Diagnosis and Surveillance of West Nile Virus Infection in Horses: Current Methods, Challenges, and Future Directions

West Nile virus (WNV) is a mosquito-borne flavivirus of growing importance for both human and equine health in Europe. Horses are highly susceptible to neurological disease and, because they share ecological exposure with humans, they represent valuable sentinels for detecting local viral circulation within a One Health framework. However, diagnosis of WNV infection in equines is complicated by the short and low-level viraemia, which limits the sensitivity of molecular assays, and by serological cross-reactivity with related flaviviruses and the confounding effects of vaccination. In this narrative review, we summarise the current diagnostic tools for WNV in horses, including direct detection methods (RT-qPCR, virus isolation, antigen detection) and indirect serological approaches (IgM and IgG ELISA, virus neutralisation tests), and discuss their practical performance and constraints in clinical and surveillance settings. We further examine equine surveillance systems, passive clinical reporting, active serosurveys and sentinel cohorts, and their integration with vector, avian and environmental monitoring. Key challenges include methodological heterogeneity, limited access to confirmatory testing and variable cross-sector data sharing. Finally, we outline future directions, highlighting the need for harmonised laboratory protocols, innovative field-deployable diagnostics, genomic surveillance and integrated, multi-source monitoring systems to strengthen early warning capacity and improve preparedness for WNV outbreaks in equine populations.