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    Figure 1.

    Maps of Harris County, Texas showing the collection localities of West Nile virus (WNv)-positive dead birds (black dots) and WNv-positive mosquito pools (lighter stars) during June, July, August, and September 2002. The map designations are cumulative and illustrate how quickly the virus spread throughout the county. The subdivisions indicate the 268 mosquito control areas in Harris County.

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THE 2002 INTRODUCTION OF WEST NILE VIRUS INTO HARRIS COUNTY, TEXAS, AN AREA HISTORICALLY ENDEMIC FOR ST. LOUIS ENCEPHALITIS

KRISTY M. LILLIBRIDGECenter for Infectious Diseases, School of Public Health, University of Texas Health Science Center, Houston, Texas; Mosquito Control Division, Harris County Public Health and Environmental Services, Houston, Texas; Department of Pathology and Center for Biodefense and Emerging Infectious Diseases, University of Texas Medical Branch, Galveston, Texas

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RAY PARSONSCenter for Infectious Diseases, School of Public Health, University of Texas Health Science Center, Houston, Texas; Mosquito Control Division, Harris County Public Health and Environmental Services, Houston, Texas; Department of Pathology and Center for Biodefense and Emerging Infectious Diseases, University of Texas Medical Branch, Galveston, Texas

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YVONNE RANDLECenter for Infectious Diseases, School of Public Health, University of Texas Health Science Center, Houston, Texas; Mosquito Control Division, Harris County Public Health and Environmental Services, Houston, Texas; Department of Pathology and Center for Biodefense and Emerging Infectious Diseases, University of Texas Medical Branch, Galveston, Texas

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AMELIA P. A. TRAVASSOS DA ROSACenter for Infectious Diseases, School of Public Health, University of Texas Health Science Center, Houston, Texas; Mosquito Control Division, Harris County Public Health and Environmental Services, Houston, Texas; Department of Pathology and Center for Biodefense and Emerging Infectious Diseases, University of Texas Medical Branch, Galveston, Texas

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HILDA GUZMANCenter for Infectious Diseases, School of Public Health, University of Texas Health Science Center, Houston, Texas; Mosquito Control Division, Harris County Public Health and Environmental Services, Houston, Texas; Department of Pathology and Center for Biodefense and Emerging Infectious Diseases, University of Texas Medical Branch, Galveston, Texas

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MARINA SIIRINCenter for Infectious Diseases, School of Public Health, University of Texas Health Science Center, Houston, Texas; Mosquito Control Division, Harris County Public Health and Environmental Services, Houston, Texas; Department of Pathology and Center for Biodefense and Emerging Infectious Diseases, University of Texas Medical Branch, Galveston, Texas

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TAWEESAK WUITHIRANYAGOOLCenter for Infectious Diseases, School of Public Health, University of Texas Health Science Center, Houston, Texas; Mosquito Control Division, Harris County Public Health and Environmental Services, Houston, Texas; Department of Pathology and Center for Biodefense and Emerging Infectious Diseases, University of Texas Medical Branch, Galveston, Texas

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CHRISTINA HAILEYCenter for Infectious Diseases, School of Public Health, University of Texas Health Science Center, Houston, Texas; Mosquito Control Division, Harris County Public Health and Environmental Services, Houston, Texas; Department of Pathology and Center for Biodefense and Emerging Infectious Diseases, University of Texas Medical Branch, Galveston, Texas

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STEPHEN HIGGSCenter for Infectious Diseases, School of Public Health, University of Texas Health Science Center, Houston, Texas; Mosquito Control Division, Harris County Public Health and Environmental Services, Houston, Texas; Department of Pathology and Center for Biodefense and Emerging Infectious Diseases, University of Texas Medical Branch, Galveston, Texas

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ADIL A. BALACenter for Infectious Diseases, School of Public Health, University of Texas Health Science Center, Houston, Texas; Mosquito Control Division, Harris County Public Health and Environmental Services, Houston, Texas; Department of Pathology and Center for Biodefense and Emerging Infectious Diseases, University of Texas Medical Branch, Galveston, Texas

