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Usutu Virus RNA Was Found Nearly Three Times as Often as West Nile Virus RNA in Southern France

Wild Barn Swallow in flight in France
Research · Disease, parasites & microbiology

A two-year study of 2,533 wild birds from 69 species in southern France found Usutu virus (USUV) RNA in 12.0% of birds and West Nile virus (WNV) RNA in 4.3%. USUV and WNV are mosquito-borne orthoflaviviruses in which birds act as important amplifying hosts; infection can range from subclinical to severe neurological disease. Birds were sampled in the Camargue wetlands and around Montpellier during 2024 and 2025. The study used RT-qPCR to look for viral RNA in cloacal swabs and droppings, so the percentages describe molecular detections in those samples. RNA detection shows that viral genetic material was present, but does not by itself show clinical disease or the amount of infectious virus.

USUV was detected more often than WNV

USUV RNA was detected more often than WNV RNA in both resident and migratory birds. Among common resident species, great tits (Parus major) had detection rates of 17.6% for USUV and 5.1% for WNV, while house sparrows (Passer domesticus) reached 21.5% and 13.4%. Among migratory birds, barn swallows (Hirundo rustica) were positive in 12.4% and 2.4% of samples, respectively. European bee-eaters (Merops apiaster) showed similar rates for the two viruses, 9.5% and 8.8%.

The authors caution that species-level patterns partly reflect how many individuals were sampled, so raw detection counts should not be treated as a ranking of biological susceptibility.

West Nile and Usutu virus RNA detections by migratory and sedentary wild bird species
West Nile virus and Usutu virus RNA-positive detections among migratory and resident bird species sampled in 2024 and 2025. Sample size for each species is shown in parentheses. Figure 4 from Beaubaton et al. (2026), PLOS Neglected Tropical Diseases, CC BY 4.0. Source.

Detection was higher in 2024

Virus detection was substantially higher in 2024 than in 2025. USUV RNA fell from 14.0% to 10.5% between the two years, while WNV RNA dropped from 7.7% to 2.0%. In 2024, birds tested positive from late May, before the first recorded human WNV cases in early July, positive mosquito pools in early August and equine cases in early September. In 2025, USUV was detected in birds from March and WNV from late April.

Age mattered more than migration status

The models did not support a clear difference in RNA detection between migratory and resident birds. Age was more informative: younger birds, especially nestlings and recently fledged individuals, showed higher positivity for one or both viruses than older age classes. The authors conclude that active bird surveillance can complement mosquito, veterinary and human monitoring, although its value for predicting outbreaks still needs longer and more standardized time series.

Sources and image credit

  1. Beaubaton, R., Revel, J., Pigeyre, L., Lepeule, A., Joly, J., de Franceschi, C., Charmantier, A., Vollot, B., & Simonin, Y. (2026). 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.Primary peer-reviewed open-access article published 3 September 2026. Among 2,533 unique wild birds from 69 species, USUV RNA was detected in 12.0% and WNV RNA in 4.3%. Species-level examples included great tit, house sparrow, barn swallow and European bee-eater; the authors caution that raw species detection counts partly reflect uneven sampling intensity. Detection was higher in 2024 than 2025, and avian detections preceded human, mosquito and equine WNV detections during the 2024 season.

Image: Barn swallow in flight at Angoulême, Nouvelle-Aquitaine, France, 19 September 2024. Photo: TRinaud / Wikimedia Commons. CC BY 4.0. Cropped from the original to centre the bird. Representative French species image; not a study bird. · CC BY 4.0 · Source

About this content: This story was produced with AI assistance within an editorial workflow developed by Wildlife Vagabond. Editorial responsibility remains with Wildlife Vagabond.How AI is used

Disease, parasites & microbiology

SPECIES IN THIS STORY

Species in this story

Barn Swallow Hirundo rustica Explore species Great Tit Parus major Explore species House Sparrow Passer domesticus Explore species European Bee-eater Merops apiaster Explore species

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