Adult Mosquito Collection

Figure 1

Figure 1. . Mosquito trap locations, Staten Island, 2000. NPS = National Park sites.

Adult mosquitoes were collected one night per week from May 2 through November 17 by using CDC miniature light traps baited with dry ice and CDC gravid traps at 21 locations on SI (Figure 1) and 80 locations in the other boroughs. During August 14 to September 2, adult mosquitoes were collected 5-7 nights per week on SI as part of an evaluation of the efficacy of adult mosquito control. All traps were set in the late afternoon and evening and retrieved the following morning. Specimens were immediately frozen with dry ice.

Adult mosquitoes were identified and sorted to species whenever possible. If the condition of a specimen did not permit species identification, specimens were grouped as Cx. pipiens/restuans or by genus alone. Up to 50 adult mosquito specimens were pooled (pools of Cx. salinarius had up to 100 specimens) by trap site and date. Mosquito pools collected from May 2 to June 2 were submitted to the Centers for Disease Control and Prevention laboratory in Fort Collins, Colorado, and pools from June 3 to November 17 were submitted to the Arbovirus Laboratory of the New York State Department of Health (NYSDOH) for testing.

Minimum infection rate (MIR) was used as the index of virus activity in mosquito populations. MIR is calculated as the number of WN virus-positive pools per number of adult mosquitoes tested and is expressed as the number positive per 1,000 tested. Weekly MIRs were calculated for each species collected on SI and the other boroughs. Mosquito data include only those specimens that were sorted and tested as of December 2000.

Dead Bird Reporting and Collection

Dead bird reports were taken from the public by two systems. An interactive voice-response telephone system allowed callers to leave detailed information about the bird(s) being reported in a recorded message. Reports could also be entered onto a form on the NYCDOH web site. In both systems, the reporter was asked to provide the date the bird was initially found; its exact location, including street address, borough (county), and zip code; species; and cause of death (if known). The density of reported dead birds per square mile of surface area was determined by week for SI and the other boroughs.

Dead bird reports were reviewed before data entry; those meeting predetermined criteria were collected for WN virus testing. Collection was limited to birds recently dead (found in the previous 24 to 48 hours) and in relatively good condition (e.g., little or no decay). The primary criterion for collection was the species of bird. Initially only crows (American or Fish), sparrows, Blue Jays, or any cluster of five or more birds were collected for testing. Collection was expanded to include other species, resources permitting, if they were in good condition. Collection of other species focused on raptors (especially Merlins and American Kestrels) or species not typically reported (e.g., Black Skimmer, Belted Kingfisher). Collected birds were sealed in plastic bags and stored with freezer packs in the field. All specimens were submitted to the Wildlife Pathology Unit of the New York State Department of Environmental Conservation for necropsy; tissue specimens were submitted to the Arbovirus Laboratory of the NYSDOH for WN virus testing.

Laboratory Testing of Mosquitoes and Birds

Adult mosquito pools and avian tissues were tested for WN virus with one set of primers/probes by real-time reverse transcription-polymerase chain reaction (RT-PCR) (TaqMan, ABI Prism 7700 Sequence Detector, Applied Biosystems, Foster City, CA). Confirmatory tests were performed by using a second TaqMan primer/probe set, standard RT-PCR, virus isolation in cell culture, and immunofluorescence assays (avian tissues). A sample was confirmed positive when at least two different tests were positive. Details on virus testing are described elsewhere (4,5).

Human and Equine Case Surveillance

NYCDOH conducted citywide enhanced passive and active hospital-based physician and laboratory surveillance for human WN virus infections, including all three acute-care hospitals on SI. Additional details on human surveillance are described elsewhere in this journal by Weiss et al. (6). Surveillance for equine illnesses was by enhanced passive surveillance. Additional details regarding equine surveillance are described elsewhere in this journal by Trock et al. (7).

Adult Mosquito Collection and Testing

A total of 24,068 adult mosquitoes from 23 species were tested in 967 pools from SI; 131 pools from 6 species had laboratory evidence of WN virus. NYSDOH reported WN virus isolation from Aedes vexans, Cx. pipiens, Cx. salinarius, Ochlerotatus triseriatus, and Psorophora ferox, and WN viral RNA detection in Anopheles punctipennis (8). During the same period, a total of 51,044 adult mosquitoes from 26 species were tested in 1,958 pools collected from the other boroughs; 39 of these pools had laboratory evidence of WN virus (Table). These are the first reports of Ae. albopictus and Cx. erraticus from New York State.

Figure 2

Figure 2. . Minimum infection rate (MIR) of Culex pipiens/restuans, Cx. salinarius, Aedes sp., and Ochlerotatus sp., dead bird densities, West Nile-infected human and equine cases by week, Staten Island and other boroughs,...

Laboratory evidence of WN virus was first detected from mosquitoes in a Cx. pipiens pool collected July 7 and in a Cx. salinarius pool collected July 17, both from SI. The first human patient on SI became ill on July 20. Additional human cases on SI had onset of disease between July 28 and September 2. Human cases occurred when the MIR among Cx. pipiens and Cx. pipiens/restuans ranged from approximately 5 to 16/1,000 (July 7 to September 22) and the MIR among Cx. salinarius was at least 4/1,000 (6 of 7 weeks from July 15 to September 1). The MIR among Cx. salinarius peaked at 9/1,000 in the week ending August 11 (Figure 2).

Dead Bird Reports and Testing

Overall, 14,849 dead birds were reported in NYC during 2000, including 4,910 (33%) from SI. Of the SI birds, 235 (4.8%) were collected and submitted for testing; 60 (26%) had laboratory evidence of WN virus. These included 45 American Crows, 5 Blue Jays, 2 Snowy Owls, and 1 each of the following species: American Kestrel, Canada Goose, Common Grackle, Fish Crow, Greater Black-backed Gull, Mourning Dove, Northern Mockingbird, and sparrow.

The density of total dead birds per square mile on SI began to increase in the week ending July 7 and peaked at >14 dead birds/square mile per week during the week ending July 21. The density of dead birds remained >5/square mile until the week ending September 1. This period of increased weekly density of dead birds coincided with the period of elevated MIRs among Cx. pipiens, Cx. pipiens/restuans, and Cx. salinarius (Figure 2).

Human and Equine Cases

Ten human cases of neurologic WN virus infection were reported from SI, and four cases were reported from three other boroughs. Onset of illness for the SI cases ranged from July 20 to September 2 and from August 15 to September 13 for the other boroughs. Four confirmed equine cases were reported, two on SI (onset August 17 and September 8) and two from another borough (onset August 27 and October 1).