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    Annual seroconversion rates to West Nile virus in sentinel chicken flocks in Florida counties. The locations of the sentinel chicken flocks are also shown.

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    Dead bird reports per 100,000 population in Florida counties. The number of birds reported was higher in counties within the epicenters than in other parts of the state.

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    Dead doves reported singly per 100,000 population in Florida counties.

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    West Nile virus (WNV) surveillance results from Leon County, one of the Florida counties with a human case in 2001. Peak numbers of dead corvid reports and clinical horse cases preceded the onset of the human case. Reports of dove mortality did not correlate with other WNV surveillance findings.

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    Distribution of confirmed human and equine West Nile (WN) cases in Florida. The counties included in the two transmission foci are also highlighted.

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    Epidemiology curve of human West Nile virus encephalitis cases in Florida in 2001. The first two cases were residents of Madison County in week 28. The last diagnosed case, with disease onset in week 44, was reported from Marion County.

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    Trock SC, Meade BJ, Glaser AL, Ostlund EN, Lanciotti RS, Cropp BC, Kulasekera V, Kramer LD, Komar N, 2001. West Nile virus outbreak among horses in New York State, 1999 and 2000. Emerg Infect Dis 7 :745–747.

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    Cherry B, Trock SC, Glaser A, Kramer L, Ebel GD, Glaser C, Miller JR, 2001. Sentinel chickens as a surveillance tool for West Nile virus in New York City, 2000. Ann N Y Acad Sci 951 :343–346.

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    Bernard KA, Maffei JG, Jones SA, Kauffman EB, Ebel GD, Dupuis II AP, Ngo KA, Nicholas DC, Young DM, Shi PY, Kulasekera VL, Eidson, M, White DJ, Stone WB, NY State West Nile Virus Surveillance Team, Kramer LD, 2001. West Nile virus infection in birds and mosquitoes, New York State, 2000. Emerg Infect Dis 7 :679–685.

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SURVEILLANCE RESULTS FROM THE FIRST WEST NILE VIRUS TRANSMISSION SEASON IN FLORIDA, 2001

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  • 1 Bureau of Epidemiology, Florida Department of Health, Jacksonville, Florida; Bureau of Laboratories-Tampa, Tampa, Florida; Department of Health, Tampa, Florida; Division of Animal Industry, Florida Department of Agriculture and Consumer Services, Tallahassee, Florida; Division of Health Affairs, College of Medicine, Florida State University, Tallahassee, Florida

After West Nile virus (WNV) was first detected in Florida in July 2001, intensive surveillance efforts over the following five months uncovered virus activity in 65 of the state’s 67 counties with 1,106 wild birds, 492 horses, 194 sentinel chickens, and 12 people found infected with the virus. Thirteen of 28 mosquito isolations came from Culex mosquitoes. As seen in the northeastern United States, wild bird mortality was the most sensitive surveillance method. However, unlike the predominantly urban 1999 and 2000 epizootics, the Florida transmission foci were rural with most activity detected in the northern part of the state. All human cases were preceded by the detection of WNV in animals; however, only eight of the twelve cases were preceded by reports of WNV activity in the county of residence. West Nile virus-positive animals detected by multiple surveillance systems preceded seven of these cases by two weeks or more.

