Outbreak of Western Equine Encephalitis Virus Infection Associated with Neurological Disease in Horses Following a Nearly 40-Year Intermission Period in Argentina

Abstract

Western equine encephalitis virus (WEEV) is a mosquito-borne arbovirus (genus Alphavirus,family Togaviridae) that has re-emerged in South America in late 2023, causing severe disease in bothhorses and humans after a nearly 40-year intermission period. We here describe the virological,serological, pathological, and molecular features of WEEV infection in horses during the 2023–2024outbreak in Argentina. WEEV-infected horses developed neurological signs with mild to severeencephalitis associated with minimal to abundant WEEV-infected cells, as demonstrated by WEEVspecific in situ hybridization. The distribution of viral RNA was multifocal, with predominancewithin neuronal bodies, neuronal processes, and glial cells in the medulla oblongata and thalamicregions. Phylogenetic analysis of partial nsP4 sequences from three viral isolates obtained from threedifferent provinces of Argentina support grouping with other temporally current WEEV strains fromUruguay and Brazil under a recently proposed novel lineage.