Usutu Virus: A Silent Spread That Has Experts Concerned

The Usutu virus, a flavivirus transmitted by mosquitoes of the genus Culex, is attracting increasing attention in Europe. Initially identified in Africa, it is now well established in several European countries. In birds, it causes massive mortality, particularly among blackbirds (Turdus merula), whereas in humans, the majority of infections go unnoticed. However, severe neurological cases have been reported, particularly among immunocompromised individuals. Climate change and shifting ecosystems are fueling the geographic spread of the Usutu virus, which is advancing toward northern Europe. The parallel circulation of the West Nile virus further complicates surveillance and public health challenges.  

A Major “One Health” Study in the Netherlands

A study published on August 23 in *Nature Communications* reveals active circulation of the Usutu and West Nile viruses in the Netherlands. The researchers adopted a “One Health” approach combining surveillance of wild birds and equines, monitoring of wetlands and migratory routes, and advanced genomics analyses to trace the evolution and spread of viral strains. This integrated approach made it possible to precisely map infection hotspots and demonstrate that these viruses circulate not only in isolated natural areas but also near populated areas. The findings point to an increased zoonotic risk that calls for an urgent strengthening of biosurveillance programs.  

A Confirmed Outbreak in Denmark

Meanwhile, in August 2025, the Statens Serum Institut confirmed the detection of the Usutu virus in two young blackbirds found dead in the Copenhagen area. This is the second consecutive documented outbreak in Denmark, following 59 confirmed cases in 2024. According to Outbreak News Today, these new cases were detected through a systematic biosurveillance program for dead birds conducted by the institute and the University of Copenhagen. The recurrence of these detections in the Nordic countries indicates that the virus has become firmly established in the region and is spreading geographically at an alarming rate.  

A Flavivirus to Monitor Within a “One Health” Framework

The Usutu virus belongs to the same complex as the West Nile virus and the Japanese encephalitis virus. In birds, it causes high mortality rates, particularly among blackbirds and certain migratory species. Culex pipiens mosquitoes, the primary vectors, thrive in wetlands and suburban areas and are taking advantage of the longer warm seasons associated with climate change to colonize new regions. In humans, the majority of infections are asymptomatic, but severe cases affecting the central nervous system remain a cause for concern. Co-circulation with the West Nile virus, transmitted by the same mosquitoes, complicates early detection and requires enhanced diagnostic and surveillance capabilities.  

Alarming Detections

The gradual establishment of the Usutu virus in Northern Europe is linked to a combination of factors. Climate change is contributing to longer and wetter summers, increasing mosquito density and the duration of their activity. Wetlands and migratory corridors concentrate reservoir birds and facilitate viral spread. The ecological adaptation of mosquitoes to urban areas increases the risk of human exposure. Finally, the co-circulation of other flaviviruses, such as West Nile virus, exacerbates epidemiological pressures and complicates response plans. These dynamics call for the intensification of integrated biosurveillance programs linking human health, animal health, and environmental health to prevent future outbreaks.  

A Surveillance Model for Northern Europe

The “One Health” approach implemented in the Netherlands illustrates the effectiveness of integrated systems that combine detailed genomics analyses, the integration of environmental data, and multisectoral cooperation among veterinary laboratories, public health agencies, and ecological observatories. Experts recommend extending this model to all of Northern Europe in order to anticipate the future spread of the Usutu virus, limit potential impacts on bird populations, and reduce the risk of transmission to humans.