63.3 65.4

All outdoors (dry lot or pasture only) 28.6 24.8

1-5. Categories for Insect Control Methods Currently in use in Equine 

Housing Area:

Screen on barn or stalls 18.2 11.7

Chemicals (spray, foggers, topical repellent on 

horse, other chemicals)

34.1 35.8

Mechanical (zappers, tape, traps) 40.9 36.2

1-6. Mosquito Habitat - Within 1⁄4 Mile of Equids, in Past 30 Days:

Ditches with standing water 44.9 47.5

Other standing water 87.8 72.3

If Other Standing Water is Yes, Specifically:

Temporary pond 30.2 31.5

Waste lagoon 0.0 0.0

Fresh water marsh 34.9 34.3

Salt marsh 7.0 2.7

Swamp or pond 58.1 65.7

1-7. Primary Water Delivery Systems for Equids:

Ground water 

(running water, ditch or surface)

6.3 4.1

Man-made water delivery (bucket, trough, 

bucket and trough, automatic waterer)

93.7 95.9

100.0 100.0

1-8a. If Barn = Yes, then (N= 44 94)

Barn is designed to minimize roosting 

birds

12.5 19.3

1-8b. Any Birds in Barn: 88.6 78.5
West Nile Virus Report USDA: APHIS: VS

- 8 - August 2001

Percent Premises vs Presence of 

Wild Bird Congregations

69.4

93.9

45.8

53.5

30.7

96.0

0%

20%

40%

60%

80%

100%

Blackbird roost Windrows Waterfowl

congregation

Wild Bird Congregations

Case

Control

4Note: There appeared to be a marginally significant association between 

infection status of premises and the presence of a blackbird roost, p=0.07, 

OR= 1.91, 95% CI: (0.89 <OR< 4.12), and the presence of waterfowl 

congregations, p=0.06, OR=1.97, 95% CI: (0.90<OR<4.34) within 1⁄2 

mile of equids in the last 30 days.

4Note: The observed association between premises’ infection status and 

the percent of premises with any dead birds was not statistically

significant (p=0.18).

*Note: This question was asked to determine if any equids on premises 

since 1999 (when WNV was first recognized in the US) had neurological 

signs.

1-9. Various Specific Wild Bird Congregations:

Blackbird roost within 1⁄2 mile of equids 

in last 30 days

45.8 30.7

Windrows within 1⁄2 mile of equids 93.9 96.0

Wild waterfowl congregations within 1⁄2 

mile of equids in last 30 days

69.4 53.5

1-10. Dead Wild Birds Noted on Premises Since May 1, 2000:

Crows 2.0 1.0

Blue Jays 4.1 0.0

Hawks 0.0 1.0

Other * 28.6 17.8

Any dead birds 28.6 18.8

* 12 premises reported Sparrows, 4- Starlings, 4- Pigeons/Mourning Doves,

2- Geese, 2- Blackbirds, plus a few other types of birds only reported once.

1-11a. Clinical Signs in Equids on Premises Since August 1, 1999*:

Any of below 83.7 2.0

Apprehension 32.7 0.0

Depression 57.1 2.0

Listlessness 59.2 2.0

Head shaking 18.4 1.0

Lip paralysis 16.3 1.0

Ataxia 77.5 2.0

Weak hind legs 63.3 1.0

Inability to stand 42.9 1.0

Limb paralysis 18.7 0.0

Paresis 30.6 0.0

Combo for WNV 

one or more of the following: increased 

apprehension, depression or listlessness 

and one or more of other 7 clinical signs.

67.3 2.0
USDA: APHIS: VS West Nile Virus Report

August 2001 - 9 -

4Note: Equids with clinical signs of WNV were predominantly but not exclusively on case premises i.e., not 

all equids with clinical signs of neurologic disease were positive to serologic testing for WNV antibody. One 

or more equids died on 39 percent of case premises. The majority of case premises had WNV confirmed 

cases in September and October of 2000. Data collector assessment of mosquito habitat and bird

congregation habitat was not different between case and control premises.

1-11b. Of Premises with Equids Noted to have Above Signs, Percent of

Premises Where: (N= 41 2)

At least one equid died 39.0 0.0

At least one equid was tested for WNV 97.6 0.0

At least one equid was positive on test 

for WNV

97.6 0.0

1-11c. Percent of Premises (with Clinical Signs) by Date of Onset of Clinical 

Signs for 1st Equid:

August 9.8 100.0

September 53.7 0.0

October 36.5 0.0

100.0 100.0

1-11d. Of Equids with Clinical Signs of WNV:

Earliest date noted 8/14/00 8/15/00

Latest date noted 10/30/00 8/15/00

1-12a. Data Collector Assessment of Mosquito Habitat:

 (N= 49 100)

Low 16.3 25.0

Moderate or high 83.7 75.0

100.0 100.0

1-12b. Data Collector Assessment of Bird Congregation Habitat:

Low 34.7 26.0

Moderate or high 65.3 74.0

100.0 100.0

1-12c. Data Collector Assessment of Cleanliness of Operation:

Poor 2.1 0.0

Fair 23.4 14.1

Good or excellent 74.5 85.9

100.0 100.0
West Nile Virus Report USDA: APHIS: VS

- 10 - August 2001

Table 2. Animal Results on Case Premises

Percent case equids on case premises and percent non-case equids on case premises by the following characteristics: 

the bold numbers for each characteristic are the number of equids with data for that characteristic.

Percent of Equids by Use

7.6

11.3

15.1

66.0

15.8

11.1

25.7

47.4

0%

20%

40%

60%

80%

Pleasure Show/comp Breeding Farm/ranch

Primary Use

Case

Non-case

4Note: There was an association between horses’ infection status and 

use for pleasure vs. other use on case premises, p=0.01, OR=2.16, 

95% CI: (1.15<OR<4.07).

4Note: There was no association between age and exposure to WNV 

status (p=0.7).

Table 2

Total Number of Equids 54 540

Characteristics:

Percent 

Case 

Equids

Percent

Non-case

Equids

2-1. Primary Use: 53 540

Pleasure 66.0 47.4

Show, competition or race 15.1 25.7

Breeding 11.3 11.1

Farm, ranch or other 7.6 15.8

100.0 100.0

2-2. Age: 34 213

Less than 5 years 11.8 16.4

5 to 19 years 70.6 64.3

20 plus years 17.6 19.3

100.0 100.0

2-3. Gender: 33 180

Female 42.4 40.0

Male* 57.6 60.0

*There were no positive stallions in this 

study.

100.0 100.0

2-4. Light Color: 51 525

Yes 23.5 19.8

No 76.5 80.2

100.0 100.0
USDA: APHIS: VS West Nile Virus Report

August 2001 - 11 -

Percent of Equids that Were

Housed in Stalls

15.1

35.9 43.4

56.3

50.0

14.4

0%

20%

40%

60%

80%

Day Night Either

Stalling

Case

Non-case

4Note: There was some evidence that case equids were less likely to be 

housed in stalls, p=0.07, OR=0.60, 95% CI: (0.32<OR<10.9). This 

appeared to be especially important at night.

