A decade of West Nile virus surveillance in the host and vector populations of Denmark, 2011 to 2023.

BACKGROUNDTo spot potential introductions of West Nile virus (WNV) into Denmark, a national surveillance programme for WNV was established in 2011. The relevance of this programme was underscored in the late 2010s, when WNV was detected in areas close to Denmark.AIMWe describe the Danish WNV surveillance programme and its findings in 2011-2023.METHODSThe surveillance programme monitors mosquitoes, which are WNV vectors, and some mammalian animals and birds, which are WNV hosts. Surveillance samples are also tested for the closely related Usutu virus (USUV), which, like WNV, is a flavivirus. During the study, WNV and USUV RNAs were sought in 62 bats (sampled in 2022-2023), 5,661 mosquitoes (2011-2023), 628 dead wild birds (2011-2014 and 2019-2023), and 492 live birds (2011-2012, 2022-2023). These 492 birds were from among 3,269 live long-distance migratory birds (sampled in 2011-2023) serologically tested for WNV and USUV antibodies. Additionally, 4,978 free-ranging poultry (2011-2023) and 236 horses (2011-2013) were tested serologically.RESULTSNeither WNV nor USUV RNA was detected in bats, mosquitoes, or birds, but anti-WNV specific antibodies were detected in migratory birds, one domestic chicken and one imported horse. For migratory birds, competitive ELISAs detected anti-flavivirus antibodies in 3.9% (128/3,269) of tested samples. Across 2011-2023, the annual flavivirus seroprevalence varied from 1-13%. Using virus neutralisation assays on selected samples, anti-WNV or anti-USUV specific antibodies were detected in 25 and 11 bird samples, respectively.CONCLUSIONSFindings demonstrate that the concern about virus incursion is well founded and support continued vigilance for WNV.