Persistent onchocerciasis after 24 years of ivermectin mass treatment in the Democratic Republic of the Congo: a cross-sectional analytical study.

Abstract

Objectives: Despite more than two decades of community-directed treatment with ivermectin (CDTI), onchocerciasis remains endemic in several parts of sub-Saharan Africa, including the Democratic Republic of the Congo (DRC). Suboptimal therapeutic coverage and environmental factors may hinder progress toward elimination.

Methods: We conducted a cross-sectional analytical study between June and December 2025 in five health zones of Kasai Central, DRC. Individuals aged ≥5 years and residing in the area for at least five years were selected using multistage stratified sampling. Onchocerciasis infection was assessed using OV16 serology, clinical examination, and targeted skin snip microscopy. Associations between cumulative ivermectin exposure, environmental factors, and prevalence were assessed using regression analyses.

Results: A total of 1087 participants were included (median age 29 years; 51.2% female). Overall onchocerciasis prevalence was 19.1% (208/1087; 95% CI: 16.8-21.5), with significant heterogeneity across health zones (14.2-24.9%, P < 0.001). Individuals with fewer than ten ivermectin treatments had higher prevalence than those with ≥10 treatments (25.5% vs 11.3%, P < 0.001). In multivariable analysis, fewer than ten ivermectin treatments (aOR = 2.45; 95% CI: 1.78-3.36; P < 0.001) and residence within 5 km of a river (aOR = 1.96; 95% CI: 1.42-2.70; P < 0.001) were independently associated with infection. At the area level, higher cumulative therapeutic coverage was inversely associated with prevalence (β = -0.39 per 10% increase; 95% CI: -0.52 to -0.26; P < 0.001), while proximity to rivers was positively associated with infection. However, these area-level findings should be interpreted with caution due to the limited number of health zones included in the analysis.

Conclusion: After more than 20 years of CDTI, onchocerciasis transmission persists in Kasai Central, with marked spatial heterogeneity. Strengthening treatment coverage and targeting high-risk riverine areas are critical to accelerate elimination efforts in the DRC.