4Note: The observed association between use of insect protection at 

night and exposure to WNV status was not statistically significant 

(p=0.13).

4Note: Although there was an observed tendency for case equids not to 

have been out of the county, this was not statistically significant 

(p=0.17).

2-5. Housed in Stall: 53 540

Daytime:

 Yes 15.1 14.4

 No 84.9 85.6

Night:

 Yes 35.9 50.0

 No 64.1 50.0

Either:

 Yes 43.4 56.3

 No 56.6 43.7

2-6a. Insect Protection (Day): 51 517

Yes 52.9 50.7

No 47.1 49.3

100.0 100.0

(Type of insect protection was almost exclusively insecticide/repellent)

2-6b. Insect Protection (Night): 51 517

Yes 17.7 10.6

No 82.3 89.4

100.0 100.0

2-7. Signs Since 6/1/2000: 54 540

Apprehension 16.7 0.0

Depression 31.5 0.4

Listlessness 35.2 0.4

Head shaking 9.3 0.2

Lip paralysis 7.4 0.0

Ataxia 42.6 0.6

Weak hind legs 35.2 0.2

Inability to stand 14.8 0.0

Limb paralysis 5.6 0.0

Paresis 11.1 0.0

Any of the above 46.3 0.6

Combination for WNV 38.9 0.4

2-8. Travel History: 51 454

Yes 9.8 17.4

No 90.2 82.6

(Has been out of the county in previous 

45 days)

100.0 100.0
West Nile Virus Report USDA: APHIS: VS

- 12 - August 2001

II. Spatial Analysis

A total of 64 premises were identified as having one or more equids infected with WNV. During the outbreak, 

56 premises were confirmed to have one or more equids infected with WNV and, of these premises, 95% of the 

case sites were from New Jersey (48.2%), New York (26.8%), Connecticut (12.5%), and Delaware (7.1%). 

Rhode Island, Massachusetts, and Pennsylvania each had one case premises. Of the 56 premises identified

during the outbreak, 41 premises were included in the case-control study. Serological testing of equids on farms 

selected as control sites resulted in finding eight additional premises with a subclinically infected equid. Figure 1 

shows the number of case premises in relation to the states affected.

The geographic distribution of premises with case equids is shown in Figure 2. The geographic extent of equid 

infections in 2000 ranged from Middlesex County, Massachusetts, in the north and east, to Kent County,

Delaware, in the south, to Dauphin County, Pennsylvania, in the west. Based on minimum polygon fitting

methods, the estimated geographic center of the epizootic was at 40.76o latitude and -73.99o longitude, which is 

located in the vicinity of Manhattan, New York. Figure 2 also shows, as points in red, the location of premises 

used in the case-control study. The location of all case and control sites was determined from GPS observations, 

using standardized methods. However, in a few instances, premises coordinates were not available and, using the 

Getty gazetteer, geographic coordinates of the nearest town were used instead. Gazetteer-referenced locations 

are shown in Figure 2 as green points.

The temporal pattern of case incidence is illustrated by the epidemic curve, shown in Figure 3. The outbreak 

peak occurred about the first week of October followed by a sharp reduction in cases in mid-October. Figure 4 

shows that the affected area received frequent rainfall during the time of the epizootic. The amounts received 

were most likely sufficient to maintain larval breeding sites for those mosquito species found associated with 

WNV (Nasci, 2001). Mean daily minimum temperatures in the affected area are shown in Figure 5. After 

August 6, the mean daily minimum temperatures remained consistently below 60o F and by August 8 minimum 

temperatures were frequently below 50o F.

A comparison between weather observations, averaged for August, September, and October, and expected 30-

year climatic values for precipitation and minimum temperature is shown in Table 3. This table indicated that 

observed weather conditions were nearly normal relative climate expectations for each of the months compared. 

In October, there was a 53.1 percent decrease in observed precipitation from expected levels.

Table 3. Comparison of weather observations with expected climate values for precipitation and minimum 

temperature in the 32,900 mi2, affected area.

Month

Climate Trend 

Average Total 

Precipitation (mm)

Actual Average 

Total Precipitation 

(mm)

Climate Trend 

Average Minimum 

Temperature (oF)

Actual Average 

Minimum 

Temperature (oF)

August 102.5 104.6 60.0 59.0

September 97.3 100.3 52.4 51.1

October 89.4 41.9 41.3 40.2

The geographic organization of the temporal and spatial pattern of case occurrence is illustrated in Figure 6. 

Temporal data were based on the day of onset of clinical signs. Of the first five cases reported during the 

outbreak’s initial seven days, the first case occurred in Rhode Island, followed by southern New Jersey, then 

eastern Massachusetts, followed again by southern New Jersey, and then a case was reported in central

Connecticut. Table 4 shows the straight-line distances between consecutive equid cases. The average distance 

between consecutive cases was nearly 100 miles (range 8 - 286 miles) and the average number of days between 

reported cases was slightly less than 2 (range 0 - 13; median = 1 day). The relationship between consecutive case 

distances and the time between case reports is shown in Figure 7. No specific spatial or temporal pattern was 

observed between consecutive cases, regardless of the duration between reported cases.
USDA: APHIS: VS West Nile Virus Report

August 2001 - 13 -

The location of control premises relative to each case site and to other WNV case premises is shown in Figure 8. 

The average distance between a case site and the respective control premises selected is shown in Table 5. 

Control premises were generally located about 2 miles (range = 0.29 to 4.59 miles) from each case site. Also, the 

mean distance between a case and associated controls was less than the mean distance between a case and the 

nearest other case by about 10 miles.

Table 4. Days and distances between pairs of consecutive equine cases

Case 

Order 

Distance 

(mi)1,2

Days Between 

Cases3

Case 

Order

Distance 

(mi)1,2

Days Between 

Cases3

1-2 212 4 21-22 98 1

2-3 268 0 22-23 39 1

3-4 286 1 23-24 54 0

4-5 205 2 24-25 15 2

5-6 119 3 25-26 69 3

6-7 79 3 26-27 116 2

7-8 82 2 27-28 145 0

8-9 67 0 28-29 26 3

9-10 140 8 29-30 72 0

10-11 162 2 30-31 8 0

11-12 10 0 31-32 15 0

12-13 229 1 32-33 49 1

13-14 108 0 33-34 73 0

14-15 17 0 34-35 182 2

15-16 101 1 35-36 146 0

16-17 78 0 36-37 86 1

17-18 14 4 37-38 139 0

18-19 9 0 38-39 96 1

19-20 22 2 39-40 48 13

20-21 111 0 40-41 86 5

1 Lambert Conformal Conic Projection for the continental United States used as the 

projection for distance calculations. Euclidean distances only were calculated.

