Zoonotic Alphaviruses in Costa Rica: Long-Term IgM Serosurveillance in Horses from 2009–2019

Objective: This study aimed to determine the annually circulation of Venezuelan (VEEV), Eastern (EEEV), and Western (WEEV) equine encephalitis viruses, identify patterns in case occurrence, and assess the associations with environmental and geographic factors. Methods: Serum samples from 548 ill equines exhibiting fever, depression, and/or neurologic symptoms were analyzed over 11 years (2009–2019) using IgM MAC ELISA. Logistic models were used to determine the associations between VEEV seropositivity and environmental conditions. Results: VEEV was detected annually in 23.4% (128/548) of the cases, while EEEV appeared intermittently (1.5% [8/548]). No WEEV cases were detected, likely due to avian migratory flyway routes. VEEV exhibited a 6-year peak pattern linked to La Niña in Guanacaste province. Eleven human alphavirus cases were confirmed between 2013 and 2019, including 1 fatal VEEV case in a child. Conclusion: These findings underscore the need for continued equine alphavirus surveillance within the One Health framework.