INTRODUCTION

After the West Nile virus (WNV) (Flaviviridae, flavivirus) epizootic in New York City had been detected in 1999, scientists soon predicted that the WNV might spread to southeastern United States with birds migrating from the northern WNV transmission foci.1 This mode of virus spread over large distances has been documented for other arboviruses in the United States,2 and for WNV in other parts of the world.3 The potential spread to Florida was of particular interest because of the mild climate that has been able to support enzootic transmission of Eastern equine encephalomyelitis virus (EEEV) and St. Louis encephalitis virus (SLEV) in that state. St. Louis Encephalitis virus is closely related to WNV, and like WNV, the SLEV enzootic transmission cycle appears to involve birds and mosquitoes of the genus Culex; Culex quinquefasciatus in northern Florida4 and Cx. nigripalpus in the central and southern parts of the state.5 Because of its history of widespread SLEV outbreaks with hundreds of human cases, Florida has had an arbovirus surveillance program in place for more than 40 years. The core of this program is the Florida Sentinel Chicken Arboviral Surveillance Network, which was established in 1978.6 Data on seroconversion rates to SLEV among sentinel chickens, together with mosquito surveillance data collected by 48 mosquito control districts across the state, help public health and mosquito control officials assess the intensity of epizootic virus transmission in an area and determine appropriate responses needed to prevent human disease.7 In anticipation of the arrival of WNV, the Florida arbovirus surveillance program was expanded in 2000 to include wild bird mortality reporting and testing. The decision was based on the surveillance findings in New York and other northeastern states where WNV induced mortality among crows was used to predict the human risk for acquiring the virus.8

As expected, wild bird mortality surveillance provided the first evidence of WNV activity in Florida. The virus was first detected in a crow collected in June 2001 in Jefferson County, a rural county (estimated population in July 2001 = 13,138)9 in the eastern part of the panhandle. In this paper, we give an overview of the results of WNV surveillance during the first transmission season in Florida. During the period of this study (January 1–December 31, 2001), we detected a widespread outbreak with 492 horse and 12 human cases, and a transmission focus that spanned over 31 northern Florida counties. Three of the human cases were infected in Monroe County (the Florida Keys), which formed a second transmission focus at the southern tip of the state.

MATERIALS AND METHODS

Wild bird reporting.

A web-based dead bird mortality reporting system was launched on May 1, 2000. The dead bird mortality database was designed by Florida Department of Health (DOH) and managed by the Florida Fish and Wildlife Conservation Commission (FWCC). The vast majority of the dead bird reports were entered directly by the public or by county health department (CHD) or FWCC staff. After September 25, 2001, the public could also report dead birds by calling a toll-free hotline number. Instructions for dead bird reporting were disseminated in several ways. Pamphlets and posters with reporting information were produced and distributed locally by CHD staff. In 2001, a press release reminding the public to report dead birds was sent out on March 14. After the virus had been identified in the state on July 6, 37 additional press releases, alerting the public to take precautions against mosquito bites, were sent to media in the state over the following five months. All releases had instructions on dead bird reporting.

Wild bird testing.

County health departments were encouraged to identify and submit birds of any species collected within 24 hours of death for testing. Initially, bird necropsies were performed at the Florida Department of Agriculture and Consumer Services (DACS) Animal Disease Diagnostic Laboratory (Kissimmee, FL) and fresh heart tissues were forwarded on ice to the DOH Bureau of Laboratories in Tampa for reverse transcriptase-polymerase chain reaction (RT-PCR) and virus isolation testing.10,11 After August 31, both necropsies and virus testing were preformed at the DOH Tampa laboratory. Shipping and testing were provided free of charge to the submitter.

Sentinel chicken serology.

Two hundred twelve flocks, with 3–12 adult sentinel chickens in each, were maintained in 30 Florida counties (Figure 1). Serum samples for baseline analyses were collected and analyzed before chickens were placed in the field. Once in the field, 0.5 mL of blood was collected from each sentinel chicken up to four times per month. Weekly serum samples were usually collected from each chicken during peak SLEV transmission months (July–December). Serum samples were shipped to the DOH laboratory in Tampa for analysis of antibodies to SLEV, EEEV and WNV. Specimens were processed by extraction with protamine sulfate-acetone and screened with a hemagglutination inhibition (HI) antibody test12 for group antibody to flavivirus, using SLEV (TBH28) SM sucrose-acetone–extracted antigen. Positive specimens (titer ≥ 1:10) were tested for IgM antibodies to WNV in an IgM-capture enzyme-linked immunosorbent assay (MAC-ELISA).13 West Nile virus IgM-negative birds were re-bled within a week to confirm the flavivirus infection. Repeat HI-positive chickens without IgM antibodies to WNV were classified as SLEV infected. When available, sera negative or equivocal for antibody to WNV in the MAC-ELISA were assayed in a serum neutralization plaque reduction (PRNT) assay against both SLEV and WNV.14 Weekly seroconversion rates were calculated by dividing the number of positive birds in a county by the total number of tested birds in the same county during a specific week in 2001. Annual seroconversion rates (ASR) were calculated by dividing the number of positive birds in a county by the total number tested birds in the same county in 2001.

