Progress in eliminating onchocerciasis in the WHO Region of the Americas: Report from the Inter-American Conference on Onchocerciasis, 2024

Overview

Human onchocerciasis (river blindness) is caused by the parasitic worm Onchocerca volvulus, transmitted by Simulium black flies that breed in fast-flowing rivers and streams. In the human host, adult male and female O. volvulus worms become encapsulated in subcutaneous fibrous “nodules”, and fertilized females produce embryonic microfilariae that migrate to the skin, where the black fly vectors ingest them during a blood meal. In the vector, the microfilariae develop into the infectious third larval stage, at which time they can be transmitted to the next human host via subsequent bites. The parasite has no environmental reservoir or natural nonhuman host. Microfilariae cause severe itching and disfiguring skin disease, and they may enter the eye, causing vision loss and blindness in some individuals. Ivermectin (Mectizan^®) is a safe, effective oral microfilaricide donated by Merck Sharp and Dohme since 1987 to control or eliminate onchocerciasis through repeated community-wide mass drug administration (MDA). 

The Onchocerciasis Elimination Program for the Americas (OEPA) was created at the end of 1993 following Resolution CD35.R14, adopted by the 35th Directing Council of the Pan American Health Organization in 1991, which called for the elimination of onchocerciasis-related eye disease by 2007. Through Resolutions CD48.R12, 2008, CD55.R9, 2016 and CD57.R7, 2019, the Directing Council further advanced the goal of elimination of transmission.

As of 2025, the current population at risk for onchocerciasis (39701 individuals)8 is spread across 715 communities, scattered over approximately 230000 km² of Amazon highland savannah and rainforest. Known as the Yanomami focus area (YFA), the region is a cross-border transmission zone between Brazil and Venezuela. The focus area is named after the Yanomami, who comprise one of the largely untransformed indigenous groups in the world, with a high rate of population movement and relatively recent contact with outsiders. Therefore, respect for their culture is an essential precondition for effective work by welfare, scientific and medical programmes, thus driving a unique programme implementation approach by the OEPA.