Research by Wageningen Bioveterinary Research (WBVR) and the Dutch Wildlife Health Centre (DWHC) shows that foxes in the province of Friesland can become infected with highly pathogenic avian influenza (HPAI H5N1). This occurs mainly during periods when the virus is circulating among wild birds. Testing hunted foxes for possible infection therefore proves to be a valuable addition to existing avian influenza monitoring in mammals.
Indicator
On behalf of the Ministry of Agriculture, Fisheries, Food Security and Nature, WBVR and DWHC examined a total of fifty foxes from population management and damage control programmes in Friesland in 2024 and 2025. The animals were tested for the presence of the avian influenza virus as well as for antibodies against the virus. This enabled researchers to identify both active infections and previous exposure to avian influenza.
Genetic material of the HPAI H5N1 virus was detected in nasal and throat swabs from two foxes. In one of these animals, the virus could be fully genetically characterised. It concerned an H5N1 virus closely related to viruses detected in wild birds and other avian species during the same period.
Active infection
Pathological examination revealed severe pneumonia in one fox, which was likely caused by avian influenza. The virus was not detected in brain tissue.
Previous exposure
In addition to virological testing, serological analyses were performed. Antibodies against the avian influenza virus were detected in almost half of the examined foxes, indicating that these animals had been exposed to an avian influenza virus. In a proportion of these foxes, antibodies specifically against H5 were identified, pointing to previous exposure to highly pathogenic avian influenza. All seropositive foxes were found exclusively during periods when avian influenza was also detected in wild birds.
Importance of surveillance
The findings from this study in foxes underline the importance of continuous surveillance of avian influenza in wild mammals, particularly in wetland areas and during periods of increased virus circulation among birds. Virological testing can serve as an early warning system, while serology provides insight into trends and spread over time.
KM (Kim) Bouwman-te Selle, PhD
This contribution was selected for its informative value in the context of avian influenza surveillance. Although the study focuses on wildlife, the findings provide relevant insights into the circulation of the HPAI virus and into monitoring strategies supporting the poultry production chain.