Mosquito trapping and testing.

Most mosquito control districts in Florida run integrated pest management programs with both surveillance of larval and adult mosquito populations. However, in 2001, only the Lee County Mosquito Control District routinely tested mosquitoes for arbovirus infections. After the detection of WNV in the state, mosquito collections were made in Duval, Holmes, Jefferson, Madison, Monroe, Okaloosa, Pinellas, Santa Rosa, Sarasota, and Washington counties. Unfed female mosquitoes were identified to species and shipped on dry ice to the DOH laboratory in Tampa for WNV RT-PCR and virus isolation testing.10,14 In addition, staff from Centers of Disease Control and Prevention (CDC) (Fort Collins, CO) collected mosquitoes from Jefferson and Monroe counties in July and October 2001. West Nile virus testing of these mosquitoes was done at the CDC.

Neurologic illness in horses.

Veterinarians were encouraged to report and test horses with neurologic signs for antibodies to WNV in a letter sent out from the State Veterinarian and State Public Health Veterinarian to all veterinary practitioners in Florida on June 27, 2000. During the summer and fall of 2001, DACS staff gave nine presentations to local veterinary societies across the state and information about reporting was also available on DACS and DOH websites. In addition to the passive reporting system set up for horse cases, the United States Department of Agriculture (USDA) Area Veterinarians were brought in from out-of-state to assist the state veterinarian’s office with active West Nile Virus case finding in north central Florida and the eastern part of the panhandle during July and August 2001. Arbovirus testing of symptomatic horses was provided free of charge. Sera were screened for flavivirus and alphavirus antibodies with the HI test at the Animal Disease Diagnostic Laboratory. Flavivirus-positive samples were sent to the National Veterinary Services Laboratories (Ames, IA) for confirmatory testing with MAC-ELISA and PRNT.15 The USDA case definition was used in confirming all cases.15

Neurologic illness in humans.

Cases of human encephalitis are reportable in Florida. Information on diagnosis and reporting of arboviral infections for providers was sent to CHDs in June 2000 for distribution in their respective communities. This information was also available on the DOH website. A “blast” fax with these reporting instructions was sent out directly to Florida hospitals and community health centers on August 24, 2001. Department of Health Bureau of Laboratories offered free diagnostic testing of all serologic specimens submitted for arbovirus testing. Sera were screened for flavivirus antibodies in an HI assay, using the SLEV antigen as described earlier in this report. Positive sera were tested for specific antibodies to SLEV or WNV by MAC-ELISA and PRNT. Submitted cerebrospinal fluid samples were tested by MAC-ELISA. West Nile virus encephalitis cases were classified as confirmed or probable according to the CDC and the Council of State and Territorial Epidemiologists (CSTE) case definition.16

Definition of transmission foci.

Florida had two WNV transmission foci. The largest included 31 counties in the panhandle and north central parts of the state. The area spanned from Holmes, Washington, and Bay counties in the west to Levy, Marion, Putnam, and St. Johns counties toward the south. Counties included in this focus had evidence of mammalian virus transmission with more than five horse cases and in counties with sentinel chicken flocks, an ASR of 20% or higher. Rural counties (population < 50,000), with limited arbovirus surveillance programs, bordering these counties were also included in the definition. The second WNV transmission focus was in Monroe County (the Florida Keys) where three human cases were diagnosed. No horse cases were detected; however, the county’s horse population is very small (Jeter WC, unpublished data). Monroe County did not maintain sentinel chicken flocks.