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RHIA PASCUACenter for Infectious Diseases, School of Public Health, University of Texas Health Science Center, Houston, Texas; Mosquito Control Division, Harris County Public Health and Environmental Services, Houston, Texas; Department of Pathology and Center for Biodefense and Emerging Infectious Diseases, University of Texas Medical Branch, Galveston, Texas

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TAMRA MEYERCenter for Infectious Diseases, School of Public Health, University of Texas Health Science Center, Houston, Texas; Mosquito Control Division, Harris County Public Health and Environmental Services, Houston, Texas; Department of Pathology and Center for Biodefense and Emerging Infectious Diseases, University of Texas Medical Branch, Galveston, Texas

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DANA L. VANLANDINGHAMCenter for Infectious Diseases, School of Public Health, University of Texas Health Science Center, Houston, Texas; Mosquito Control Division, Harris County Public Health and Environmental Services, Houston, Texas; Department of Pathology and Center for Biodefense and Emerging Infectious Diseases, University of Texas Medical Branch, Galveston, Texas

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ROBERT B. TESHCenter for Infectious Diseases, School of Public Health, University of Texas Health Science Center, Houston, Texas; Mosquito Control Division, Harris County Public Health and Environmental Services, Houston, Texas; Department of Pathology and Center for Biodefense and Emerging Infectious Diseases, University of Texas Medical Branch, Galveston, Texas

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Harris County, Texas, is an endemic area of St. Louis encephalitis (SLE); and an active surveillance program that monitors SLE virus activity in mosquitoes, birds, and humans has been in place there for the past 28 years. In June of 2002, West Nile (WN) virus appeared in Houston and quickly spread throughout the region. This report describes the results of 12 years of SLE surveillance in Harris County and the contrasting pattern of WN virus activity, when it arrived in 2002. Our data indicate that both SLE and WN viruses can coexist, despite their ecologic, antigenic, and genetic similarities, and that both viruses will probably persist in this geographic region.

INTRODUCTION

St. Louis encephalitis (SLE) virus is endemic, and occasionally epidemic, in the Houston, Texas metropolitan area. The first documented SLE epidemic in the area occurred in 1964. During that year, 243 human cases were confirmed with 27 deaths.1,2 A second epidemic occurred during the summers of 1975 and 1976, when a total of 58 cases and 11 deaths were confirmed. Subsequent SLE outbreaks occurred in 1980, 1986, and 1990–1991 when 53, 28, and 65 cases were reported, respectively.3 In 1975, Harris County, which geographically covers most of the Houston metropolitan area, established an SLE surveillance program that has continued uninterrupted for the past 28 years.4 The surveillance program is managed by the Harris County Mosquito Control (HCMC) Division, which monitors SLE virus activity between April and November of each year by determining the infection rate in field-collected mosquitoes (Culex quinquefasciatus) and the prevalence of antibodies in wild birds.

Following the introduction of West Nile (WN) virus into the United States in 19995 and in response to the expanding front of virus activity across the southeastern region of the country, the HCMC Division expanded its arbovirus surveillance program to also include the detection of WN virus activity in mosquitoes and wild birds. In addition, in 2001 the HCMC Division established an avian mortality surveillance system6 and coordinated the collection and submission of dead birds to the World Reference Center for Emerging Viruses and Arboviruses (WRCEVA) at the University of Texas Medical Branch (UTMB) in Galveston for WN virus testing.

The SLE and WN viruses are both members of the Japanese encephalitis (JE) group of the genus Flavivirus, family Flaviviridae; and they have a similar natural history.7,8 Both are maintained in nature in a cycle involving birds and Culex mosquitoes. The two viruses also are serologically related; and experimental studies in laboratory animals suggest that infection with one virus partially protects against infection with the other.9,10 Consequently, there was considerable interest in determining what would happen when WN virus moved into a geographic region where SLE virus was already endemic. This paper reports the results of our surveillance of both viruses during the summer of 2002, the season that WN virus first appeared in the Houston metropolitan area.

MATERIALS AND METHODS

Study area.

