Distribution of metacercariae and cyprinoid fish abundance in Nong Han Wetland, Thailand.

Opisthorchiasis, caused by , remains a public health concern in Southeast Asia, particularly in Thailand. This study investigates the prevalence and intensity of metacercariae in cyprinoid fish from the Nong Han Wetland in Northeast Thailand, and assesses fish abundance and environmental factors influencing parasite distribution. Fish were sampled monthly at eight sites during the rainy (June - October 2024) and dry (November 2024 - March 2025) seasons. Fish species were identified, and their relative abundance and species diversity (Shannon-Wiener index (H')) were assessed. Cyprinoid fish were digested with 0.25 % pepsin-HCl to detect, identified and quantify metacercariae under a microscope. Concurrent water quality paramteters (temperature, pH, dissolved oxygen, and fecal coliform levels) were analyzed to assess their correlation with parasite distribution. A total of 1429 fish from 36 species were collected, with (Family Belontiidae) being the most abundant. Among 392 cyprinoid fish (Family Cyprinidae) from 11 species (H' = 1.8), 15.1 % were infected with metacercariae found in 6 species. showed the significant highest prevalence (20.3 %). Infection intensity was highest in and (6.0 metacercariae/infected fish). Higher fecal coliform levels and proximity to human settlements correlated with infection. Dry season prevalence (17.8 %) and intensity (5.1 ± 2.6) were significantly greater than the rainy season (11.0 % and 2.6 ± 1.1,  ≤ 0.001). A strong correlation existed between fecal coliforms and prevalence ( = 0.981,  ≤ 0.001), with hotspots near human settlements. The findings highlight the influence of fecal contamination, seasonality, and human factors on transmission, emphasizing the need for targeted interventions.