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Dutch Wild Boar Showed Evidence of West Nile Virus Infection Before First Human Cases

Wild boar sow with piglets in Deelerwoud, Netherlands
Research · Disease, parasites & microbiology

Serological evidence from Dutch wild boar (Sus scrofa) indicates that West Nile virus was circulating in the Netherlands in or before 2018, two years before the country detected its first human cases in 2020. The study also examined Usutu virus. Both are mosquito-borne orthoflaviviruses maintained mainly in transmission cycles between birds and mosquitoes. Antibodies record a past immune response to infection or exposure; they do not mean that an animal still had an active infection when its blood was sampled.

Cross-reactive antibodies complicate the picture

Researchers analysed 1,504 serum samples collected through Dutch wild boar surveillance from 2018 to 2021. The samples were first screened with ELISA and positive or doubtful samples were examined with virus-neutralisation tests. Because West Nile and Usutu viruses are closely related, antibodies to one can cross-react with the other and make serological results difficult to interpret. The researchers therefore used a three-level Bayesian hierarchical model to estimate past infection probabilities while accounting for direct antibody responses, cross-reactivity and measurement error.

WNV evidence preceded the first human cases

The model found evidence of West Nile virus infection throughout the four-year period. Estimated probabilities of past infection ranged from 7.8% in 2020 to 13.0% in 2019. For Usutu virus, the estimated probability fell from 24.0% in 2018 to 5.4% in 2021. The authors stress that these modelled probabilities should not be read as direct seroprevalence estimates: cross-reactivity cannot be eliminated completely, and because viral RNA was generally not detected, the exact timing of infection remains uncertain.

Wild boar can act as sentinels

Wild boar are dead-end hosts and do not contribute to onward transmission of these viruses, but that makes their antibodies useful in another way. Their exposure to mosquitoes, relatively limited home ranges and inclusion in existing wildlife surveillance mean they can act as sentinel animals for past virus circulation. The authors conclude that adding orthoflavivirus serology to wild-mammal surveillance could improve early detection and risk assessment for West Nile and Usutu viruses.

Sources and image credit

  1. Streng, K., de Freitas Costa, E., van Oort, S., Hakze-van der Honing, R., Bron, G., Grøntvedt, C. A., van der Poel, W. H. M., Hozé, N., & Counotte, M. (2026). Evidence of West Nile Virus Infections in Wild Boars (Sus scrofa) in the Netherlands, 2018–2021. Zoonoses and Public Health.Primary open-access research article first published 6 September 2026. Full text checked. The study analysed 1,504 wild boar sera from 2018–2021 using ELISA, virus-neutralisation tests and a three-level Bayesian hierarchical model that accounts for direct and cross-reactive antibody responses and observation error. It found evidence of West Nile virus infection in or before 2018; modelled past-infection probabilities ranged from 7.82% in 2020 to 13.02% in 2019, while estimated Usutu virus infection declined from 24.00% in 2018 to 5.37% in 2021.

Image: Wild boar sow with piglets in Park Deelerwoud, Netherlands, 29 May 2011. Photo: Henk Monster / Wikimedia Commons. CC BY 3.0. Representative Dutch species image; not animals from the virus study. · CC BY 3.0 · Source

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