Researchers who sampled 19,105 birds at 30 locations in Huelva, Sevilla, Cádiz and Málaga between 2003 and 2024 found that urban birds could provide an early warning of West Nile virus circulation in Spain. Antibodies were more common in city birds during years when people contracted the virus. That pattern points to a potential surveillance signal alongside mosquito sampling and reports of human cases.
Birds are not a diagnostic test for humans. In years with human West Nile cases in Andalusia, researchers found increased serological evidence among urban birds, but no comparable rise among birds from natural habitats.
The research teams at EBD-CSIC, CISA-INIA-CSIC and CIBERESP found evidence that West Nile virus has circulated in Andalusia since at least 2003. Its presence is not limited to isolated epidemic seasons.
Antibodies against the pathogen appeared in 2,182 birds from 41 species. The study, published in One Health, found that exposure varied with age, body size, migration patterns and habitat. The clearest link to human outbreak years appeared among birds living in urban areas.
That difference matters because virus activity does not rise evenly across cities, forests, scrubland and other habitats. The common blackbird (Turdus merula) and house sparrow (Passer domesticus) are useful for regular checks because they stay close to homes, gardens and public parks.
The study also links exposure to several biological traits. Adult birds from larger species had antibodies more often. The researchers suggest that their greater heat output, carbon dioxide emissions and odour signals may attract more mosquitoes. Beak length was also associated with exposure. Migratory species differed from sedentary ones, while first-year urban birds showed higher seroprevalence than some birds from forests and scrubland.
Human cases show why earlier environmental signals could matter. Spain recorded 77 confirmed cases and eight deaths in 2020. In 2024, the figures rose to 158 infections and 20 deaths.
Spain and Andalusia already combine veterinary, entomological and human surveillance. The system includes risk mapping and mosquito control. As earlier reporting on Spanish public-space disputes shows in a different context, location can shape the response. Here, the relevant divide is between urban and non-urban habitats.
Bird monitoring cannot replace mosquito sampling or medical reporting, but it can add another signal. The researchers recommend a One Health model that connects urban-bird surveillance with mosquito monitoring and human case reporting. In Andalusia, the PEVA strategic plan has also expanded this broader surveillance structure and has been in operation since 2024.
Blackbirds and sparrows could show where the virus is circulating before the full number of human cases becomes clear. The study covered 19,105 birds over two decades.
Antibodies show that birds had previous contact with the virus. They do not diagnose infection in people. Laboratory testing, mosquito sampling and official epidemiological reporting remain necessary.
