Onchocerciasis-associated skin and ocular disease and the impact of ivermectin treatment on morbidity prevalence: a modelling study

Background Despite decades of control interventions in sub-Saharan Africa, morbidity associated with Onchocerca volvulus infection still exerts a substantial burden of disease, arising from cutaneous, ocular and neurological manifestations. Methods We developed and integrated a morbidity sub-model into our previously published individual-based, stochastic transmission model, EPIONCHO-IBM, including both reversible (severe itch, reactive skin disease (RSD)), and irreversible (atrophy, depigmentation, hanging groin) cutaneous sequelae, and eye disease (blindness, visual impairment). We modelled the relationship between onchocerciasis skin disease (OSD) and infection prevalence using pre-intervention data from northern Nigeria, and between onchocerciasis ocular disease (OOD) and infection intensity using data from the Onchocerciasis Control Programme in West Africa. We simulated the impact of ivermectin mass drug administration (MDA) upon OSD and OOD using data from Cameroon, Central African Republic, Nigeria, Sudan and Uganda. Results Modelled age-specific OSD and OOD prevalence at baseline aligned well with reported prevalence estimates across the simulated range of endemicity levels but underestimated irreversible OSD in older age groups. Under MDA, we captured trends in infection prevalence, severe itch and irreversible OSD but underestimated reductions in RSD and blindness prevalence. Conclusions Integrating morbidity outcomes into transmission dynamics modelling will help improve estimates of onchocerciasis disease burden and inform the effectiveness and cost-effectiveness of current and alternative interventions.