Surveillance for West Nile virus in wild birds from northern Europe.

A total of 1935 migratory birds from 104 different species were captured in southeastern Sweden in 2005-2006 and tested for antibodies against West Nile virus (WNV). Overall, 46 birds (2.4%; binomial confidence limits, 1.8-3.2) were positive by blocking-ELISA, but only 2 (0.10%; binomial confidence limits, 0.0-0.4) had antibodies detectable by both blocking-ELISA and WNV neutralization test. ELISA-positive birds included long- and short-distance migrants likely exposed to WNV while wintering in or migrating through areas enzootic for WNV. Exposure to a cross-reactive Flavivirus was suspected for short-distance migrants of the Turdidae family, but no cross-neutralization with tick-borne encephalitis and Usutu viruses was observed.