Descriptive statistics and Fisher’s exact P values were calculated with Epi Info version 6.04B (CDC, Atlanta, GA).

RESULTS

Bird mortality.

A total of 17,659 dead bird reports that detailed 23,276 dead bird sightings were reported between January 1 and December 31, 2001. The majority (90%) of these were received after July 6, when we announced Florida’s first WNV-positive bird. Seventy two percent (n = 16,749) were reported singly. Of the 17,893 birds identified to family, birds from the dove family (order Columbiformes, family Columbidae) were most commonly reported (36% of all reports), followed by birds from the crow family (order Passeriformes, family Corvidae) (n = 5,087, 28%). Fifty-three percent of reported corvids (12% of all bird reports) were crows, either American crows (Corvus brachyrhynchos) or fish crows (Corvus ossifragus), and 47% were blue jays (Cyanocitta cristata). Only 3% of the dead bird sightings reported from Monroe County were crows; 6% dead corvids. Dead bird reporting intensity was greater in densely populated counties than in rural counties. However, when the number of dead bird reports was calculated per 100,000 residents, the number of reports per county correlated well with the intensity of WNV activity in the county measured by other surveillance systems (Figure 2). Analyses of population-adjusted dead bird reporting of particular bird families showed that while the number of dead corvid reports/100,000 population, especially dead crow reports, appeared to be greater in WNV transmission foci, the correlation between WNV activity and dead dove reporting was less clear. The highest dead dove mortality was reported from counties along the Gulf Coast. More than one-third (37%) of the birds were reported in clusters, and testing confirmed an outbreak of trichomoniasis in doves collected in Pinellas County in July and a paramyxovirus outbreak among doves in panhandle counties in October (Breault T, unpublished data). Most (85%) dead corvids were found singly. The distribution of the 3,988 dead doves reported singly is shown in Figure 3.

The weekly number of dead corvid reports also correlated better with local virus activity in sentinel chickens and mammals reported during the same time period (Figure 4). The highest number of dead crow sightings per 100,000 population in a particular week was reported from Madison County during the week of July 8 (week 28) (197.5 reports per 100,000). Reports of crow mortality peaked coincident with the onset of symptoms in the two reported human WNV encephalitis cases from Madison County. In other northern Florida counties with single human WN cases, the peak number of dead crow reports ranged from 5.5 to 25.6 birds per 100,000 per week. The peak number of weekly dead corvid (primarily crow and blue jay) reports for these counties ranged from 6.4 to 41.6 birds per 100,000. Corvid mortality peaked on average 2.8 weeks (range = 1–6 weeks) prior to disease onset of their human case. Peak corvid mortality in Monroe County (n = 4, 5.5/100,000) was reported during the week of September 1 (week 36), two weeks after the disease onset of the county’s second human case.

Dead bird testing.

Of the 7,669 birds (16 orders and 50 families) tested for WNV in 2001, 1,106 (14.4%) birds were found infected with the virus. The WNV-positive birds found in 65 of the 67 Florida counties represented 10 orders and 25 families (Table 1). The highest virus recovery rates were seen in corvids with 49.8% (n = 838) of crows and 29.9% (n = 728) of blue jays testing positive for WNV. The mean percentage positive corvids was significantly higher in the northern Florida transmission focus than in other counties (59% versus 7%; P < 0.001). All counties in the transmission focus had a 40% or higher virus recovery rate from corvids with one exception. None of the 32 corvids tested from St. Johns County during July and August 2001, were positive for the virus. The proportion of positive doves was also significantly higher in counties in the transmission focus (11% versus 6%; P < 0.001). In Monroe County, only four corvids were tested for WNV infections; a single blue jay tested positive. Among other species tested from this county, ringed turtle doves had the highest virus recovery rate at 41.7% (n = 12). West Nile virus-positive wild birds were the first indicators of virus activity in 54 Florida counties.

