Onchocerciasis: diagnostic target product profile to support preventive chemotherapy. WHO, 2021.
Overview
Onchocerciasis, also known as river blindness, affects an estimated 21 million people, with 99% of cases reported in 31 sub-Saharan countries (WHO, 2020a). The disease is caused by the filarial worm Onchocerca volvulus, which is transmitted by Simulium flies. Adult worms live in nodules, some of which are subcutaneous. Conversely, the embryos (microfilariae) can migrate through the skin, causing debilitating pruritus and skin disease, and to the eyes, leading to progressive and permanent blindness. Onchocerciasis is also hypothesized to lead to neurological disorders including epilepsy (Chesnais, 2020), nodding syndrome (Geelhand de Merxem, 2020) and stunted growth.
One of the challenges posed by the current diagnostics is the difficulty of comparing epidemiological data obtained with different techniques, i.e. skin biopsies, versus rapid evaluation and monitoring of onchocerciasis, versus serology. Rather than having different diagnostics for mapping, monitoring and stopping decisions, it would be more effective to have a single platform that could support all these functions, while providing longitudinal data. All activities described in the WHO 2016 guideline are to be applied to onchocerciasis “transmission foci” or “transmission zones”, but currently there are no WHO-recommended protocols to delineate such foci. Addressing onchocerciasis in areas endemic for L. loa, where ivermectin can lead to severe adverse events, remains a major problem (Vinkeles Melchers, 2020). Unless new treatments are identified that are suitable for use within MDA campaigns and not microfilaricidal, diagnostics for L. loa infection are needed to exclude those at risk of severe and serious adverse reactions to microfilaricides. This holds particular importance for areas in which onchocerciasis is hypo-endemic and L. loa is co-endemic, which will need to be included in interventions for elimination of onchocerciasis, where the risks associated with distribution of ivermectin may outweigh the benefits.