Surveillance of West Nile virus infections in humans and animals in Europe, monthly report

Abstract

In 2025, and as of 2 July 2025, no countries in Europe reported any locally acquired1 human cases of WNV infection with known place of infection.

In the previous five years, the first locally acquired cases of the WNV transmission season usually had symptom onset in June. However, the absence of notification of locally acquired cases of WNV in the EU/EEA and EU-neighbouring countries is not unexpected at this time of the year. This could either be due to the absence of WNV infections in humans or due to a delay in diagnosis and reporting of cases of WNV infection. Furthermore, a majority of WNV infections in humans remain asymptomatic or pauci-symptomatic.

From the veterinary perspective, 2 WNV outbreaks among equids and 3 outbreaks among birds have been reported in Europe in 2025. The earliest start date of an outbreak among equids and birds was on 15 January 2025 in Germany and 16 February 2025 in Italy, while the latest onset of an outbreak among equids and birds was, respectively, on 12 June 2025 in Hungary and 11 June 2025 in Italy.

The number of outbreaks in birds and equids reported during this first period of 2025 is below the mean monthly outbreak count for the same time frame (calculated from 2015–2024). During the same period in 2024, 16 outbreaks were reported. In 2025, as of 2 July, this is the lowest number of outbreaks in birds and equids reported during the same period since 2022. All three countries (and their associated regions) reported WNV outbreaks in birds and/or equids in 2024 and in prior years, indicating endemic WNV activity in these regions.

In temperate regions like Europe, WNV transmission typically occurs from mid-June to mid-November, when mosquito activity is highest. Off-season reports of WNV outbreaks in birds and equids should be carefully evaluated as they raise questions about the timing of infection. The two early-season WNV outbreak reports (Germany's equid case in January and Italy's bird case in February) require cautious interpretation, as they may reflect residual detection (e.g. lingering antibodies or viral RNA from prior infections) rather than active transmission in 2025.

The absence of reported human West Nile virus infections in Europe as of 2 July 2025, alongside a notably lower number of outbreaks in birds and equids compared to 2024, suggests a reduced level of viral circulation in the environment during the early transmission season in 2025. Natural fluctuations in virus prevalence can occur year to year, influenced by immunity levels in bird populations and ecological conditions.

Human cases are expected to occur in the coming weeks.