Chickens.

Of the 2,128 chickens monitored for antibodies to arboviruses statewide, 194 birds in 21 counties developed specific antibodies to WNV (Figure 1). Annual seroconversion rates ranged from 57.5% and 54.5% of all tested birds in Duval and Leon counties to 1.2% of all tested birds in Orange County. The weekly sentinel chicken seroconversion rates peaked during the week of September 16 (week 38) in Duval County (58.9%) and the week of September 22 (week 39) in Leon County (27.5%).

Detections of antibodies to WNV in sentinel chickens were the first indication of WNV activity in three counties. In these counties, WNV-positive wild birds were identified on average 1.5 weeks, and WNV-positive horses were identified on average 2 weeks, after the first WNV-positive chicken. In counties where birds or horses were the first indicators of WNV activity, chicken seroconversions lagged on average 7.7 weeks.

Mosquitoes.

Twenty-eight mosquito pools tested positive for WNV from six counties. The majority of the isolations were made from mosquitoes in the genus Culex (Table 2). Detailed descriptions of the mosquito data will be published elsewhere (Godsey MS and others, unpublished data and Hribar LJ and others, unpublished data).

Horses.

Nine hundred eighty-two cases of neurologic illness consistent with arbovirus disease were reported to the Florida DACS. Four hundred ninety-two of 948 horses were laboratory confirmed with acute WNV encephalitis. The first confirmed horse from Jefferson County had onset of illness on June 26 (week 26). The outbreak subsequently spread east and west resulting in WN horse cases in 40 Florida counties (Figure 5). The largest number of cases was diagnosed in Marion County (n = 95). The first horse case in Marion County had disease onset on August 4 (week 31). West Nile virus-positive horses were the first indicators of WNV activity in six counties. In these counties, positive wild birds were detected on average three weeks later and sentinel chickens seroconverted on average 7.5 weeks after the first horse case. In counties where wild bird mortality was the first indicator of WNV activity, the lag time between the first positive bird and horse cases was on average four weeks.

Humans.

Five hundred eighteen persons with a neurologic illness consistent with arboviral encephalitis were tested for antibodies to WNV by the Florida DOH Bureau of Laboratories. Twelve cases from nine counties (2 in Madison, 3 in Monroe, 1 in Jefferson, 1 in Clay, 1 in Leon, 1 in Palm Beach, 1 in Washington, 1 in Putnam, and 1 in Marion Counties) with onset dates ranging from July 13 to October 28 were confirmed (Figure 6). The median age of the cases was 50.5 years, eight of the cases were male, and nine were hospitalized (median age = 64 years). None of the cases were fatal. All human cases followed the detection of the first WNV-positive wild bird in the state. Eight of the twelve cases were preceded by detected WNV activity in the county of exposure. For seven of these cases, WNV-positive wild birds and multiple horse cases were reported four weeks or more prior to the onset of the human case in the county itself or in surrounding counties. In Putnam and Leon counties, antibodies were detected in sentinel chicken flocks an average of 6.5 weeks prior to disease onset. In Palm Beach County, multiple horse cases were detected one week prior to human disease onset. The first WNV antibody-positive sentinel chicken in Palm Beach County was detected one week post disease onset of the human case. In Madison and Monroe counties, human cases were the first evidence of WNV activity. Neither of these two counties maintained sentinel chicken flocks.

West Nile virus activity persisted throughout the fall with chicken flocks testing positive for antibodies to WNV as late as December 18 (week 51), confirmed horse cases reported on December 26 (week 52), WNV-positive birds collected on December 26, and positive mosquito pools collected on December 10 (week 50).

