Spatio-temporal Usutu virus model explains Eurasian blackbird Turdus merula population trends

Emerging vector-borne pathogens (VBPs) pose significant global health threats, occasionally causing widespread outbreaks with high wildlife mortality. Understanding climatic and anthropogenic drivers behind transmission dynamics in wildlife populations is key to mitigate health risks. Citizen science data may allow us to investigate the spread of pathogen outbreaks as it can be collected over a wide range within short time periods. In this study, we explored the use of wildlife mortality data to model the annual occurrence of VBPs. Making use of retrospective Eurasian blackbird Turdus merula mortality data collected within a citizen science program, we employed an ecological niche modelling framework to model the annual distribution of Usutu virus (USUV) in the Netherlands. Based on these modelled USUV occurrence probabilities, we assessed the impact of the USUV occurrence on blackbird populations. Our results reveal a rapid south–north spread of USUV across the Netherlands within three years with an average expansion rate of 91 km year^-1. This spread led to significant population declines in blackbirds, reducing population numbers by 30% compared to pre-USUV levels. Our study demonstrated that citizen science bird mortality surveillance can generate useful data to model the spatial transmission intensity of VBP over time that follows the declines in wildlife populations. Higher winter and spring temperatures, which may have facilitated early development of mosquito populations and resulted in greater mosquito abundance later in the year, had a positive impact on the USUV occurrence estimates. While our approach proved successful in modelling pathogen distribution using a well-monitored species like the blackbird, it may be less effective for pathogens that cause minimal wildlife mortality or affect species that are harder to detect. This study provides a first example of a spatio-temporal estimation of USUV transmission intensity in Europe, offering a framework for investigating the annual impact of USUV outbreaks on wildlife populations.