Elimination of human onchocerciasis: progress report, 2023–2024

Overview

Onchocerciasis, more commonly known as river blindness, is a neglected tropical disease caused by the parasitic worm Onchocerca volvulus. The disease is transmitted to humans through the bite of an infected blackfly (Simulium spp.), which breeds in fast-flowing rivers and streams. The blackfly vector ingests microfilariae (immature worms) when it bites an infected person. Inside the fly, the microfilariae develop into infective larvae that are then transmitted to another human during subsequent bites. Once inside the human host, the larvae mature into adult worms, forming nodules under the skin.

Onchocerciasis is most prevalent in sub-Saharan Africa, where >99% of the world’s cases are found, although it also occurs in certain regions of Latin America and Yemen. The disease is endemic in areas where blackflies thrive, particularly in communities located near rivers and streams where the flies breed. The geographical association with rivers gives the disease its common name, “river blindness”.

The lifecycle of O. volvulus within the human body is complex and prolonged. Adult worms live in their subcutaneous nodules for up to 15 years. During this time, the female worms produce millions of microfilariae that migrate through the skin, eyes and other tissues and are responsible for most of the symptoms associated with onchocerciasis. As they move through the skin, they cause intense itching, rashes and depigmentation, leading to a condition known as “leopard skin”. Chronic inflammation from the presence of microfilariae can also cause skin thickening and atrophy, resulting in lizard skin-like changes.

The most devastating consequence of onchocerciasis is, however, on the eyes. When microfilariae migrate to the eyes, they can trigger an immune response that leads to inflammation and damage to ocular tissues. Over time, this can result in vision impairment and eventually blindness. It is estimated that onchocerciasis is the second leading infectious cause of blindness worldwide, after trachoma.

The socioeconomic impact of onchocerciasis is profound. Infected individuals often suffer from severe itching, disfiguring skin conditions and vision loss, which can significantly impair their ability to work and perform daily activities. This, in turn, leads to loss of income and increased dependence on others, perpetuating cycles of poverty in affected communities. The stigma associated with the visible manifestations of the disease contributes to social exclusion. Early exposure to onchocerciasis in childhood has also been associated with epilepsy, known as onchocerciasis-associated epilepsy.

Efforts to eliminate onchocerciasis have been under way for several decades. The mainstay is mass drug administration (MDA) of ivermectin, an antiparasitic medication that effectively kills microfilariae and prevents progression of the disease. Ivermectin primarily targets the microfilariae (immature worms) in the skin, killing them and preventing the release of new microfilariae by adult female worms. This is known as the embryostatic effect. While ivermectin does not kill adult worms, it effectively reduces the microfilarial load, which helps to manage symptoms and reduce transmission until it is suppressed, preventing uptake of larvae by flies and subsequent incident cases. If ivermectin MDA is discontinued prematurely, transmission will resume, as adult worms may still be alive.

Ivermectin is also known to cause some damage to the adult worm, including damage to its reproductive capacity. Transmission is interrupted when all adult worms are dead or can no longer reproduce. Ivermectin has been distributed in affected communities through large-scale public health campaigns, often supported by international organizations and pharmaceutical donations. In addition to MDA, vector control strategies, such as spraying of insecticides and clearing of vegetation by community members (a strategy called “slash and clear”) to reduce blackfly populations and nuisance, have also been used in some areas. These interventions have significantly reduced the prevalence of onchocerciasis in many regions, resulting in elimination in some countries and a significant reduction in transmission in others. Challenges remain, however, particularly in remote and conflict affected areas where access to health services is limited and in Loa loa co-endemic areas where a different strategy is required.