Harris County, Texas is located in the northern portion of the Gulf of Mexico coastal plain, a 50-mile swath along the Texas Gulf Coast. Harris County, which includes the city of Houston, covers a geographic area of more than 1,788 square miles, 27% of which is devoted to farming and ranching. The natural vegetation varies from mainly forested areas in northern and eastern Harris County to the predominant prairie grassland in the southern and western parts of the county. Surface water includes a number of lakes, rivers, and streams dominated by an extensive network of bayous and human-made canals that are part of the flood management system. Elevation ranges from 0 to 310 feet above sea level. The total population of Houston is more than 3.4 million people, making it the fourth largest city in the United States.

Due to its abundant rainfall, soil composition, and relatively low elevation, Houston is subject to periodic flooding. Houston’s flood control/drainage infrastructure consists of two parts: a series of six major bayous and an elaborate but aging system of storm sewers and underground tunnels that capture the floodwaters. The storm sewers carry rain and other surface and drainage water but exclude wastewater and polluted industrial wastes. This elaborate drainage system creates ideal biotic and abiotic conditions conducive for mosquito larval development, particularly during relatively dry periods when stagnant water remains in the storm sewer system. Approximately 99% of the mosquitoes that develop in these storm sewers are Cx. quinquefasciatus.4 These mosquitoes use the storm drains as daytime resting sites as well as for larval development.

Collection and trapping of mosquitoes.

Since 1979, the HCMC Division has used CO2-baited Centers for Disease Control (CDC)-type traps for collecting adult mosquitoes in storm sewers. Those traps, or parts used for in-house fabrication, were obtained from Hausherr’s Machine Works (Toms River, NJ) or the John W. Hock Company (Gaines-ville, FL).

During the period from April 1990 to November 1996, light traps were set at 268 distinct locations underground in storm sewers and above ground in residential areas without storm sewers. Trapping was done weekly, bi-weekly, or monthly depending on the historical level of SLE found in each area.4 Traps were set out between 1:30 PM and 5:00 PM and were picked up the next morning between 7:30 AM and 10:30 AM. The mosquitoes were brought to the processing laboratory alive in the light trap collection bags and frozen at −70°C. They were then placed on a chill table (BioQuip Products, Gardena, CA), identified and sorted into pools of 50 or less, and taken to the HCMC Division Virology Laboratory for detection of SLE virus. From 1997 to 2001, collections were made at only 248 locations. Twenty trap sites were eliminated in areas without storm sewers or that had had little or no SLE virus activity since 1979.

After the arrival of WN virus in June 2002, the trapping methods were again changed. The number of mosquito sampling locations was reduced to 100; these 100 sites were trapped weekly. In addition to CDC light traps in storm sewers, 15 gravid traps baited with hay infusion water11 were placed in back yards in residential areas considered at highest risk for WN virus activity. Gravid traps were set out and picked up using the same schedule described above for the CDC light traps.

Virus detection in mosquitoes.

After collection, live mosquitoes were transported to the HCMC Division laboratory where they were sorted and identified on chill tables and separated into pools of 50 females or less for arbovirus assay.4,11 Prior to 2002, the mosquito pools were only assayed for SLE virus, using an antigen capture enzyme immunoassay (EIA) described previously.12 The capture antibody, monoclonal antibody 4A4C-4, was used at a 1:4,000 dilution and the detecting antibody, monoclonal antibody 6B6C-1, was used at a 1:3,000 dilution. Both of these reagents, as well as the WN virus monoclonal 3.91D, were kindly provided by the Division of Vector-Borne Infectious Diseases, Centers for Disease Control and Prevention (Fort Collins, CO). In 2002, a WN virus antigen detection assay13 was added using monoclonal antibody 3.91D (1:2,000 dilution) as the capture antibody and monoclonal antibody 6B6C-1 (1:10,000 dilution) as the detection antibody. All SLE antigen EIA-positive pools were inoculated intracerebrally into litters of newborn mice and then confirmed by complement fixation (CF) test.14 In 2002, with the overwhelming number of WN virus antigen-positive mosquito pools, only 227 of the 858 antigen-positive pools were confirmed by mouse inoculation and CF test. Some of the other WN virus antigen-positive pools were confirmed at UTMB by culture in Vero cells and/or reverse transcriptase-polymerase chain reaction (RT-PCR) as described later in this report.

