West Nile virus (WNV)

In the Netherlands, locally acquired animal and human cases of West Nile virus (WNV) were first identified in 2020 via multidisciplinary WNV monitoring and research activities focusing on mosquitoes, birds, horses, and humans.

We investigated how different activities contributed to WNV detection and response in the Netherlands between 2020 and 2023, to determine best practices, challenges, and opportunities for improvement.

We identified WNV monitoring and research activities in the Netherlands from 2020 to 2023 and analysed their timeliness to detect and react to WNV circulation. An after-action review (AAR) was conducted with national WNV experts to assess best practices and challenges in the multidisciplinary approach.

In 2020, WNV circulation was discovered in an infected bird through a wild live bird research survey and subsequently through mosquito research and monitoring. Thirty-five days after finding the WNV-PCR-positive bird, the first autochthonous human case was uncovered. Between 2021 and 2023, research projects in animals, including sentinel chickens, detected ongoing local enzootic WNV circulation. The AAR highlighted rapid information sharing and interpretation, enabled by multidisciplinary collaborations, as best practice. However, differing institute priorities could sometimes lead to diverging views on follow-up actions.

Research and monitoring activities in mosquitoes and animals, particularly wild birds and sentinel chickens, enabled early detection of WNV circulation. Real-time testing could provide early warning of human cases, enabling timely responses. Therefore, these research and monitoring activities should be maintained. Multidisciplinary collaboration enabled rapid detection and response, and addressing remaining challenges could further strengthen effectiveness.

West Nile virus (WNV) and Usutu virus (USUV) outbreaks in Europe pose growing public health concerns. In Austria, human WNV and USUV infections occur nearly every year since 2009 with notable case number variations.

We analysed annual incidences and spatiotemporal distributions of human WNV and USUV infections in Austria in 2009–2024.

Annual incidence rates of laboratory-confirmed WNV and USUV cases recorded through the national surveillance were calculated, stratified by age, sex, clinical presentation, exposure place and virus sequence.

During 2009–2024, recorded case numbers were highest in 2024, with 37 WNV (19 male/18 female; median age: 62 years, range: 18–88) and 27 USUV infections (18 male/9 female; median age: 59 years, range: 20–69). Nineteen WNV cases developed West Nile neuroinvasive disease, while no USUV cases had neurological disease. Thirty-four of the WNV cases and all USUV cases were locally acquired. In northern Burgenland, an eastern Austrian region with an avian hotspot and only sporadic cases previously reported, WNV and USUV incidences respectively rose from averages of 0.6 and 1.0 per 100,000 in previous years to 6.6 and 4.2 per 100,000 in 2024. All 25 sequences analysed in 2024 from locally acquired WNV cases were of lineage 2. Among 15 USUV sequences, 14 belonged to the Europe-2 and one to the Africa-3 clade.

Human WNV and USUV infection increases in a previously low-incidence region underscore their increasing public health impact in Austria. Strengthening surveillance and response measures is essential for early detection, guiding prevention efforts, and ensuring blood donor safety.

The first autochthonous human West Nile virus (WNV)-positive cases in Germany were confirmed in 2019. Risk minimisation measures (RMM) were introduced in 2020; no WNV transfusion-transmitted infections have been reported to date.

To analyse German suspected WNV-positive blood donations during annual seasons 2020–23 to review donor testing requirements.

WNV look-back procedures were initiated as per German regulations and additional donor data were collected. Blood samples were analysed by metagenomic next-generation sequencing (mNGS), individual donor nucleic acid amplification technique (ID-NAT)-based testing and antibody (Ab) testing.

Seventy-four cases were followed up after WNV-positive donor mini-pool screening. Forty-five (83%) of 54 samples tested with the cobas WNV assay and 14 (29%) of 49 samples tested with the RealStar WNV assay showed a reactive ID-NAT-based result; the viral load ranged between 70,251 IU/mL and values below quantification limits. Fifteen (23%) of 64 samples serologically tested were reactive with at least one of the three Ab tests performed; the previous WNV-negative donation was nearly always documented > 28 days before. Of 73 samples sequenced, mNGS detected WNV in 26 (36%) and other flaviviruses in 14 (19%) cases.

In some suspected cases where a WNV infection was not confirmed, mNGS demonstrated a cross-reaction with other flaviviruses. Ab testing could only detect WNV in late stages of infection. A NAT-based WNV donor screening with a detection limit of at least 120 IU/mL seems to be a sufficiently effective RMM at present. However, a continuous re-evaluation of test strategy is always required.