Hektoen International

A Journal of Medical Humanities

Onchocerciasis, a continuing cause of blindness

Simulium black fly. Photo by Robert Webster / xpda.com on Wikimedia. CC BY-SA 4.0.

Among the various illnesses that deprive humans of their sight, onchocerciasis ranks highest. It affects millions of people in sub-Saharan Africa, as well as substantial numbers in Yemen and isolated areas of Brazil and Venezuela. Transmitted by infected blackflies that breed near fast-flowing streams and rivers, it is commonly known as “river blindness.”

The disease begins when an infected Simulium blackfly bites a human and deposits microlarvae under the skin. The microlarvae mature into adult parasites over the next six to twelve months and reside in subcutaneous nodules where they can survive for as long as fifteen years. Their females produce millions of microfilariae, which migrate through the skin, eyes, and other tissues, provoking an inflammatory response.

Involvement of the skin by the parasite causes intense itching, rashes, depigmentation, thickening, and subcutaneous nodule formation. Onchocerciasis-associated epilepsy has been described as presenting with seizures between the ages of three and eighteen. Debilitating visual impairment and blindness occur when microfilariae enter the eye, causing conjunctivitis, keratitis, and iritis. Over time, sclerosing keratitis and optic neuritis can develop, leading to irreversible loss of vision.

The complications of onchocerciasis have been known for centuries. In 1875, the Italian anatomist Giovanni Battista Grassi first described the microfilariae of Onchocerca volvulus in skin samples from a patient. A few years later, in 1893, the German parasitologist Rudolf Leuckart identified the adult worms in subcutaneous nodules. In the early twentieth century, Sir Patrick Manson identified the blackfly as the vector of transmission.

Early efforts to control onchocerciasis focused on insecticide spraying but had little impact on eliminating the disease. More successful was the discovery of ivermectin, a broad-spectrum antiparasitic agent that is highly effective in killing microfilariae and preventing their transmission. In 1987, Merck & Co. began to donate ivermectin (branded as Mectizan) to affected countries, marking the beginning of large-scale mass drug administration (MDA) programs. Doxycycline has now also been shown to be useful in killing a high percentage of female worms but not the microfilariae. A newly developed drug, moxidectin, is even more effective and safer than ivermectin in reducing skin microfilariae loads for longer periods.

Onchocerciasis has already been declared eliminated in Colombia, Ecuador, Guatemala, and Mexico. There remains, however, a need for even more effective and shorter treatment regimens and for improved access to the drug in remote and conflict-affected areas. The development of drug resistance also remains a potential danger. Thus, continued political commitment, community engagement, and international support remain essential for reaching the goal of totally eliminating the disease.

GEORGE DUNEA, MD, Editor-in-Chief

Summer 2024



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