Antibody surveillance in live birds.

St. Louis encephalitis virus activity in the local wild bird population was monitored by antibody surveillance. Live birds were captured in mist nets at randomly selected sites in the county. Cracked corn and millet seeds were sprinkled around the nets to attract birds. After capture, birds were bled from the jugular vein (0.5–1.0 mL depending on size), identified, and released. Their sera were subsequently tested for the presence of antibody to SLE virus by a hemagglutination-inhibition (HI) test.14 Netting of wild birds was done weekly from April until November in most years; however, in 2002 collections were done irregularly as time permitted because the HCMC Division personnel were heavily involved in vector control activities. In 2002, WN virus antigen was added to the protocol and bird sera were tested for antibodies to both SLE and WN viruses in the same test. Antigens for the HI test were prepared from brains of newborn mice injected intracerebrally with each virus; the infected brains were processed by the sucrose-acetone extraction method.14 Initially, all sera were screened by HI at a 1:20 dilution at pH 6.6 with four units of antigen and a 1:200 dilution of goose erythrocytes.14 If a positive HI reaction was obtained at a dilution of 1:20, the serum was then titrated against both antigens at two-fold dilutions from 1:20 to 1:5,120.

Testing of dead birds for WN virus.

In anticipation of the arrival of WN virus, an avian mortality surveillance system6 was established during the summer of 2001. Media reports and other public messages instructed citizens to report dead birds to the HCMC Division. Initially, the HCMC Division attempted to collect dead birds throughout the county; but in 2002, because of increased avian mortality and the overwhelming public response, 19 drop-off locations where citizens could leave dead birds were established in different locations in Harris County. The birds collected were screened by species and condition (state of decomposition) and then were frozen at −75°C. Because of the large number of dead birds collected in 2002, it was decided to focus mainly on corvids (American crows and blue jays). Accordingly, 30–50 selected dead birds were submitted to the WRCEVA at UTMB each week between April and November for virus testing.

At the UTMB, a filtered homogenate of brain from each dead bird was inoculated into a flask culture of Vero cells, using culture methods and medium previously described.15 If a viral cytopathic effect was observed, some of the cells were scraped off and spotted onto 12-well glass slides for examination by an indirect fluorescent antibody technique (IFAT),14 using mouse polyclonal and monoclonal antibodies to West Nile virus. Fluid from all IFAT-positive cultures was also examined by CF test for confirmation. A small subset of the WN virus-positive cultures were also confirmed by an RT-PCR.16

Detection of human cases of SLE and WN virus infection.

Human cases were mainly identified through passive surveillance and were investigated by the City of Houston Department of Health and Human Services (HDHHS) or Harris County Public Health and Environmental Services, depending on the jurisdiction of the patient’s residence at the time of hospitalization. Physicians who suspected WN or SLE virus infection in their patients sent sera or cerebrospinal fluid specimens to the HDHHS laboratory for testing by an IgM-enzyme-linked immunosorbent assay.14 IgM-positive specimens were forwarded to the Centers for Disease Control and Prevention (Fort Collins, CO) for confirmatory testing by plaque reduction neutralization test and/or EIA.

Antibody surveillance in stray dogs.

As another indicator of WN virus activity in Harris County, we examined 154 blood samples from stray dogs (≥ 1 year of age) housed at a large animal shelter in the Houston metropolitan area. After coagulation, the serum was removed, frozen at −20°C, and subsequently examined by the HI test against SLE and WN viral antigens. The blood samples were collected between April 14 and June 9, 2003. The dogs sampled were strays who were killed because of failure to identify an owner.

RESULTS

St. Louis encephalitis virus activity in Harris County mosquitoes, 1990–2002.