2 Mean (± SD) distance in miles was 97.0 (± 72.1).

3 Mean (± SD) days between consecutive cases was 1.7 (± 2.5).

Table 5. Mean distance between a case premises and associated control premises, and the mean distance between a 

case premises and the nearest other case premises

Total 

number 

cases

Total 

number 

controls

Mean (± SE) number control 

premises per case

Mean (± SE) distance from 

a case to controls (mi)

Mean (± SE) distance 

from case to nearest 

other case

41 101 2.5 (± 0.4) 2.2 (± 0.2) 12.7 (± 2.6)

The relationship between county-level equid population estimates, displayed as a quintile distribution, and the 

location of affected premises is shown in Figure 9. Affected premises were generally found in those counties 

with greater estimated numbers of equids. Both linear regression and analysis of variance (ANOVA)

comparisons of case numbers per county with equine census estimates showed statistically significant

associations between number of cases and estimates of equid numbers (p<0.01), equid density (p<0.05), and the 

numbers of equid farms per county (p<0.05).

A comparison of the number of reported clinical human cases per county and the location of equid cases is

shown in Figure 10. The geographic range of equine cases was considerably greater than the extent of human 

cases. It is noteworthy that, in spite of the wider spatial distribution of infected equids, the geographic center of 

equid cases was located near the geographic center of those counties reporting human cases. Linear regression 

and ANOVA comparisons between counties reporting clinical human cases and counties with infected equids did 

not show a statistically significant relationship (p>0.05).
West Nile Virus Report USDA: APHIS: VS

- 14 - August 2001

Figure 11 shows a comparison between WNV isolations from mosquitoes collected from counties in the affected 

area and the location of case equine premises. About half of the affected premises were located in counties 

where no isolations of WNV were made from mosquitoes. Linear regression and ANOVA comparisons of

equine cases per county with the number of virus isolations from mosquitoes did not show a statistically

significant association (p>0.05).

A comparison between counties reporting one or more WNV positive dead birds and the location of premises

with infected equids is shown in Figure 12. This illustration shows that counties with larger numbers of infected 

dead birds appear to be associated with areas having the most equine case sites. Regression models were created 

to determine if the number of infected dead birds reported from a county could be used to predict which counties 

would be most likely to have premises with infected equids. Both linear and spatial linear regression models 

showed that WNV infected dead bird numbers were statistically significant (p<0.001) in predicting which

counties are most likely to have one or more equine cases. Illustrated in Figure 13 is a plot of the calculated 

predictive values from the spatial linear regression model to indicate the potential influence of dead bird numbers 

on the probability of having one or more WNV affected premises. This figure highlights those counties where 

equid cases might have been expected, based on the dead bird count.

As a preliminary characterization of the WNV affected region, a comparison of case premises relative to

described ecological regions is shown in Figure 14. As can be seen here, most of the case premises are located in 

the Mid-Atlantic Coastal Plain and the New England Lowlands of the Eastern Broadleaf Forest (Oceanic)

Province. Ecological characteristics of each ecoregion section with positive case sites are described in Table 6. 

Approximately 92 percent of all case premises were Eastern Broadleaf Forest (Oceanic) Ecoregion Province and, 

within the next smaller ecoregion classification, about 72 percent of case premises were found in plateau

ecoregion sections of either the Lower (Southern) New England or the Upper Atlantic Coastal Plain.

Table 6. Ecoregion characteristics and the distribution of case premises by ecoregion section

Ecoregion 

Division

Ecoregion 

Province Province Description Ecoregion Section

No. (%) 

Case 

Premises

Hot 

Continental

Eastern Broadleaf 

Forest (Oceanic)

Mostly plateau with some hilly and mountainous 

landforms; climate has a strong annual temperature cycle 

with abundant rainfall greater in summer months; soils 

are characteristically Alfisols1.

Hudson Valley 6 (10.0)

Hot 

Continental

Eastern Broadleaf 

Forest (Oceanic) As above

Lower 

(Southern)

 New England

20 (33.3)

Hot 

Continental

Eastern Broadleaf 

Forest (Oceanic) As above

Northern 

Appalachian 

Piedmont

6 (10.0)

Hot 

Continental

Eastern Broadleaf 

Forest (Oceanic) As above Upper Atlantic Coastal Plain 23 (38.3)

Hot 

Continental 

Regime

Central Appalachian 

Broadleaf ForestConiferous ForestMeadow

Low mountain landforms, 300–6,000 feet, and dissected 

plateaus; distinct climate and seasons; precipitation is 

even throughout the year; soils are Ultisols1 on ridges 

and Inceptisols1 in steep areas.

Northern Ridge 

and Valley 1 (1.7)

Subtropical Outer Coastal Plain 

Mixed Forest

Flat and irregular plain gently sloping to sea level, 

mostly less than 300 ft.; abundant rainfall distributed 

throughout the year; soils are mostly Ultisols1

, 

Spodosols1, and Entisols1.

Middle Atlantic 

Coastal Plain 4 (6.7)

1 Soils classes (Donahue, et al., 1990)

Alfisols - High clay content soil that is not too highly leached, found in forested areas.

Inceptisols - Young soils that are in early stages of development. Only the more rapidly formed horizons present.

Ultisols - Clay soils that are leached, tend to be more red and yellow in color.

Spodosols - Light gray colored soils that are highly leached, found in humid forested areas.

Entisols - Very young soils with little or no horizon development. 
USDA: APHIS: VS West Nile Virus Report

August 2001 - 15 -

Because terrain elevation is a principal characteristic of an ecoregion, we next compared case locations with

terrain elevation data. Digital Terrain Elevation Data (DTED) in a grid/raster format were used to estimate land 

elevation and terrain features in and around each case equid site. Figure 15 shows the location of case sites

relative to the DTED elevation grid. Most cases occurred in the coastal lowland areas. To compare

quantitatively, grid cell values were extracted for the point location of each case premises. Figure 16 shows the 

elevation values associated with each case site. The mean (±SD) elevation value for all case sites was 192.4 (± 

29.3) feet (range 10 – 784 feet).

Several methods were used to consider whether equid cases were clustered and to characterize the strength of 

spatial associations among any clusters found. Preliminary tests to detect spatial clustering used the K-function 

statistic as an estimate of case aggregation. The K-hat value was calculated for each case site and then compared 

with the results of a random, binomial simulation. Monte Carlo methods, with 99 iterations, were used to

simulate expected K-hat values for an equal number of points that showed complete spatial randomness. Figure 

17 shows a comparison between K-hat values calculated for case sites (circles) and predicted K-hat values (lines 

to show the mean, and upper and lower 95% confidence intervals), if the process were completely random. Most 

of the K-hat values calculated for case premises fell outside of the values predicted for a random process,

suggesting that case sites are spatially clustered. It is possible that equine populations may be aggregated into 

clusters, however demographic information to assess population dispersal was not available.

