Epidemiological, Clinical and Phylogenetic Characteristics of West Nile Virus in Bulgaria, 2024

West Nile Virus (WNV), a mosquito-borne pathogen, is a growing public health challenge across Europe. Environmental and anthropogenic factors have led to the spread of the virus to higher geographic latitudes, as well as to increased viral circulation and genetic diversity. Aims: This study aimed to describe the epidemiological, clinical, and laboratory characteristics of WNV cases in Bulgaria during 2024 and to investigate WNV phylogenetics. Epidemiological, clinical and laboratory data from 32 patients with confirmed or probable WNV infections were collected and analysed. Complete viral genomes were obtained from two samples using whole genome sequencing (WGS). Phylogenetic analysis was performed using the Nextstrain WNV analysis pipeline. Severe disease was observed in 21 patients, with three fatalities reported in older males with comorbidities. Phylogenetic analysis revealed that Bulgarian strains clustered within the Central/Southern European clade of lineage 2, closely related to Greek strains. Evidence suggested localised viral evolution following cross-border introduction from Greece. Our study provides a detailed clinical and laboratory characterization of the human WNV cases detected in Bulgaria in 2024. Improved diagnostic workflows, expanded laboratory resources and increased molecular surveillance are essential to better understand the burden of WNV infections in Bulgaria, as well as to follow the evolution and spread of the virus.