West Nile Virus Infection in Birds and Mosquitoes, New York State, 2000

West Nile VirusThe emergence of West Nile (WN) virus in 1999 in four U.S. states (1) was followed by its spread to 12 states in 2000 ( 2).An enzootic cycle was established between birds and mosquitoes, and WN disease was observed in humans, horses, and birds in both years (2,3).Bird deaths due to WN virus are unusual outside North America, with the exception of deaths of geese in Israel (4) and pigeons in Egypt (5).In 1999 in North America, WN disease, characterized by meningoencephalitis and myocarditis, was observed in 14 species of wild and captive birds (6).WN virus has been detected in a number of mosquito genera in North America, including Culex and Aedes species (2,7).Vector competence has been confirmed experimentally for some North American species, including Cx. pipiens, Ae. vexans, and Ae.japonicus (8,9).We have summarized surveillance data for WN virus in dead birds and mosquitoes for New York State in the 2000 transmission season.A quantitative and kinetic analysis of data within and outside the epicenter is shown for both the bird and mosquito samples.Vertebrate and invertebrate WN virus infections are compared for counties in the epicenter. Materials and Methods Bird and Mosquito SamplesDead birds were collected and mosquitoes were trapped by local county health units and other agencies as part of the New York State WN virus surveillance effort.Dead birds were necropsied at the Wildlife Pathology Unit at the Department of Environmental Conservation.Kidney, brain, heart, liver, or spleen were harvested and stored at -70°C.Additional avian tissue samples sent to the National Wildlife Health Center were not included in this analysis because selection criteria and testing procedures differed.Mosquitoes were trapped, speciated, grouped into pools of 5 to 50, and stored at -70°C.For some pools, Cx. pipiens and Cx.restuans were not separated but were pooled together and denoted as Cx.pipiens-restuans.The Aedes genus is being reclassified into two genera, Aedes and Ochlerotatus (10), but is classified as Aedes in this manuscript.Avian tissue samples and mosquito pools were sent to the Arbovirus Laboratory at the Wadsworth Center for WN virus testing.The transmission season was defined as May 15 to October 31, 2000, with the first and last positive samples collected on May 22 and October 31, 2000, respectively.Two positive birds were found earlier in the year (a hawk on February 6 and a crow on April 1), but they were not followed by other positive specimens and did not therefore appear to represent the beginning of the mosquito-borne WN virus transmission season. WN Virus TestingSamples were processed and WN virus assays were performed as described by Shi et al. (11).Briefly, RNA was extracted from bird tissue or pools of <50 mosquitoes, and real-time reverse transcription-polymerase chain reaction (RT-PCR) was performed (TaqMan, ABI Prism 7700 Sequence Detector, Applied Biosystems, Foster City, CA).Confirmatory tests included a second TaqMan primer-probe set, standard RT-PCR, virus isolation in cell culture, and West Nile Virus Infection in Birds andMosquitoes, New York State, 2000