The strength of the spatial associations within the case site cluster was measured by calculating Geary’s C

statistic of autocorrelation. An irregular lattice method was used in which the number of equid cases per county 

was evaluated over the entire, seven-state region. Also, the value of Geary’s C for counties within the region was 

compared with a value calculated similarly if all counties were affected randomly. Geary’s C index of spatial

autocorrelation was 2.04, which was statistically significant (p<0.001) when compared with values estimated for 

a random association. To determine if case sites were clustered within the affected region, the SatScan method 

was employed, using a Bernoulli model applied to case and control premises. Results of this test yielded an 

overall relative risk of 1.6 for the region, plus a log likelihood ratio of 8.2. Comparison with stochastic

calculations showed that case premises are clustered and that this cluster is statistically significant (p = 0.025).

The first-order properties of the stationary point pattern of case sites are demonstrated in an intensity plot shown 

in Figure 18. Intensity calculations were based on the mean number of case sites per square mile. The binning 

method was applied in these calculations, followed by a loess function to smooth the results. Although outlier 

points created some background noise, approximately five to six case clusters can be observed with this method.

Fourier polygon modeling methods were used to show which equid case sites might be included in potential

clusters, depending on the level of sensitivity selected. Four levels of sensitivity were considered: low, lowmedium, medium-high, and high. Going from low to high, each level was used to define a smaller and smaller 

center of WNV activity based on the distance between case premises. The location of polygons calculated from 

the Fourier model is shown in Figures 19 and 20. At a low sensitivity, six possible clusters of cases were 

bounded and these were located in: northern Delaware, southern New Jersey, central New Jersey, northern New 

Jersey, eastern Connecticut, and east-central Connecticut. Case premises in Rhode Island, Massachusetts, and 

Pennsylvania were considered as outliers in the clustering process. As the polygon model sensitivity was

increased, fewer clusters of case sites were bounded. At the highest level of sensitivity, a single center of activity 

was located over central Monmouth County, New Jersey. These results correspond well with observations

obtained from the intensity analysis.

Respondents were asked to report occurrence of blackbird roosts because land residents were most likely to 

observe these bird sites. Blackbird roosts were defined as small wood lots (generally three to five acres) where 

many blackbirds congregate at night. Because the case-control survey suggested possible associations between 

proximity to blackbird roosts and congregations of waterfowl, we compared the location of respondents who said 

yes to either of these situations with the location of the cluster polygons. Figure 19 shows the location of 

survey respondents who reported being within one-half mile of a blackbird roost. Five of six clusters, defined at 

the low sensitivity level, had one or more premises reporting proximity to a communal bird roost (CBR). Of the 

five clusters located in proximity to a CBR, two clusters had two CBR premises, one cluster had three CBR 

premises, and the cluster located near Monmouth County, New Jersey, had eight CBR premises.
West Nile Virus Report USDA: APHIS: VS

- 16 - August 2001

The geographic distribution of case-control respondents reporting nearby congregations of waterfowl is shown in 

Figure 20. This figure also shows the spatial relationship between a predicted cluster, described above, and 

premises located in proximity to waterfowl congregations (WFC). In this case, all six clusters, defined at the low 

sensitivity level, had one or more premises with nearby WFC. Of the six clusters located in proximity to a WFC, 

one cluster had one WFC premises, two clusters had two WFC premises, three clusters had three WFC premises, 

and the cluster located near Monmouth County, New Jersey, had ten WFC premises.

The spatial pattern of vegetation types was examined using biweekly composites of NOAA satellite sensor data 

acquired during the time of the epizootic. NDVI values derived from AVHRR sensor information were used to 

compare shifts in vegetation biomass between September 8 and October 19. Shifts in NDVI values derived from 

vegetation in the affected area can be seen in Figure 21. Table 7 shows a comparison of central NDVI values 

(N=86,445 cells) obtained from the biweekly data. These measures indicate that, during the epizootic, there was 

a decline in chlorophyll reflectance, which was most probably due to a seasonal transition.

Table 7. Middle points in the distribution of scaled NDVI values

Time period Mean (± SD) Mode Median

Sept 8 – 21 149.8 (± 10.5) 155 152

Sept 22 - Oct 5 146.3 (± 9.6) 150 148

Oct 6 – 19 134.1 (± 7.9) 136 135

Spectral patterns within the NDVI values for the WNV affected region were analyzed using statistical methods of 

the ISODATA classifier. Scaled index values from the three biweekly composites were grouped into 10 classes, 

a noise reducing 3x3 modal filter was applied, and each class was assigned a different color. The results, shown 

in Figure 22, show that each of the 49 case sites is associated with one or two specific classes. The hierarchical 

classification structure, based on the transformed divergence method, is also shown in Figure 22. Using grid 

analysis methods along with the geographic coordinates of case locations, classes 2, 4, 5, 6, 7, and 9 were

identified as being near each case site. However, as shown in Figure 23, class 6 was most frequently associated 

with equid case sites. Grid analysis methods were also applied to the polygon boundaries of the predicted cluster 

outlines. Figure 24 shows the spectral patterns associated with each of the predicted clusters. In all but one of 

the clusters, class 6 occurred more frequently than other classes. Because the ISODATA classifier is an

unsupervised method, predominate vegetation types in each class are presently unknown.
USDA: APHIS: VS West Nile Virus Report

August 2001 - 17 -

Figure 1. The number of case premises in each affected state

Figure 2. The geographic distribution of premises with one or more infected equids relative to county 

locations in affected states.

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West Nile Virus Report USDA: APHIS: VS

- 18 - August 2001 6 Aug 13 Aug 20 Aug 27 Aug 3 Sept 10 Sept 17 Sept 24 Sept 1 Oct 8 Oct 15 Oct 22 Oct 29 Oct 5 Nov 5 Nov

Week of clinical onset

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Figure 3. Epidemic curve for equid cases in the 2000 epizootic

8/1/00 8/9/00 8/17/00 8/25/00 9/2/00 9/10/00 9/18/00 9/26/00 10/4/00 10/12/00 10/20/00 10/28/00

Date

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Figure 4. Mean daily precipitation in inches for the WNV affected area.
USDA: APHIS: VS West Nile Virus Report

August 2001 - 19 -

8/1/00 8/9/00 8/17/00 8/25/00 9/2/00 9/10/00 9/18/00 9/26/00 10/4/00 10/12/00 10/20/00 10/28/00

Date

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o F)

Figure 5. Mean daily minimum temperatures (oF) for the WNV affected area.

Figure 6. Chronological order of case premises being reported. The first case is located in southeastern 

Rhode Island.

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West Nile Virus Report USDA: APHIS: VS

- 20 - August 2001

Figure 7. Distance (miles) between consecutive cases

Figure 8. The location of premises used in the case-control study relative to other case premises not included 

in the study.

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Days between cases

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USDA: APHIS: VS West Nile Virus Report

August 2001 - 21 -

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Estimated number of equids per county

16 - 365

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585 - 859

860 - 1281

1304 - 10908

# Case-control positive sites

# Other positive sites

Figure 9. Estimated number of equids per county from the USDA’s Census of Agriculture and the location 

of premises with infected equids

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Figure 10. Counties reporting clinical human cases compared with the location of infected equids
West Nile Virus Report USDA: APHIS: VS

- 22 - August 2001

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Figure 11. Counties reporting West Nile virus isolations from mosquitoes are compared with the location of 

infected equids. Not shown are virus isolates from two pools of mosquitoes collected in Erie County, 

Pennsylvania. Testing of mosquitoes for WNV varied among state and counties in the affected region.