DISCUSSION

The 2001 WNV outbreak in Florida differed from the 1999–2000 outbreaks described in New York and surrounding areas in several ways. The northern Florida transmission focus was large and rural. More than five times as many horse cases were diagnosed in Florida than in the northeastern United States during the previous two years combined. Unlike the surveillance results described from New York,17,18 sentinel chicken seroconversions and horse cases in or near counties with human cases preceded all but one of the human cases. In addition, the peak weekly sentinel chicken serocon-version rates for WNV were similar to the weekly serocon-version rates to SLEV seen during the SLE outbreaks in 1993 and 1997.19 This suggests that sentinel chicken surveillance, as is the case in SLEV surveillance, will become a useful predictor of WNV activity in the state. A WNV horse vaccine was introduced in 2001. However, based on our experience with EEE and EEE vaccine use in Florida, it is likely that surveillance of arboviral encephalitis cases in horses will remain a valuable tool in assessing the risk for human WNV exposure.

In Palm Beach County, neither horses nor chickens were particularly useful in predicting human risk for WNV encephalitis in the county. However, the Palm Beach human case is still somewhat puzzling to us. We have counted the case because the 19-year-old man, a native of Mauretania, fits both the clinical and laboratory criteria of a confirmed case as defined in the CDC/CSTE WNV encephalitis case definition. Despite these findings, the man’s attending physicians concluded that his central nervous system symptoms were due to a psychotic rather than infectious process. This conclusion was based on results from the medical evaluation including evidence of a transient fever, a normal cerebrospinal fluid, and normal differential white blood cell counts. This clinical diagnosis is supported by the man’s history of very limited outdoor activities during his eight-month stay in the United States. Unfortunately, this patient was lost to follow-up and we were not able to verify whether the case recalled having prior illness compatible with WN fever during the months before his move to the United States. West Nile virus has been reported from Nigeria,20 but little is known about the incidence of human disease in west Africa.

As in the northeastern United States, WNV caused significant bird mortality in Florida, particularly among crows and blue jays. The proportion of dead crow sightings (12% of all bird reports) was lower in Florida than has been previously described from New York8 and Connecticut.21 However, despite the lower number of dead crow reports in Florida, corvid mortality was the most sensitive predictor of WNV activity in the state in 2001. The peak number of weekly dead crow sightings per 100,000 population was also comparable with the population adjusted weekly dead crow sightings described from Connecticut in 2000.21 Whether the assessments of crow and corvid mortality will become valuable tools in predicting human risk for WNV infections in future years remains to be seen.

Few dead corvids were reported from southern Florida. The lack of virus activity in chickens and horses in the southern Florida area suggest that the lower number of reports may be correlated with lower levels of WNV transmission. However, the differences may also, at least in part, be due to differences in the make up of local bird populations. Although corvids can be found in all areas of the state, the exact size of the population in various parts of Florida is unknown. The virus recovery rates in crows and blue jays were similar to those reported from New York State in 2000.22

Our data show dove mortality to be less useful than corvid or overall bird mortality for monitoring WNV activity. High dove mortality was detected in areas of the state with limited WNV activity. Experimental studies have also shown doves to be more resistant to the virus, and more likely to develop asymptomatic infections than crows.23 Hopefully, we will be able to better use these data in our WNV surveillance programs as we become more familiar with other causes of bird mortality and their seasonality. With more sensitive analytical methods, we may be able to control for the background mortality of non-corvid species. As WNV becomes enzootic and herd immunity to the virus develops in the bird population, more sensitive analyses will most likely be needed to interpret corvid mortality surveillance data as well.