Table 1 summarizes the results of SLE surveillance in Harris County over a 13-year period from 1990 to 2002. During this period, a total of 2,483,512 Cx. quinquefasciatus were collected and tested for SLE virus. These mosquitoes were processed as 69,490 pools (mean pool size = 35.7). A total of 432 SLE virus-positive pools were identified over the 13-year period; SLE virus activity was detected in Cx. quinquefasciatus in 9 of the 13 years shown in Table 1. The mean annual number of SLE virus-positive pools for the period was 33.2. The yearly minimum field infection rates (number of SLE-positive pools/total mosquitoes tested × 1,000)17 for female Cx. quinquefasciatus collected in Harris County varied from 0.00 to 0.58; the mean minimum field infection rate (MFIR) for the entire period was 0.174. The latter figure is similar to the SLE virus MFIR obtained in 2002.

St. Louis encephalitis and WN virus infection in Harris County mosquitoes in 2002.

During 2002, a total of 260,138 Cx. quinquefasciatus were collected in Harris County and assayed for SLE and WN viruses. These insects were processed by EIA as 6,093 pools (average pool size =31). Twenty-eight pools (0.46%) tested positive for SLE viral antigen; three of these were confirmed by mouse inoculation and the CF test; the others were not retested. A total of 851 pools (13.97%) tested positive for WN viral antigen. As noted before, only a subset of 226 of the WN-positives were confirmed because of the sheer number. The period of detectable SLE virus activity in Cx. quinquefasciatus in 2002 was quite short. The first SLE-positive mosquito pool consisted of insects collected July 19, 2002; the last SLE-positive pool was comprised of mosquitoes collected July 31, 2002. In contrast, the period of WN virus activity in mosquitoes was much longer. The first WN-positive mosquito pool was comprised of Cx. quinquefasciatus females trapped on June 11, 2002; the last positive pool contained mosquitoes collected on November 1, 2002.

Prevalence of antibodies to SLE virus in wild birds, 1990–2002.

Table 1 shows the seroprevalence of antibodies to SLE virus among wild birds for each year between 1990 and 2002. Although the number of wild birds collected each year varied considerably, except for 2002, the prevalence of antibodies to SLE virus remained fairly constant over the 13-year period. During 2002, the HCMC Division personnel were so busy with emergency vector control activities, in response to the introduction of WN virus, that they had little time to net live birds. Table 2 shows the prevalence of SLE HI antibodies among 14 bird species commonly sampled in Harris County between 1989 and 2001. This table also illustrates the types of birds sampled in the mist-netting program. Some species (i.e., crows, hawks, and owls) are obviously missing; but these kinds of birds are not usually collected in mist nets. Of the birds sampled, blue jays and shrikes had the highest prevalence of antibodies to SLE virus.

Prevalence of antibodies to WN virus in wild birds in 2002.

During 2002, a total of 218 wild birds from Harris County tested for antibodies to arboviruses at UTMB (Table 1). The birds sampled were collected at the end of the season (October, November, and December), after WN virus activity had largely ended. The 218 birds, represented 16 different species. Overall, 31.2% had HI antibodies to WN virus, while only 3 (1.4%) had antibodies to SLE virus. The house sparrow (Passer domesticus) was the most common bird collected and represented 43.6% of the total birds sampled in 2002. Thirty-one (32.6%) of 95 house sparrows had antibodies to WN virus. Table 3 shows the antibody patterns observed in selected WN and SLE antibody-positive birds with the HI test. Although there was some cross-reactivity observed between the two flavivirus antigens in bird sera with high antibody titers, in general the infecting virus could be identified. A serum was considered as WN or SLE antibody positive if the titer to one viral antigen was four-fold or greater than the other. Very few sera had equal or only two-fold different titers to both antigens; these were considered inconclusive and were not counted.

Isolation of WN virus from dead birds.

A total of 541 dead birds were submitted to UTMB for virus culture during 2002. Relatively few dead birds were submitted between January and May, and all species were tested; but once the first WN virus isolations were announced in the media, the public response was overwhelming. Hundreds of birds were collected weekly between June and September. At that point, a decision was made to focus on corvids and raptors, since they were most likely to be positive. The goal of the avian surveillance program was to identify new areas of WN virus activity in the county, so the birds submitted for testing were not randomly selected. If several WN virus-positive birds had already been collected in a neighborhood, no further samples from that area were sent for testing; instead an effort was made to select birds from previously negative localities to map the spread of the virus in the county. This information as well as the results of mosquito testing were used by the HCMC Division to plan emergency vector control activities.4 Figure 1 shows the WN-virus-positive dead birds and mosquito pools by collection site, during June, July, August, and September of 2002. The maps are cumulative and illustrate how quickly the virus spread throughout the county.