Figure 12. Counties reporting dead birds infected with West Nile virus compared with the location 

of infected equids. Submission and testing of avian samples varied among states and counties in the 

affected region.

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USDA: APHIS: VS West Nile Virus Report

August 2001 - 23 -

Figure 13. Predictive values of a spatial linear regression model compared with the locations of premises 

with case equids

Blue Ridge Mountains

Catskill Mountains

Erie and Ontario Lake Plain

Green, Taconic, Berkshire Mountains

Hudson Valley

Lower New England

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New England Piedmont

Northern Appalachian Piedmont

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Adirondack Highlands

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New York

Connecticut

Massachusetts

Pennsylvania

New Jersey

Delaware

New Hampshire

County outline

State outline

Predicted values

0.205 - 0.297

0.297 - 0.522

0.522 - 0.922

0.922 - 1.609

1.609 - 3.444

# Case site

# Other positive site

Vermont

Rhode Island

Maryland
West Nile Virus Report USDA: APHIS: VS

- 24 - August 2001

Figure 15. Elevation in feet of the affected area compared with case site locations

0 - 100 101 - 200 201 - 300 301 - 400 401 - 500 501 - 600 601 - 700 701 - 800

Elevation (feet)

0

5

10

15

20

25

No. of equid case sites

Figure 16. Elevation in feet of equid case sites

#

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Elevation (feet)

State outline

# Case sites

Sea level

1 - 269

270 - 548

549 - 827

828 - 1106

1107 - 1385

1386 - 1664

1665 - 1944

1945 - 2223

2224 - 2502

2503 - 2781

2782 - 3060

3061 - 3339

3340 - 3618

3619 - 3897
USDA: APHIS: VS West Nile Virus Report

August 2001 - 25 -

Figure 17. K-hat statistic calculated for each case site (circles) and a binomial simulation

(lines: mean, upper CL, lower CL) of K-hat values for a random pattern

Figure 18. Intensity plot of equid case sites showing the mean number of equid case sites per square mile

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Distance (mi)

0

10

20

30

Khat

Intensity

Equid case site
West Nile Virus Report USDA: APHIS: VS

- 26 - August 2001

Figure 19. Fourier polygon models of case clusters compared with case sites reporting proximity to a 

blackbird roost

#S#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

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#S

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New York

Connecticut

Massachusetts

Pennsylvania

New Jersey

Delaware

New Hampshire

County outlines

Low sensitivity

Low medium sensitivity

High medium sensitivity

High sensitivity

Proximity to waterfowl congregations

#S No

#S Yes

Fourier polygon model

Rhode Island

Vermont

Maryland

Figure 20. Fourier polygon models of case clusters compared with case sites reporting proximity to 

waterfowl congregations

S#S

S#

S#

S#

S#

S#

#S

#S

#S

S#

S#

S#

S#

S#

#S

S#

S#

#S

S#

S#

#S SS##SS#

#S

S#

S#

S#

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S#

S#

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S#

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S#

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New Y ork

Connecticut

Massachusetts

Pennsylvania

New Jersey

Delaware

New Ham psh ire

County outlines

Low sensitivity

Low medium sensitivity

Medium high sensitivity

High sensitivity

Proximity to a bird roost

#S No

#S Yes

Fourier polygon model

Rhode Islan d
USDA: APHIS: VS West Nile Virus Report

August 2001 - 27 -

Figure 21. Biweekly composite NDVI values for September 8 – October 19, shown in gray-scale, compared 

with the location of equid case sites and state boundaries.

Class 1

Class 2

Class 3

Class 4

Class 5

Class 6

Class 7

Class 8

Class 9

Class 10

Equid case site

Isodata Class

Class Separability Dendogram

Figure 22. Isodata classified NDVI values from September 8 – October 19 and compared with the location of 

equid case sites. Inset shows class relationships and separability.

Sep 8 - 21 Sep 22 - Oct 5 Oct 6 - 19

Equid case site

State outlines
West Nile Virus Report USDA: APHIS: VS

- 28 - August 2001

2 4 5 6 7 9

Isodata class

0

5

10

15

20

Number of observations

Figure 23. Distribution of Isodata class assignments relative to observations made for each equid case site.

Figure 24. Isodata classified NDVI values from September 8 – October 19 combined with the location of 

equid case sites and outlines of predicted clusters. Inset shows the median class value for each cluster.

Class 1

Class 2

Class 3

Class 4

Class 5

Class 6

Class 7

Class 8

Class 9

Class 10

Equid case site

Predicted cluster

1

2

3

4

5

6
USDA: APHIS: VS West Nile Virus Report

August 2001 - 29 -

Discussion

I. Case-Control Study

One of the strengths of this study is that it represents a large number of equine premises and equids. As part of 

the case-control study a total of 150 premises contributed data and a total of 1,487 equids were tested for

presence of exposure to WNV. The number of equids bled per premises ranged from one to 73, with a median of 

seven equids per premises bled as part of the case-control study.

This study also represents a joint effort of the field veterinary personnel in the northeastern region, personnel

from the USDA: APHIS: VS Area offices, the USDA: APHIS National Veterinary Services Laboratories, the 

USDA: APHIS: VS Centers for Epidemiology and Animal Health, State Veterinary Offices, and from USDA:

APHIS: VS Emergency Programs. Major contributions to the study were made by data collectors in the field, by 

equine owners who contributed information about their premises and their equids, by private veterinary

practitioners who reported suspect cases to state veterinary offices and assisted in collection of blood samples on 

some control premises, and by the equids themselves that contributed biologic samples (i.e., blood samples). 

A limitation of the study is the lack of complete data. Some information for individual equids was not provided 

and data were available from only a limited number of premises from one geographic region that had 

approximately one third of the clinical cases (for a description of these cases, see: Trock, et. al, 2001).

Findings of note include a marginally significant association between the presence of a blackbird roost and the 

presence of waterfowl congregations within 1⁄2 mile of the equine premises and having one or more equids with 

evidence of exposure to WNV. It may be useful to map blackbird roosts and wild waterfowl congregations to 

identify geographical areas of greater risk. Potentially focusing vector-control efforts at these sites (blackbird 

roosts and waterfowl congregations) may reduce the risk of exposure for equids in areas where WNV is present. 

Insect control methods at the premises or at the animal level were not significantly different between cases and 

controls. This could be the result of a failure to request data in enough detail, (i.e. determine ingredients of insect 

sprays, etc.) or a true lack of benefit of insecticide applied to horse (difficulty in complete coverage and requires 

frequent application). Based on survey results, it would appear that most insect control efforts were directed at 

fly control which may not be effective in reducing exposure to mosquitoes (likely vector of WNV). 

