Antibodies to flaviviruses in wild ducks captured in Hokkaido, Japan: risk assessment of invasive flaviviruses.

Recently, the distribution of Japanese encephalitis virus (JEV) and West Nile virus (WNV) has expanded into new territories. The invasion of WNV into Japan is of great concern. The migration of birds is suggested to be involved in the expanded distribution of these flaviviruses. In this study, 92 wild ducks--20 Anas poecilorhyncha (migratory breeders), 50 Anas platyrhynchos (undetermined), 16 Anas acuta (winter visitors), and 6 Anas penelope (winter visitors)--were captured in autumn of 2005 and 2006, in the central part of Hokkaido, a low JEV activity area. A seroepidemiologic analysis of flavivirus infections was conducted with 90% and 50% focus reduction neutralization tests (FRNT(90) and FRNT(50)). Of the 92 serum samples, 1 (1.1%) and 5 (5.4%) tested positive for WNV-specific and JEV-specific antibodies, respectively, in the FRNT(90), and 61 (66.3%) and 79 (85.9%) tested positive for WNV and JEV, respectively, in the FRNT(50). These results indicate that wild ducks in this study had been exposed to flaviviruses. The results together with the recognized distribution of flaviviruses and migratory routes of individual duck species strongly suggested that the birds captured in this study had been exposed to flaviviruses, including WNV, on the flyway, not in Hokkaido.