Widespread circulation of West Nile and Usutu viruses in sedentary and migratory avifauna: A Two-Year Study (2024-2025) of active surveillance in South of France

This two-year study (2024–2025) in southern France reveals widespread circulation of West Nile and Usutu viruses among over 2,500 sedentary and migratory birds, with Usutu prevalence exceeding West Nile and overall infection rates higher in 2024, underscoring the critical role of mixed avifauna in sustaining local enzootic cycles and the need for integrated year-round surveillance to mitigate zoonotic risks.

The Gist

Imagine two invisible, microscopic travelers: the West Nile virus and the Usutu virus. Think of them as a pair of cousins who love to hitchhike on mosquitoes (specifically the Culex pipiens kind) and use birds as their main "all-inclusive resorts" to multiply and spread.

For a long time, these cousins lived in Africa, but over the last twenty years, they've moved into Europe. They likely rode the "highways" of the sky, traveling on the backs of migratory birds that fly back and forth across the Mediterranean.

The Study: A Two-Year Bird Watch
Between 2024 and 2025, scientists in Southern France decided to play detective. They knew this region was a perfect "party zone" for these viruses because it has wetlands, city edges, and farms where birds love to hang out. They set up a massive surveillance net (literally using mist nets and bird boxes) to catch and check over 2,500 wild birds. They looked at both birds that stay in one place year-round (sedentary) and those that travel long distances (migratory).

What They Found
The results showed that these viruses are everywhere, like a common cold in a crowded school. However, they found a few key patterns:

• The Usutu "Superstar": The Usutu virus was much more common than West Nile. It was found in many different types of birds, especially the small songbirds you see in gardens and parks, like great tits, house sparrows, and barn swallows.
• The Weather Factor: The viruses were much more active in 2024 than in 2025. The scientists suspect that changes in the weather or the environment acted like a "volume knob," turning the virus activity up one year and down the next.
• The Travelers vs. The Locals: The study suggests a team effort in spreading the virus. The migratory birds act like the "importers," bringing the virus in when they arrive for the season. Once they are there, the resident birds (the locals who don't leave) keep the virus alive and circulating locally, even when the travelers have moved on. This helps the virus survive the winter.

The Big Picture
The main takeaway is that a mix of traveling and local birds is essential for keeping these viruses moving around Southern Europe. Because of this, the scientists say we need to keep a close, year-round eye on these specific birds and their habitats. By watching the birds, we can get an early warning if the viruses are getting ready to jump to humans or other animals, allowing us to manage the mosquito populations better before a problem gets out of hand.

Technical Summary

Technical Summary: Widespread Circulation of West Nile and Usutu Viruses in Sedentary and Migratory Avifauna (2024–2025)

Problem Statement
West Nile virus (WNV) and Usutu virus (USUV) are neurotropic Orthoflaviviruses that share an enzootic transmission cycle involving Culex pipiens mosquitoes as vectors and birds as amplifying hosts. While originally identified in Africa, both viruses have become endemic across Europe over the last two decades, likely introduced and disseminated via migratory bird species along Mediterranean flyways. In avian populations, infection outcomes are highly variable, ranging from subclinical infections to fatal neuroinvasive disease, dependent on viral strain, host immunity, and species-specific susceptibility. Southern France represents a critical hotspot for these viruses due to diverse avian habitats that support year-round viral maintenance. The study addresses the need to understand the prevalence and dynamics of these viruses within both sedentary and migratory bird populations in this region to better comprehend local transmission cycles.

Methodology
This study conducted active molecular surveillance over a two-year period (2024–2025) in Southern France. The surveillance targeted more than 2,500 wild birds, encompassing both sedentary and migratory species across multiple passerine and non-passerine taxa. Sampling was executed across diverse ecological zones, including wetlands, urban fringes, and agricultural areas. Birds were captured using mist nets and bird boxes to ensure representation of various species and behaviors. The study specifically analyzed samples to detect the presence of WNV and USUV, comparing infection rates between resident and migratory individuals and across the two study years.

Key Results
The analysis revealed widespread circulation of both WNV and USUV across the diverse avian species sampled.

• Species Distribution: Infection was observed primarily among passerines, with notable detection in great tits, house sparrows, and barn swallows.
• Viral Prevalence: USUV was detected at higher rates than WNV in both study years.
• Temporal Variation: Overall prevalence was markedly higher in 2024 compared to 2025, a fluctuation the authors attribute to potential climatic or ecological drivers.
• Ecological Dynamics: The data supports a dual-dynamic model where migratory individuals likely act as vectors for seeding viral introductions during seasonal passages, while resident populations sustain local enzootic cycles, thereby facilitating the overwintering persistence of the viruses.

Significance and Claims
The paper claims that these findings highlight the pivotal role of mixed avifauna in driving arbovirus dynamics within Mediterranean Europe. By demonstrating the co-circulation of WNV and USUV in both resident and migratory birds, the study underscores the complexity of local transmission networks. The authors emphasize the necessity for integrated, year-round surveillance strategies that specifically target high-risk species and habitats. They assert that such enhanced monitoring is essential for predicting spillover risks and informing vector control strategies to mitigate zoonotic threats, rather than relying solely on seasonal or single-species surveillance.