For case premises (one or more equids with evidence of exposure to WNV) at the animal level, there was a 

difference in use of the equid and housing between those equids with evidence of exposure to WNV and those 

with no evidence of exposure. Perhaps pleasure horses were more likely to be exposed to the vector for various 

reasons. For example, one potential reason could include trail-riding activities.

The clinical signs consistent with WNV infection were observed in a small percent of serologically negative

equids. This is not surprising as the neurologic signs of WNV infection are not unique and can be caused by 

other agents or diseases. However, in the northeast region, any horse with neurologic disease should be

investigated for a diagnosis of WNV infection based on the rarity of neurologic signs in seronegative horses. 

Even seropositive horses though, do not necessarily show neurologic signs. Less than half of the case horses in 

this study showed signs suggestive of WNV infection.
West Nile Virus Report USDA: APHIS: VS

- 30 - August 2001

II. Spatial Analysis

The geographic extent of equids infected in the 2000 WNV outbreak was considerably greater than the area 

involved in the 1999 outbreak when only Suffolk and Nassau counties, located on Long Island, New York, were 

affected. In that outbreak, 19 premises with equids were involved compared with the 64 premises in 2000 

outbreak. The temporal pattern of clinical onset was similar in both epizootics, beginning in mid- to late August 

and ending approximately the last week of October. Meteorological data for the affected region showed that this 

area received sufficient rainfall to support the development of aquatic stages of mosquitoes. However, low

temperatures in mid-October may have reduced mosquito-biting activity to a minimal level, thus bringing an end 

to the outbreak.

The geographic tracking of new case sites showed that consecutive cases appeared to occur randomly. Predicting 

where a new case will occur, based on the location of previous case sites, is presently not possible without

knowing the specific factors that influence disease spread. Although the temporal and spatial occurrence of new 

cases appears to be haphazard, a key event that must occur first is the establishment of a focus of infection and 

transmission in an area near susceptible equids. The habitat within a focus must be suitable to establish both 

mosquito-to-bird and bird-to-mosquito transmission cycles. It is possible that the timing of consecutive cases is 

related to the establishment of WNV in areas where equid exposure to infected, mammal feeding, mosquitoes is

most likely to occur. If this is the situation, then it may be possible to identify WNV foci based on the presence 

of specific habitat components, such as vegetation and standing water. However, predicting when a case is likely 

to occur will probably remain unreliable.

In the case-control study, all premises selected as control sites were located near a case premises. It is probable 

that case and control premises shared similar environments and risks of exposure. The serological detection of 

eight previously unrecognized, subclinical WNV infections as part of the control group evaluation raises the idea 

that infection risk may be increased for equids located near a natural focus of infection. To support or refute 

such a relationship, additional testing of horses on premises not located near an equine clinical case of WNV

infection needs to be conducted.

Equid population estimates, based on animal numbers, equid density, and the numbers of farms with equids, 

suggest that when more animals are exposed there is a greater chance that one or more equids will become 

infected. The accuracy of the U.S. Census of Agriculture demographic data for equids is unknown; however, it 

may represent an underestimate of the actual population size and number of equid farms. Accurate and timely 

equid census data would provide a better means of estimating vector-equid contact rates in an area. Vector-host 

contact information is important in planning outbreak mitigation efforts. 

Considering the relatively wide geographic range of sites reporting wild bird and equid infections, it is surprising 

that WNV infections in humans and mosquitoes were not found over a more extensive area. However, the 

number of dead birds reported infected with WNV seemed to have predictive value in indicating where equid 

cases would be expected to occur. In our analysis, all free-ranging, avian species were combined as a total

number of WNV infected dead birds per county. It is possible that one or more bird species may be a better 

indicator of virus activity in an area rather than using combined totals for all avian species. Another

consideration is that accuracy and consistency in reporting dead birds is dependent on surveillance systems

developed by states and counties. These reporting systems may not be timely enough to provide sufficient data 

for predictive modeling. Further studies are needed regarding how to interpret dead bird counts relative to the 

risk of WNV infection in equids.

Most case premises were within ecoregions characteristic of Bailey’s (1995) Eastern Broadleaf Forest category 

and these sites were generally situated in a coastal plain. Comparisons with the DTED model showed that most 

cases occurred in areas below 200 feet in elevation. Hydrologic basins in these areas are often poorly drained 

and are frequently characterized by well-developed wetlands. The specific role that wetlands might serve in the 

transmission of WNV to equids was not be determined by the case-control study. It is possible that marshes, 

swamps, or similar wetland habitat might act as focus of infection that establishes, at least temporarily, WNV
USDA: APHIS: VS West Nile Virus Report

August 2001 - 31 -

activity in a suitable area. Virus transmission to equids could then occur if competent vectors and a sufficient 

number of susceptible hosts are present.

Evidence of spatial clustering was supported by several separate measures. Ripley’s K-statistic showed that the 

pattern of case sites was not randomly arrayed. A high level of spatial autocorrelation was demonstrated by 

Geary’s C index. This indicated a strong spatial association among all case sites. In addition, the SatScan 

statistic, based on the use of likelihood ratios, provided further evidence of clustering. According to Kulldorff 

and Nagarwalla (1995), the SatScan method can detect clusters of any size, located anywhere within a study 

region. This method is not restricted to clusters conforming to administrative boundaries. Further work with the 

SatScan method is needed to identify those case sites that can be assigned to specific sub clusters.

A possible association between case sites and proximity to a CBR or a WFC was identified in the case-control 

survey. Additional support of this association was provided by the spatial analysis. Most case premises

reporting proximity to a CBR or a WFC were related to case clusters, as defined by the Fourier polygon

modeling method. When two or more case respondents replied affirmatively to the CBR or WFC questions, it is 

not known whether each one is referring to the same object of risk. It is possible that CBR and WFC are 

indicative of another, as yet unknown, risk factor that is associated with virus transmission. Recent evidence by 

Hodgson, et al. (2001) suggests that defensive behavior by European starlings (Sternus vulgaris) in response to 

mosquito (Culiseta melanura) biting frequently ends in interrupted feeding. Consequently, a partially fed

mosquito is more likely to feed on more than one bird in a communal roost, leading to a more rapid transmission 

of an ornithonotic arbovirus, such as WNV. This mosquito feeding behavior could lead to the swift

dissemination of WNV throughout a population of avian hosts and produce a focus of infection in short amount 

of time.

Satellite imagery was used to characterize vegetation in the affected region. Biweekly composites of NDVI data 

showed a seasonal, downward shift in chlorophyll biomass. These data were used to classify vegetation in the 

affected region. This approach was used to convert highly variable sensor data into specific classes of vegetation

based on standardized statistical methods. Identification of vegetation classes in the vicinity of affected premises 

showed categories common to each site. Because the ISODATA classification algorithm is an unsupervised

technique, it is necessary to establish the identity of each class from field observations. Therefore, information 

about the vegetation feature common to each sub-cluster has yet to be determined.