West Nile virus was isolated from brain samples of 307 of the 541 dead birds submitted for testing in 2002. Table 4 shows the species composition of the culture-positive birds. Although the sample of dead birds was not random, nonetheless the predominance of blue jays was impressive. This species comprised 91.5% of the virus-positive birds. Both blue jays and crows are common in Harris County, but most of the dead birds taken by citizens to the drop-off centers were blue jays. The first bird isolation of WN virus was made from a blue jay collected on June 10, 2002; the last bird isolation was from a blue jay collected on October 15, 2002.

Human cases of severe arboviral disease in Harris County.

During the 12-year period from 1990 to 2001, a total of 86 human cases of SLE was reported from Harris County (Table 1). In 2002, a total of 113 confirmed or probable human cases of SLE and WN virus infection was reported by HDHHS. Of these cases, 105 (93%) were due to WN virus and 8 (7%) were due to SLE virus. The first human case of WN virus infection in Houston was identified on July 23, 2002.

Prevalence of antibodies to SLE and WN viruses in stray dogs.

Sera from 154 adult stray dogs in Houston were examined by the HI test for antibodies to SLE and WN viruses. The median age of the canines tested was two years, with a range of 1–10 years. The group included 20 different breeds that could be identified. Six (3.9%) of the 154 stray dog sera had four-fold or greater antibody titers to SLE viral antigen. In contrast, 87 (56.5%) of the canine sera had four-fold or greater antibody titers to WN viral antigen. One dog exhibited a 1:20 titer for both SLE and WN antigens; since the etiology was uncertain, it was not counted.

DISCUSSION

Two of our initial questions in 2001 were 1) could WN virus become established in a SLE-endemic region such as Harris County, Texas, and 2) if it did, would WN virus eventually displace SLE virus from the region? The answer to the first question was answered by the 2002 surveillance data; once WN was introduced into the region, it spread rapidly throughout Harris County. The answer to the second question is still uncertain, although the available data suggest that both viruses will persist. As indicated by our surveillance results (Tables 1–4), SLE and WN viruses were both active during 2002. In 2003, both viruses were again active; in fact, there was more SLE virus activity recorded in Harris County in 2003 than in 2002 (Parsons R, Tesh RB, unpublished data). So after two years, there is no indication that one of the viruses will displace the other.

In retrospect, this is not unexpected since other members of the JE serocomplex (genus Flavivirus) coexist in some regions of the world. Both JE and WN viruses are endemic and occur together in southern India where they share common vertebrate hosts (birds) and vectors (mosquitoes of the Cx. vishnuii group).18,19 Murray Valley encephalitis, Kunjin (a variant of WN), and JE viruses now coexist in northern Australia and New Guinea, indicating that even three members of this serologic group can occur together.20–22 In fact, Mackenzie and others23 have noted the similarities in the recent emergence of JE virus in the Australasian region with the emergence of WN virus in North America.

Several characteristics of the ecology of WN virus in Harris County were different from the ecologic pattern observed in northeastern and central regions of the United States. First, the principal vector was different. The principal vectors of WN virus in the northeast and midwest have been Cx. pipiens and Cx. restuans.24 Culex pipiens is absent from Harris County, and Cx. restuans is not very abundant, particularly during the hot summer months. Culex quinquefasciatus, the “southern house mosquito”, is the dominant Culex species in the Houston metropolitan region; all of our mosquito isolations of WN virus in 2002 were made from that species.

