Prevalence of West Nile Virus and Usutu Virus Serum Antibodies in Dutch Bird Ringers

In August 2020, West Nile Virus (WNV) was isolated from a live common whitethroat for the first time in The Netherlands. Follow-up sampling showed that the virus could also be detected in mosquitos from the same location during at least a whole month of sampling. On 15 October 2020, one case of West Nile virus infection has been reported in a man who was likely infected in the Utrecht region. This is the first time that a locally acquired human case of WNV infection has been reported in The Netherlands. Six additional cases have been identified, one of which from the region Arnhem. West Nile virus infection is a mosquito-borne zoonosis. The disease, which has spread across the Northern Hemisphere in the past three decades, is now found on an annual basis in many European countries where the centre of gravity lies in Southern-European countries. Recently, WNV was reported for the first time in Germany. The virus is transmitted among birds through the bite of infected mosquitoes and incidentally infects humans and other mammals, such as horses. Around 80% of human WNV infections are asymptomatic. The most common clinical presentation is West Nile fever but, older people and immunocompromised persons are at higher risk of developing neuro-invasive disorders (West Nile neuroinvasive disease). Currently, there are no prophylaxis or specific treatment against the disease in humans. In addition, Usutu virus (USUV) was detected in The Netherlands in 2016. USUV is another flavivirus, related to WNV, and also capable of infecting humans. Disease associated with USUV infection in humans appears to be milder and only limited number of cases have been identified. During their bird catching activities, bird ringers are intensively exposed to mosquito bites at the natural habitat of the birds and at the same time of the day when mosquitoes are particularly active. The aim of this study is therefore to determine the prevalence of WNV and USUV serum antibodies in bird ringers in The Netherlands.