Conclusions

Based on the spatial analysis of case-control and other data, exposure of equids to WNV is a

geographically clustered event. Within regions of virus activity, exposure of individual equids appears to 

be chance event. Consequently, immunoprophylaxis is warranted for equids in regions where foci of 

WNV are likely to be found. This is similar to the approach used in protecting equids from infections with 

the viruses that cause eastern equine encephalitis and western equine encephalitis. Other recommended 

mitigation methods include reducing the size of vector mosquito populations, especially in areas near

communal bird roosts or waterfowl congregations.

Because there are many unanswered questions about equid exposure to infected mosquitoes in and around 

epizootic foci, future studies of affected, equid premises should include an ecological assessment of the 

surrounding area. It is important to determine which species of mosquitoes feeding on equids are also 

infected with WNV. In addition, the infection status of free-ranging birds in the vicinity of an affected 

premises needs to be determined. The location of communal bird roosts or congregations of waterfowl 

relative to a site with an infected equid needs to be determined more precisely.
West Nile Virus Report References USDA: APHIS: VS

- 32 - August 2001

References

Anderson, J.E. 1982. The home range: a new nonparametric estimation technique. Ecology, 63:103-112.

Bailey, R.G. 1995. Description of the Ecoregions of the United States. 2

nd ed. Misc. Publ. No. 1391 (rev.), 

Washington, DC: USDA Forest Service. 108 pp.

Bailey, T.C. and A.C. Gatrell. 1995. Interactive spatial data analysis. Essex, England: Addison Wesley Longman 

Limited, xiv + 413 pp.

Cressie, N.A.C. 1993. Statistics for spatial data. New York: John Wiley and Sons, Inc., xx +900 pp.

Daly, C. and R.P. Neilson. 1992. A digital topographic approach to modeling the distribution of precipitation in 

mountainous terrain. In, Jones, M.E. and Laenen, A. (eds.) Interdisciplinary Approaches in Hydrology and

Hydrogeology. American Institute of Hydrology, pp. 437-454.

Daly, C. and R.P. Neilson, and D.L. Phillips. 1994. A statistical-topographic model for mapping climatological

precipitation over mountainous terrain. Journal of Applied Meteorology 33:140-158.

Diggle, P.J. 1983. Statistical analysis of spatial point patterns. London: Academic Press.

Donahue, R. L., R. H. Follett, R. W. Tulloch. 1990. Our soils and their management: increasing production through 

environmental soil and water conservation and fertility management. Danville, Illinois: Interstate Publishers, xiv + 

594 pp.

Eidenshink, J.C. 1992. The 1990 conterminous U.S. AVHRR data set. Photogrammetric Engineering and Remote 

Sensing, 58(6): 809-813.

Geary, R.C. 1954. The contiguity ratio and statistical mapping. The Incorporated Statistician 5, 115-145.

Hodgson, J.C., A. Spielman, N. Komar, C.F. Krahforst, G.T. Wallace, and R.J. Pollack. 2001. Interrupted bloodfeeding by Culiseta Melanura (Diptera: Culicidae) on European Starlings. J. Med. Entomol., 39 (1): 59-66.

Harvey, M.J, and Barbour, R.W. 1965. Polygon fitting of Microtus ochrogaster as determined by a modified 

minimum area method. Journal of Mammology 46:398-402.

Jensen, J.R. 1996. Thematic information extraction: Image classification. In, Introductory digital image processing, 

2

nd ed., chapter 8. Upper Saddle River, New Jersey: Prentice Hall. Pp. 236-238.

Kulldorff, M.K. Rand, G. Gherman, G. Williams, and D. DeFrancesco. 1998. SaTScan v2.1: Software for the 

spatial and space-time scan statistics. Bethesda, MD: National Cancer Institute.

Kulldorff, M. and N. Nagarwalla. 1995. Spatial disease clusters: Detection and inference. Statistics in Medicine, 

14: 799-810.

Nasci, R.N. 2001. Mosquito vector associations in the United States: 1999-2000. Proceedings of the International 

Conference on the West Nile virus, April 5-7, 2001, sponsored by the New York Academy of Sciences.

Ostlund, Eileen N. 2001. Equine West Nile Encephalitis, United States. Emerging Infectious Diseases, Volume 7 

(4): 665-669. 

Tou, J. T. and R. C. Gonzales. 1974. Pattern recognition principles. Isodata Algorithm. In, Pattern classification by 

distance functions, chapter 3. Reading, Massachusetts: Addison-Wesley, pp. 97-104.

Trock, Susan C. 2001. West Nile Virus Outbreak Among Horses in New York State, 1999 and 2000. Emerging 

Infectious Diseases, Volume 7 (4): 745-747. 

Venables, W.N. and B.D. Ripley. 1999. Modern applied statistics with S-Plus, 3rd ed. New York: Springer-Verlag, 

xi +501 pp.
USDA: APHIS: VS Appendix A: Sample Questionnaire West Nile Virus Report

August 2001 - 33 -

Appendix A: Sample Questionnaire

Section 1 – Premises Information

Section 1 - WNV Premises Information

Premises Identification:

Premises Identification Number: _______________________________________________

Latitude (in decimal degrees) Longitude (in decimal degrees)

Interviewer name and telephone number: _____________________________________________________

Premises Information

1) How many of the following equids are present on the premises?

Type Number on premises

Horses

Mules

Donkey/burro

Ponies

Miniature horses

Other (Specify ___________________________)

TOTAL

2) How many total outdoor acres have been 

available for equine turnout since June 1, 2000? Acres.

According to the Paperwork Reduction Act of 1995, no persons are required to respond to a collection of 

information unless it displays a valid OMB control number. The valid OMB control number for this

information collection is 2 hours. The time required to complete this information collection is estimated to 

average 2 hours per response, including the time to review instructions, search existing data resources, 

gather the data needed, and complete and review the information collection.

United States

Department of

Agriculture

Animal and

Plant Health

Inspection

Service

Veterinary

Services

West Nile Virus

Data Collection
West Nile Virus Report Appendix A: Sample Questionnaire USDA: APHIS: VS

- 34 - August 2001

3) How many of the following domestic animals are present on the premises? How are they housed?

Species Number

Housing:

Use code 1-3

1 -Outdoors only

2 - Indoors only

3 - Access to both

Distance from equids

(in feet)

Cattle

Pigs

Sheep

Goats

Chickens

Turkeys

Waterfowl

Other fowl 

(specify____________)

Mosquito Abatement in Equid Building

4) How many equids were primarily housed in each of the following ways, during the day and/or twilight 

to midnight, since June 1, 2000?

Housing Type Number Equids – Day Number Equids - Twilight to 

Midnight

Stalls only

Stalls with runs

Dry lot paddock only

Dry lot paddock with shelter

Pasture only

Pasture with shelter

Other

TOTAL

5) Which of the following insect control methods are currently being used in equid housing area?

