Serologic evidence of West Nile Virus in wild ducks captured in major inland resting sites for migratory waterfowl in South Korea

The rapid global expansion of West Nile virus (WNV) has recently raised concerns regarding its possible spread into South Korea. To date, WNV infection in wild birds in South Korea has not been identified. Bird migration is thought to be involved in spreading WNV, and wild birds are the possible routes of introduction of WNV infection. To assess the risk of WNV infection in South Korea, we conducted a nationwide WNV surveillance of wild birds, with an emphasis on migratory ducks from WNV-affected areas. Our chief aim was to determine whether birds with the potential to introduce WNV are present in South Korea by testing migrating and resident wild birds for WNV antibodies. We collected blood samples from 1531 wild birds representing 57 bird species at several major inland resting sites for migratory waterfowl in South Korea. A seroepidemiological analysis of WNV and Japanese encephalitis virus (JEV) infections was conducted using plaque-reduction neutralization tests (PRNTs) for each virus. To search for recent WNV infections, sera were also evaluated by IgM antibody capture ELISA. Of the 1531 serum samples, 5 (0.3%) tested positive for WNV-specific antibodies, and 70 (4.6%) tested positive for JEV-specific antibodies. A total of 9 (0.6%) samples were positive for both WNV and JEV antibodies; these samples were interpreted as having a flavivirus exposure. All birds that had neutralizing antibodies specific to WNV were negative for IgM, which indicates the likelihood of a relatively old infection. Along with the recognized distribution of flaviviruses along several duck species’ migratory routes, our findings strongly suggest that some of the birds captured in this study had been exposed to WNV or JEV.