Epidemiologic, Entomologic, and Virologic Findings during Reemergence of Western Equine Encephalitis Virus, Argentina.

During November 2023-April 2024, a total of 1,543 horses with neurologic disease from rural areas in 17 provinces of Argentina were reported to national animal health authorities. Nested reverse transcription PCR, quantitative real-time reverse transcription PCR, and sequencing of equine brain necropsies confirmed western equine encephalitis virus (WEEV) infection in 23 horses and 1 sheep. Phylogenetic analyses identified a lineage previously detected in Argentina in 1957, overlapping areas affected during the 1980s epizootics. Overall, we aspirated 1,362 female mosquitoes (12 species) from 1 of the most affected areas; Aedes (Ochlerotatus) albifasciatus (70.1%) and Ae. (Ochlerotatus) scapularis (26.8%) mosquitoes were the most abundant species. Three of 27 mosquito pools (n = 1,089 mosquitoes) were identified as WEEV positive (2 Ae. albifasciatus, 1 Ae. scapularis). Increased rainfall, agricultural expansion, vector proliferation, and low vaccination coverage were likely key factors contributing to the reemergence of WEEV.