An explanatory sequential mixed methods study of Opisthorchis viverrini infection incidence and behavioral determinants over twenty years in rural Thailand

Opisthorchis viverrini infection remains a significant public health issue in rural Thailand and is strongly linked to cholangiocarcinoma. For twenty years, the Na-yao community, a central Thai settlement founded by northeastern migrants, has been subject to traditional control interventions that rely primarily on periodic stool examinations, treatment, and health education. This study assesses long-term infection patterns and socio-behavioral determinants following these interventions. An explanatory sequential mixed-methods design was used. The quantitative strand included a prospective cohort study (n = 246) to estimate incidence density and identify risk factors, using stool examinations and Poisson regression models. The qualitative strand included focus group discussions and in-depth interviews, and framework analysis was used to explore sociocultural factors influencing infection control. The quantitative strand reported an incidence density of 6.38 per 100 person-years (95% CI 3.72–10.21), which decreased significantly from 2004 levels but has recently stabilized transmission. Analysis confirmed that traditional uncooked fish menus, including Koi pla and Larb pla dip, remain the main sources of infection. Koi pla (aIRR = 9.22, p < 0.001) has been recognized as a major risk factor since the 2004 study. In contrast, Larb pla dip (aIRR = 2.78, p = 0.04) was recently identified in this community as a distinct vector. Although long-term interventions have reduced infection rates, transmission persists because of deeply rooted culinary traditions.