Crow deaths have been proposed as a sentinel surveillance system for WN virus in the northeastern United States.25 However, our experience has been different; most of the observed bird mortality in Harris County in 2002 was in blue jays (Cyanocitta cristata). Both the American crow and the blue jay are common birds in eastern Texas, so the observed difference cannot be explained by a scarcity of crows. Instead, it may reflect differences in the roosting habits of crows and blue jays26 and in the feeding habits of Cx. pipiens and Cx. quinquefasciatus.27

The intensity of WN virus transmission that occurred in Harris County during the summer of 2002 was also very high, as indicated by the large number of WN virus antigen-positive Cx. quinquefasciatus pools. Of the 6,093 pools tested, 851 (13.97%) were positive. The MFIR was 3.3 per 1,000 Cx. quinquefasciatus tested. This infection rate corresponds to a MFIR of 2.1 reported for mixed pools of Cx. pipiens/Cx. restuans collected at several sites of WN virus activity in the New York City metropolitan area in 2000.28 In contrast, the MFIR for SLE virus in the Harris County mosquitoes in 2002 was only 0.11 per 1,000. These data are supported by the contrasting antibody rates for the two viruses observed in wild birds and stray dogs in Harris County. The WN virus antibody rate in wild birds bled at the end of 2002 (October–December) was 31.2%; the corresponding SLE antibody rate was 1.4%. Likewise, the prevalence of antibodies to WN virus in stray dogs (alive during 2002 but bled in early 2003) was 56.5%, while the corresponding rate for SLE was 3.9%. During 2002, a total of 105 WN human cases and 8 SLE human cases were reported from the Houston metropolitan area, but there are no data currently available on the actual prevalence of human infection with these two arboviruses in Harris County. It will be interesting to observe if the same high level of WN virus activity continues in the region in subsequent years.

The first isolation of WN virus in Harris County (and in Texas) in 2002 was made from a dead blue jay collected on June 10th; the last evidence of WN virus activity in the county was a WN virus antigen-positive pool of Cx. quinquefasciatus collected on November 1. Sporadic collections of mosquitoes and dead birds were made by the HCMC Division during November and December 2002 and in January, February, and March 2003; but no further evidence SLE or WN virus activity was obtained (Parsons R, Tesh RB, unpublished data). Regular mosquito and dead bird surveillance was started again in April 2003. The first evidence of WN virus activity in Harris County in 2003 was isolation of the virus from a dead mourning dove (Zenaida macroura) collected on April 30th. The first WN virus-positive mosquito pool (Cx. quinquefasciatus) was collected on May 20th. In contrast, the first SLE virus-positive Cx. quinquefasciatus pool was collected on June 24, 2003. But both viruses are again active in Harris County in 2003; in fact, there was more SLE virus activity detected in 2003 than in 2002 (Parsons R, Tesh RB, unpublished data). Based on these initial surveillance data, we suspect that both viruses will persist in the region. Future studies will attempt to elucidate the seasonal pattern of activity of both viruses in Harris County and to develop a statistically based early warning system for human infection and disease. We will also attempt to determine whether these two flaviviruses overwinter locally or are reintroduced into the region each spring.

Table 1

Results of St. Louis encephalitis (SLE) virus surveillance activities in Harris County, Texas (1990–2002)

Year Total birds tested % of birds with SLE antibodies Number of Culex quinquefasciatus tested Number of SLE-positive mosquito pools (MFIR*) Total human cases of SLE
* MFIR = minimum field infection rate (number of SLE-positive pools/total Cx. quinquefasciatus tested.
1990 9,183 3.9 244,486 44 (0.18) 24
1991 7,281 7.4 218,607 89 (0.41) 41
1992 5,862 7.9 278,150 65 (0.23) 9
1993 2,928 6.0 284,273 35 (0.12) 1
1994 4,265 5.3 185,629 38 (0.12) 1
1995 4,882 4.7 194,614 0 (0.00) 0
1996 5,461 3.7 200,064 20 (0.10) 2
1997 2,917 3.4 148,429 0 (0.00) 0
1998 2,419 2.8 150,990 40 (0.26) 4
1999 2,411 3.4 125,279 0 (0.00) 0
2000 2,384 1.9 178,412 0 (0.00) 0
2001 4,252 2.1 124,810 73 (0.58) 5
2002 218 1.4 260,138 28 (0.11) 8
Table 2

Prevalence of antibodies to St. Louis encephalitis (SLE) virus in selected bird species tested in Harris County, Texas between 1989 and 2001

