Persistent focal onchocerciasis transmission near the Maridi Dam, South Sudan, despite intensified elimination interventions: Repeated Ov16 serosurveys in young children, 2019–2026

Background Maridi County in South Sudan is an onchocerciasis hotspot with a high burden of onchocerciasis-associated epilepsy. Elimination activities were strengthened through slash-and-clear vector control from 2019 and biannual community-directed treatment with ivermectin from 2021. We assessed Ov16 seropositivity in young untreated children and its temporal and geographical distribution following these interventions. Methodology/Principal findings Repeated cross-sectional Ov16 rapid diagnostic test surveys were conducted in 2019, 2023, and 2026 using the same field approach. For direct temporal comparison, analyses were restricted to children aged 3–4 years from the four sites surveyed in 2026. Overall Ov16 seroprevalence was 18.5% (36/195; 95% CI 13.4–24.8). Combined seroprevalence was 33.3% (35/105) in the two frontline villages near the Maridi Dam, Kazana 1 and Kazana 2, versus 1.1% (1/90) in the two more distant villages of Hai-Tarawa and Hai-Gabat (p < 0.001); seroprevalence reached 49.1% (26/53) in Kazana 2. Seroprevalence was 11.5% (10/87) among 3-year-olds and 24.1% (26/108) among 4-year-olds (p = 0.024). Across the same four villages and age range, overall prevalence was 14.3% (6/42) in 2019, 20.0% (14/70) in 2023, and 18.5% (36/195) in 2026 (p = 0.744). To account for unequal village representation across surveys, village-standardized prevalence estimates were also calculated by assigning equal weight to each village and closely paralleled the crude estimates. A generalized linear mixed model with village as a random intercept was fitted, adjusting for age and sex. Compared with 2019, there was no evidence of a temporal difference in seropositivity in 2023 (OR 1.32, 95% CI 0.40–4.40; p = 0.652) or 2026 (OR 1.24, 95% CI 0.43–3.60; p = 0.686). Conclusions/Significance Repeated Ov16 surveys identified continued substantial seropositivity among children aged 3–4 years, particularly in Kazana 2 near the Maridi Dam, consistent with recent O. volvulus exposure and ongoing local transmission. The findings support improving ivermectin coverage among eligible residents, sustained entomologically monitored vector control, and geographically targeted surveillance. Safe paediatric antifilarial options could help close the protection gap in children at risk of onchocerciasis-associated epilepsy.