Evidence of West Nile Virus Infections in Wild Boars (Sus scrofa) in the Netherlands, 2018-2021

Orthoflaviviruses are emerging in Europe and have potential significant impact on animal and public health. Usutu virus (USUV) and West Nile virus (WNV) have been first detected in the Netherlands in 2016 and 2020, respectively. Wild boars (Sus scrofa) have been suggested as an interesting population to detect the circulation of orthoflaviviruses. We aim to provide epidemiological insights of USUV and WNV infections in wild boars in the Netherlands. We further explore the interpretation of serological results, taking into account cross-reactivity patterns.

A total of 1504 wild boar sera from 2018 to 2021 were tested using a screening ELISA. Positive samples were confirmed using Virus Neutralization Tests. We explain the observed serology of the individuals with a 3-level Bayesian hierarchical model.

We found evidence for WNV infections in boars throughout the study period, with probabilities of past infections ranging from 7.82% in 2020 up to 13.02% in 2019. Our results show a declining trend in past USUV infections over the four-year study period from 24.00% in 2018 to 5.37% in 2021. Furthermore, we estimate direct and cross-reactive titre responses to both viruses, including the probability of developing a cross-reactive response.

This study indicates that WNV infections in wild boars occurred before the first human cases were detected in 2020. Surveillance of orthoflaviviruses in wild boars could provide a convenient method to monitor mosquito-borne viruses such as WNV and USUV.