Retrospective serological evidence of exposure to selected equine viruses in horses from Indigenous communities of Costa Rica, 2011.

Horses are essential for transportation and livelihoods in Indigenous communities of Costa Rica. However, information on their exposure to viral pathogens remains scarce. We conducted a retrospective cross-sectional serosurvey using samples collected from horses from four Indigenous communities (Alto Pacuare, Amubre, Paso Marcos, and Vereh) sampled between March and October 2011 (n = 278). Sera were tested in 2014 using commercial immunodiffusion or ELISA assays for antibodies against West Nile virus (WNV), equine herpesviruses 1 and 4 (EHV-1 and EHV-4), influenza A virus, equine arteritis virus (EAV), and equine infectious anemia virus (EIAV). Seroprevalence estimates and Wilson 95% confidence intervals were calculated, and differences among communities were assessed using Pearson's chi-square or Fisher's exact tests, as appropriate. The number of tested sera differed across assays due to sample availability. Serological evidence compatible with WNV exposure was detected in 58.9% (139/236) of horses, with the highest proportion in Amubre (83.3%). EHV-4 showed consistently high seropositivity (91.2%, 125/137), whereas EHV-1 seroprevalence was low (6.6%, 9/137). Antibodies to influenza A virus and EAV were detected at low frequencies (5.1%, 12/237, and 3.0%, 7/236, respectively). EIAV seroprevalence reached 16.6% (35/211), with the highest proportion in Paso Marcos (23.8%). Significant differences among communities were observed for WNV, influenza A virus, and EHV-4 seropositivity. These findings represent historical serological evidence of exposure in 2011 and should not be interpreted as reflecting current viral circulation. Nevertheless, they provide a retrospective baseline for comparison with future surveillance and highlight the value of including underserved Indigenous equine populations in disease-monitoring and prevention programs.