A number of Florida mosquito species become infected with WNV in the field. The relative importance of these as human virus vectors remains to be determined. Laboratory studies indicate that local Culex species (Cx. nigripalpus, Cx. quinquefasciatus, and Cx. salinarius) are competent vectors.24 Ochlerotatus taeniorhynchus mosquitoes also transmit WNV in laboratory trials but seem to be less efficient.25

This first season of WNV activity appeared first in the Florida panhandle and in the Florida Keys: both are important resting areas for migrating birds.26 The WNV outbreak then quickly spread over a large geographic area, as the SLEV does during years with epidemic transmission. However, despite the widespread distribution of WNV, it is interesting to note that little activity was recorded in central Florida, where SLEV activity is often seen.27 In 2001, the intensity and the spread of virus activity may be explained by the bird migration behaviors alone. Weather conditions, believed to be important for establishing the right ecologic conditions for SLEV outbreaks,28 may also have been a factor. The spring of 2001 was very dry in Florida. The dry season ended in early June when tropical storm Allison produced more than 10 inches of rain over the eastern part of the Florida Panhandle and southern Georgia, providing excellent breeding grounds for flood water mosquitoes. What effect, if any, the potential overlap of vectors and vertebrate hosts will have on the WNV and SLEV ecology in Florida remains to be seen.

Table 1

Birds tested for West Nile virus (WNV) infections in Florida in 2001

OrderFamilyCommon family nameNumber tested% positive
* Tested species from the crow family included the American crow (Corvus brachyrhynchos) and fish crow (Corvus ossifragus) (n = 838, 49.8% WNV positive), Blue jay (Cyanocitta cristata) (n = 728, 29.9% WNV positive), Magpie (Pica pica) (n = 1, 0% WNV positive), and raven (Corvus corax) (n = 1, 0% WNV positive).
AnseriformesAnatidaeDuck7012.9
ApodiformesApodidaeSwift30.0
TrochilidaeHummingbird1711.8
CaprimulgiformesCaprimulgidaeNightjar150.0
CharadriiformesCharadriidaePlover20.0
LaridaeGull326.2
RynchopidaeSkimmer10.0
ScolopacidaeSandpiper90.0
StercorariidaeSkua10.0
CiconiiformesArdeidaeHeron300.0
PhoenicopteridaeFlamingo10.0
ThreskiornithidaeIbis30.0
ColumbiformesColumbidaePigeon2,0447.3
CoraciiformesAlcedinidaeKingfisher30.0
CuculiformesCuculidaeCuckoo3212.5
FalconiformesAccipitridaeHawk10.0
CathartidaeAmerican vulture40.0
FalconidaeFalcon8911.2
PandionidaeOsprey110.0
GalliformesPhasianidaePheasant509.5
TetraonidaeGrouse1822.8
MeleagrididaeTurkey120.0
Guinea fowl30.0
GruiformesGruidaeCrane10.0
RallidaeRail320.0
PasseriformesAlaudidaeLark50.0
BombycillidaeWaxwing90.0
CorvidaeCrow*1,56840.0
FringillidaeFinch3808.7
HirundinidaeSwallow825.0
IcteridaeTroupial3128.3
LaniidaeShrike610.5
MimidaeMockingbird4437.4
MotacillidaePipit714.3
ParidaeTitmouse1163.6
ParulidaeAmerican wood warbler450.0
PloceidaeWeaverbird70.0
SturnidaeStarling248.3
SylviidaeOld world warbler3037.6
ThraupidaeTanager40.0
TroglodytidaeWren712.8
TurdidaeThrush2077.2
TyrannidaeTyrant140.0
VireonidaeVireo195.3
Unknown2147.5
PelecaniformesAnhingidaeAnhinga333.3
PelecanidaePelican20.0
PhalacrocoracidaeCormorant50.0
PiciformesPicidaeWoodpecker905.6
PodicipediformesPodicipedidaeGrebe60.0
PsittaciformesPsittacidaeParrot1015.9
StrigiformesStrigidaeTypical owl434.6
TytonidaeBarn owl10.0
UnknownUnknown1,11310.2
Total7,66914.4
Table 2