Common name (scientific name) Total tested Percent SLE antibody-positive
House sparrow (Passer domesticus) 29,923 3.6
Mourning dove (Zenaida macroura) 8,417 2.2
Rock dove (Columba livia) 8,088 0.5
Blue jay (Cyanocitta cristata) 5,800 11.0
Northern mockingbird (Mimus polyglottos) 4,035 6.1
Northern cardinal (Cardinalis cardinalis) 2,768 4.4
Common grackle (Quiscalus quiscula) 2,461 2.2
European starling (Sturnus vulgaris) 1,442 1.9
Inca dove (Columbina inca) 1,204 5.4
Great-tailed grackle (Quiscalus mexicanus) 827 2.5
Loggerhead shrike (Lanius ludovicianus) 379 13.2
Brown-headed cowbird (Molothrus ater) 287 1.4
Carolina wren (Thryothorus ludovicianus) 157 5.7
American robin (Turdus migratorius) 133 6.0
Table 3

Hemagglutination-inhibition (HI) test results on selected West Nile (WN)– and St. Louis encephalitis (SLE)–seropositive birds from Harris County, Texas, 2002*

Antigen (4 units)
Bird serum SLE WN
* 0 = < 1:20.
A 1:160 ≥ 1:1280
B 1:80 1:320
C 0 1:160
D 1:40 0
E 1:20 1:320
F 1:2,560 1:160
G 1:640 1:160
Table 4

Species composition of 307 West Nile virus culture-positive birds collected in Harris County, Texas in 2002

Common name Number positive Percent of total
Blue jay 281 91.5
American crow 13 4.2
House sparrow 4 1.3
Loggerhead shrike 3 1.0
Heron 1 0.3
Northern cardinal 1 0.3
Mourning dove 1 0.3
Cattle egret 1 0.3
Parakeet 1 0.3
Rough-legged hawk 1 0.3
Figure 1.
Figure 1.

Maps of Harris County, Texas showing the collection localities of West Nile virus (WNv)-positive dead birds (black dots) and WNv-positive mosquito pools (lighter stars) during June, July, August, and September 2002. The map designations are cumulative and illustrate how quickly the virus spread throughout the county. The subdivisions indicate the 268 mosquito control areas in Harris County.

Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 70, 6; 10.4269/ajtmh.2004.70.676

Authors’ addresses: Kristy M. Lillibridge, Rhia Pascua, and Tamra Meyer, School of Public Health, University of Texas Health Science Center, 1200 Herman Pressler, Houston, TX 77030, Telephone: 713-500-9358, Fax: 713-500-9359. Ray Parsons, Yvonne Randle, Taweesak Wuithinanyagool, Christina Hailey, and Adil A. Bala, Mosquito Control Division, Harris County Public Health and Environmental Services, 3333 Old Spanish Trail, Houston, TX 77021, Telephone: 713-440-3035, Fax: 713-440-4795. Amelia P. A. Travassos da Rosa, Hilda Guzman, Marina Siirin, Stephen Higgs, Dana L. Vanlandingham, and Robert B. Tesh, Department of Pathology, University of Texas Medical Branch, Galveston, TX 77551, Telephone: 409-747-2431, Fax: 409-747-2429.

Acknowledgments: We thank Dr. Raolf R. Arafat (Bureau of Epidemiology, Houston Department of Health and Human Services); Cindy Kilborn (Chief Epidemiology, Harris County Public Health and Environmental Services); and John Nix (Bureau of Animal Regulation and Control, Houston Department of Health and Human Services), for help with the human surveillance data and stray dog study. We are also grateful to Dora Salinas for help in preparing the manuscript.

Financial support: This work was supported by grant RO1-AI10984 and contract NO1-AI25489 from the National Institutes of Health and by contracts U50/CCU620541, U90/CCU620915, and U90/ CCU620539 from the Centers for Disease Control and Prevention.

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Author Notes

Reprint requests: Robert B. Tesh, Department of Pathology, University of Texas Medical Branch, 301 University Boulevard, Galveston, TX 77555-0609, Telephone: 409-747-2431, Fax: 409-747-2429, E-mail: rtesh@utmb.edu.
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