Control Method - (Use codes 0 - 2)

2 - No 1 - Yes 0 - No barn

Screens on barn openings (windows, doors, vents)

Screens on stall

Bug zappers

Insect foggers

 Number of times daily________________________

 At what time of day usually? (check all that apply)

? morning ? ? afternoon ? dusk ? night

 

Sticky tape without sugar

Sticky tape with sugar

Other 

(specify_________________________________)
USDA: APHIS: VS Appendix A: Sample Questionnaire West Nile Virus Report

August 2001 - 35 -

Mosquito Breeding Habitat 

6) Within 1⁄4 mile (about 500 yards) of where equids are kept, are there: (or has there been in the past 30 

days?)

a) Roadside or irrigation ditches where water is standing? Yes ? No ?

b) Sources of open, standing water present? Yes ? No ? 

If so, indicate the type of standing water (mark all that apply): ? temporary pond ? waste lagoon 

? freshwater marsh ? salt marsh ? swamp

7) What was the primary water delivery system for most equids for most of the time since June 1, 2000? 

Enter code in the box at right (from the list below) for primary water delivery system used.

Ë®code

1 - Running surface water (such as a: creek, stream, spring, river, or irrigation ditch

2 - Ditch 

3 - Non-running surface water (such as: pond or lake)

4 - Buckets only 

5 - Water trough only

6 - Buckets and water trough equally

7 - Automatic waterer

8 - Other, specify__________________________________________________________

Bird Habitat

8) Is there an equid barn on the premises? ? Yes ? No

a) Is the building constructed to minimize roosting wild birds? ? Yes ? No

b) Are barn swallows nesting in the barn? ? Yes ? No

 Type of bird Average Number in barn (circle category closest to estimated number)

House 

sparrows

0 1-9 10-49 50-99 100 ?

Swallows 0 1-9 10-49 50-99 100 ?

Pigeons 0 1-9 10-49 50-99 100 ?

Other (specify) 0 1-9 10-49 50-99 100 ?

9) Wild Birds in the area:

a) Are there any blackbird roosts within 1⁄2 mile of where equids are kept (within last 30 days)? ? Yes ? No

b) Is there any dense vegetation, row of bushes or trees (hedgerows, windbreaks, large willow trees, etc.)? 

Within 1⁄2 mile of where equids are kept? ? Yes ? No

c) Are there congregations of wild waterfowl within 1⁄2 mile of where equid are kept (within last 30 days)? 

? Yes ? No

10) How many dead birds were noted on the premises since May 1, 2000? How many were submitted for 

laboratory testing? To whom were they submitted?

Type Number Number Submitted Submitted to whom?

Crows

Blue Jays

Hawks

Other (specify)
West Nile Virus Report Appendix A: Sample Questionnaire USDA: APHIS: VS

- 36 - August 2001

11) Have any equids on the premises shown any of the following signs since August 1, 1999

a) 

Increased apprehension ? Yes ? No

Depression ? Yes ? No

Listlessness ? Yes ? No

b) If any are yes, did they also show:

Head shaking ? Yes ? No If so; how many equid ____________

Flaccid paralysis of 

 lower lip ? Yes ? No If so; how many equid ____________

Ataxia ? Yes ? No If so; how many equid ____________

Weakness of hind limbs ? Yes ? No If so; how many equid ____________

Inability to stand ? Yes ? No If so; how many equid ____________

Limb paralysis ? Yes ? No If so; how many equid ____________

Paresis ? Yes ? No If so; how many equid ____________

How many equids total are counted in 11b? _________________Did any die? ? Yes ? No

For equids counted in 11b:

What was the approximate date of onset for 1st equid? _____________________

Were any tested for West Nile virus? ? Yes ? No

Where were the tests done? ______________________________________

Were any tests positive for West Nile? ? Yes ? No ? Don’t Know

12) Additional comments: Use the area below (or on the back) for any additional information that was not 

covered in the questions above.

Section 2 - VMO Evaluation Sheet

Section 2 - WNV VMO Evaluation Sheet

1. Rate mosquito breeding habitat: ? low ? moderate ? high

2. Rate bird congregation habitat: ? low ? moderate ? high excellent

3. Rate overall cleanliness of the operation: ? poor ? fair ? good or excellent

Any additional comments about premises or equids?
USDA: APHIS: VS Appendix A: Sample Questionnaire West Nile Virus Report

August 2001 - 37 -

Section 3 – Blood Collection Record

 Section 3 - WNV Blood Collection Record

Premises Identification # _________________ Collection Date: _________________ Page ___________ of __________

Interviewer's Name: ___________________________ Interviewer's Phone Number: _____________________________

Number of Equids on the operation today: ________________ Total Samples Submitted ______________________ Tube #

A.

Primary 

Use 

Code 1-6

B. 

Is equid light 

in Color? 

(White, gray, 

buckskin, 

palomino, etc.) 

Yes/No

C. 

# of days equid 

has been out of 

county in past 45 

days 

If>0, complete 

travel log

D. 

Housed in 

stall -

daytime? 

Yes/No

E. 

Housed in stall 

dusk to 

midnight? 

Yes/No

F. 

Insect 

protection 

used during 

daytime? 

Use code 1-5

G. 

Insect 

protection 

used dusk to 

midnight? 

Use code 1-5

H. 

Overall 

health 

status of 

equid 

Use code 1-2

I. 

Showed signs 

since 6/1/2000 

Record all signs 

(from list below) 

shown since 6/1/2000 

If none, enter 0

Y1 N3 Y1 N3 Y1 N3

Y1 N3 Y1 N3 Y1 N3

Y1 N3 Y1 N3 Y1 N3

Y1 N3 Y1 N3 Y1 N3

Y1 N3 Y1 N3 Y1 N3

Y1 N3 Y1 N3 Y1 N3

Y1 N3 Y1 N3 Y1 N3

Y1 N3 Y1 N3 Y1 N3

Primary Use of Equid: (if 

young, what is intended use) Insect Protection:

Overall Health 

Status of 

Horse:

Signs:

1 - Pleasure 1- Chemical insecticide repellant 1 - Normal 1 - increased apprehension 6 - ataxia (including stumbling)

2 - show or competition 2 - Fly sheets 2 - Abnormal - 2 - depression 7 - weakness of hind limbs

3 - breeding 3 - Screened stall (Specify) 3 - listlessness 8 - inability to stand

4 - racing 4 - Other (specify) 4 - head shaking 9 - limb paralysis

5 - farm/ranch work 5 - None 5 - flaccid paralysis of lower lip 10 - paresis

6 - other If none, enter 0
West Nile Virus Report Appendix A: Sample Questionnaire USDA: APHIS: VS

- 38 - August 2001

Section 4 – Equine Travel Log

Section 4 - WNV Equine Travel Log

Premises ID # __________Interviewer’s Name______________________Phone # ______________

Date Completed _______________

List travel history for each equid that has left the county in past 45 days.

Tube # Traveled to (give city, 

county & state)

Purpose of travel

Use code 1 - 8

(See below)

Number of visits in 

past 45 days

Purpose of travel:

1 - trail ride 3 - race 5 - breeding 7 - sale

2 - show/competition 4 - hospital 6 - other horse premises 8 - other (specify)