West Nile virus isolations from Florida mosquitos in 2001

CountyGenus and speciesNo. of positive poolsNo. of pools*Total number of mosquitoes tested†Collection month
* Number of pools tested from the county during the same month the virus isolations were made.
†Number of mosquitoes tested from the county during the same month the virus isolations were made.
BayOchlerotatus taeniorhynchus123274September
JeffersonCuliseta melanura351504July
Culex nigripalpus225187July
Culex quinquefasciatus6431,257July
Culex salinarius3943,857July
Culex sp.1511,357July
MadisonOchlerotatus atlanticus15184July
MonroeAnopheles atropos1588September
1726October
120112November
1744December
Deinocerites cancer125354October
125491November
Ochlerotatus taeniorhynchus1543,938September
11045,350October
PinellasAnopheles crucians121602November
Culiseta melanura19157November
WashingtonCulex nigripalpus1637July
Figure 1.
Figure 1.

Annual seroconversion rates to West Nile virus in sentinel chicken flocks in Florida counties. The locations of the sentinel chicken flocks are also shown.

Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 69, 2; 10.4269/ajtmh.2003.69.141

Figure 2.
Figure 2.

Dead bird reports per 100,000 population in Florida counties. The number of birds reported was higher in counties within the epicenters than in other parts of the state.

Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 69, 2; 10.4269/ajtmh.2003.69.141

Figure 3.
Figure 3.

Dead doves reported singly per 100,000 population in Florida counties.

Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 69, 2; 10.4269/ajtmh.2003.69.141

Figure 4.
Figure 4.

West Nile virus (WNV) surveillance results from Leon County, one of the Florida counties with a human case in 2001. Peak numbers of dead corvid reports and clinical horse cases preceded the onset of the human case. Reports of dove mortality did not correlate with other WNV surveillance findings.

Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 69, 2; 10.4269/ajtmh.2003.69.141

Figure 5.
Figure 5.

Distribution of confirmed human and equine West Nile (WN) cases in Florida. The counties included in the two transmission foci are also highlighted.

Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 69, 2; 10.4269/ajtmh.2003.69.141

Figure 6.
Figure 6.

Epidemiology curve of human West Nile virus encephalitis cases in Florida in 2001. The first two cases were residents of Madison County in week 28. The last diagnosed case, with disease onset in week 44, was reported from Marion County.

Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 69, 2; 10.4269/ajtmh.2003.69.141

Authors’ addresses: Carina G. M. Blackmore, Florida Department of Health, Box 210, Jacksonville FL, 32331, Telephone: 904-791-1744, Fax: 904-791-1567, E-mail: [email protected]. Lillian M. Stark, Bureau of Laboratories-Tampa, 3602 Spectrum Boulevard, Tampa, FL 33612, Telephone: 813-974-5990, Fax: 813-974-9776. William C. Jeter, Division of Animal Industry, 407 South Calhoun Street, Room 335, Mayo Building, Tallahassee, FL 32399-0800, Telephone: 850-410-0942, Fax: 850-410-0915. Robin L. Oliveri, Lisa A. Conti, and Steven T. Wiersma, Bureau of Epidemiology, 4052 Bald Cypress Way (Bin A-12), Tallahassee, FL 32399-1720, Telephone: 850-245-4401, Fax: 850-922-9299. Robert G. Brooks, Department of Family Medicine and Rural Health, College of Medicine, Florida State University, Tallahassee FL, 32306-4300, Telephone: 850-644-3845, Fax: 850-644-9399.

Acknowledgments: We thank the county health department staff for coordinating local dead bird reporting and WNV testing; the Bureau of Laboratory and Bureau of Animal Disease Diagnostics staff for excellent laboratory support; Tom Loyless, Dr. John Smith, and the Florida Mosquito Control District staff and the CDC for mosquito collections; Gary Sprandel for assistance with the dead bird database; and D’Juan Harris for producing the maps for this manuscript.

Financial support: The study was supported by CDC grant “Epidemiology and Laboratory Capacity for Infectious Diseases,” the Florida Department of Health, the Florida Department of Agriculture and Consumer Services, and the Florida Fish and Wildlife Conservation Commission.

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