Biomedical knowledge gaps, supernatural causal beliefs and the traditional-first treatment pathway sustaining onchocerciasis transmission despite mass drug administration in Bo District, Sierra Leone: an explanatory sequential mixed-methods study

Onchocerciasis (river blindness) persists in Bo District, Sierra Leone, despite two decades of ivermectin-based mass drug administration (MDA). Whether community knowledge, causal beliefs and treatment-seeking continue to favour transmission has not been quantified at scale. Using an explanatory sequential mixed-methods design, we surveyed 1,500 adults across five endemic chiefdoms (Bagbwe, Baoma, Gbo, Selenga, Tikonko) using multistage cluster sampling, and conducted 11 key informant interviews and 8 focus group discussions analysed thematically. Biomedical perception was measured with closed items summed into a composite score (range 0-3); the principal behavioural outcome was traditional-first treatment (use of traditional remedies before modern care). Quantitative data were analysed with chi-square tests, logistic regression, and ROC analysis. Biomedical understanding was moderate and incomplete: only 46.5% correctly identified the blackfly vector and 38.1% judged community awareness sufficient, while supernatural or moral attributions accounted for 68.1% of social explanations and half the sample (50.3%) endorsed a traditional or spiritual origin (mean composite perception 1.77/3). Perception did not vary across chiefdoms (all Cramers V < .06); the only consistent correlate was prior experience of the disease, which raised the odds of adequate perception (OR = 0.77 for inadequate perception, p = .016). Traditional-first treatment was the modal pathway (55.3%), care-seeking was reactive (70.1% acted only on symptom onset) and socially governed (71.3% consulted family or peers), and traditional healers were present in most communities (56.9%). A flagship logistic model predicting traditional-first treatment had negligible explanatory power (McFadden R{superscript 2} = .005; likelihood-ratio p = .737; AUC = 0.544), with only distrust of modern medicine showing a directional effect (OR = 1.33, p = .065). Qualitative accounts explained these patterns as a coherent cultural logic rather than ignorance, rooted in naming the disease for water, spiritual aetiologies, the invisibility of the parasite, and the complementary role of healers. Aetiology-aligned health communication, engagement of traditional healers and gender-sensitive MDA delivery are needed to close the residual transmission gap.