The Human Threat From West Nile Virus and Usutu Virus in a Changing Climate.

In recent years, the closely related arboviruses West Nile virus (WNV) and Usutu virus (USUV) have spread farther north in Europe. The viruses often co-circulate and are maintained in a natural cycle between ornithophilic mosquitoes and birds. Humans and other mammals can become infected with these viruses, but are considered dead-end hosts. Both viruses can cause fever and encephalitis and may potentially be fatal to humans, birds, and horses. Usutu virus was discovered in blackbirds in Denmark for the first time in the fall of 2024 and again in 2025. Denmark is currently a European border country of WNV and the recent emergence of USUV may precede establishment of WNV. With ongoing climate change, increasing temperatures and precipitation levels are expected. Both factors affect virus transmission and vector season length, thereby increasing the risk of these viruses moving northwards. The risk calls for enhanced surveillance of both viruses to ensure timely interventions and optimal control from a One Health perspective. This review describes the similarities and differences between the viruses, the epidemiology and surveillance strategies implemented in Denmark and Europe and discusses the aspects in relation to ongoing climate changes.