Environmental contamination of eggs and its impact on transmission control in rural northeast Thailand.

Background: Environmental contamination with eggs significantly impacts on opisthorchiasis transmission and disease control. This study assessed egg contamination in underwater sediments from canals and aimed to reduce transmission in egg-positive and downstream communities by controlling human opisthorchiasis in endemic area of Northeast Thailand.

Methods: Sediment samples from 14 sites in UdonThani municipality were determined for eggs. Human fecal samples and fish were examined for opisthorchiasis and metacercaria infection, respectively. Water samples were analysed for faecal coliforms bacteria. From 2017 to 2023, transmission control efforts were evaluated by monitoring changes in human and fish infection rates.

Results: In 2017 and 2018, eggs were found in canal sediments at 153 and 86 eggs/liter, respectively. Human opisthorchiasis prevalence was 5.3%, with cyprinoid fish showing a 25.0% infection rate (11.6 cysts/infected fish) and significant fecal contamination (>5,500 CFU/100 ml). Conversely, egg-negative sites showed a 0.5% human infection rate and no metacercariae were found. After health interventions (2019 to 2023), eggs were not detected in sediments from canals, resulting in a significant reduction in human opisthorchiasis prevalence to 0.9% at the previously egg-positive site and from 12.2% to 1.7% in downstream communities ( < 0.001). Additionally, the prevalence of metacercariae in cyprinoid fish decreased from 11.4% (7.6 cysts/infected fish) to 4.5% (2.4 cysts/infected fish), indicating reduced transmission.

Conclusion: The presence of eggs in sediment was associated to human opisthorchiasis transmission and demonstrated the significant effectiveness of health interventions in controlling the disease.