West Nile Virus Transmission in Resident Birds, Dominican Republic

T initial outbreak of West Nile virus (WNV; family Flaviviridae, genus Flavivirus) in the Western Hemisphere took place in New York in 1999, with deaths observed in humans, horses, and numerous species of wild birds (1). Since then, this virus has spread rapidly across North America (2,3). Migratory birds are suspected of being responsible for the rapid spread of WNV through North America (4), and transport of WNV by Neotropical migratory birds throughout the New World has been anticipated (5). Although WNV has spread rapidly through continental areas, its ability to spread across oceanic barriers is uncertain. The many islands of the West Indies represent the wintering grounds of numerous North American migratory birds (6,7) that breed in or migrate through WNV transmission foci in the United States. The Caribbean islands tend to have high human population density, and low populations of many birds and other vertebrates are restricted to certain islands. Introduction of WNV to the West Indies would present a human and equine health concern and potentially threaten numerous endangered and endemic bird species and perhaps other wild vertebrates. Given the speculation that WNV may be disseminated by migrating birds (5,8,9), we hypothesized that the virus would be introduced to the Dominican Republic. Accordingly, we sampled apparently healthy birds there for evidence of locally acquired WNV infection. The Study Birds were studied at two sites in the Dominican Republic, on the island of Hispaniola (Figure): Parque Nacional Sierra de Baoruco (November 7–16, 2002; 18° 12' N, 71° 32' W) and Parque Nacional Los Haitises (November 18–23, 2002; 19° 00' N, 69° 30' W). Birds were collected by standard methods (10). Tissues (eye, spleen, and kidney) were removed from 89 birds of 29 species (25 resident, 4 migratory) at Sierra de Baoruco and from 58 birds of 27 species (18 resident, 9 migratory) at Los Haitises; the tissues were tested for active WNV infection. Blood samples were collected from a subsample of these birds, including 41 that represented 18 resident species at Sierra de Baoruco and 33 that represented 16 resident species at Los Haitises. Blood was not collected from migratory birds or from certain very small resident birds, such as hummingbirds. Blood and tissue specimens were frozen immediately in liquid nitrogen for transportation and then stored at –70°C. Voucher specimens (including additional tissue samples) were prepared for all birds and are deposited at the University of Kansas Natural History Museum (KUNHM). The sex and breeding condition of each bird were determined by examination and measurement of gonads. Birds’ ages were assessed by plumage, skull ossification, and presence or absence of a bursa of Fabricius. The migratory or resident status of each bird was determined on the basis of standard references (6,7). For species that had both migratory and resident populations, we based status assessment on breeding conditions (breeding birds were assumed to be resident). Serum samples were screened for flavivirus-neutralizing antibodies by plaque-reduction neutralization test (PRNT) according to standard methods (11) and by using challenge inocula of 100 plaque-forming units (PFU) WNV strain NY99-4132 and Saint Louis encephalitis virus (SLEV) strain TBH-28. Testing for neutralizing antibody