From Epizootic to Enzootic: Dynamics and Impacts of Highly Pathogenic Avian Influenza in Dutch Wild Bird Populations (2016–2025)

Abstract

Since 2016, highly pathogenic avian influenza (HPAI) has caused recurring outbreaks in wild birds in the Netherlands, with a potential shift from seasonal epizootics to year-round enzootic circulation since 2021. This study aims to create an overview of changing dynamics of HPAI outbreaks in Dutch wild bird populations, assessing temporospatial trends, species susceptibility and species-specific clinical presentation, with the goal to support wildlife surveillance programs. For this, literature on avian influenza outbreaks from 2016 - 2022 and DWHC surveillance data from 2022 - 2025 was used.

The analysis identified five outbreak events, each distinct through their timing and associated target species. During the observation period, 66 bird species tested positively on HPAI, deomnstrating a high degree of host-adaptability. In recent years, the most affected species were: Barnacle geese (Branta leucopsis), sandwich terns (Thalasseus sandvicensis) and black headed gulls (Chroicocephalus ridibundus). These species were not affected severely prior to 2020. Year-round virus incursions were first reported in 2021, and continuous circulation occurred in subsequent years. Summer outbreaks occurred coastally, when colonial seabirds gathered for their breeding season.

Symptom data from 434 PCR-confirmed HPAI cases showed predominantly neurological manifestations (92.4% of cases), with head shaking, spinning, tremors and reduced levels of consciousness being commonly reported. Ocular signs in the form of a blue/white corneal haze were observed in a fifth of cases (22.4% of cases). Variation exists between taxonomic groups. Raptors exhibited a higher rate of respiratory signs (10% of raptor cases), while non-target waterbirds displayed more general symptoms and reduced neurotropism (72.0% of non-target waterbird cases).

This study confirms a shift in HPAI dynamics from seasonal epizootics to more sustained, enzootic, circulation in Dutch wild bird populations. Quickly changing virus dynamics, with changing seasonality, a broadening host range and variable virulence, ranging from sub-clinical to highly lethal, make prediction of virus dynamics difficult. However, the results of this study highlight the value of citizen reports, in the detection and characterisation of HPAI outbreaks. Observed trends have implications for outbreak recognition, targeted surveillance, and can help shape guidelines to better recognise and manage avian influenza cases by first responders in the field.