NTDs: Onchocerciasis – another parasite, another insect bite, this time causing blindness

This post is the 12th in a series highlighting the WHO’s list of 17 Neglected Tropical Diseases (now technically 18 as mycetoma was added to the list at the 69th World Health Assembly in May 2016). To read the previous posts in this series click here.

Emmanuel Kwame, 60, lost his sight to river blindness as a young man. He lives in Asubende, Ghana, earning a living as a farmer and fisherman. Jason Beaubien/NPR

Onchocerciasis, also known as river blindness, is a Neglected Tropical Disease caused by a tapeworm (Onchocerca volvulus) which is transmitted by the bite of an infected black fly. Onchocerciasis is called river blindness because the black flies that transmit it (genus Simulium) like to live in fast-moving rivers and streams, mostly in rural areas where people rely on agriculture so the land is fertile.

The life cycle of river blindness looks complex but is not as complex as some of the other NTDs, or even that of malaria. The life cycle only involves humans and black flies, who pass the parasite back and forth. The parasite invades the eyes and skin of infected humans. Invasion of the eyes damages the optic nerve and causes irreversible blindness. Invasion of the skin can cause intense itching, skin hardening and skin discoloration. The itching can be so severe that people are unable to sleep and people try to relieve the itching by such methods as using red-hot machetes, pouring boiling water over themselves, or using broken clay pots to itch themselves. The itching has even driven some people to suicide.

River blindness occurs mainly in tropical areas, as you can see from the map below. More than 99% of infected people live in 31 African countries. Onchocerciasis is also found in Yemen and the Americas. In total, 120 million people are at risk of river blindness. While there is not vaccine or medication to prevent river blindness, we can control the vector (the black fly) and treat people infected with the parasites before permanent damage occurs. The Onchocerciasis Control Programme (OCP) has managed to treat 40 million infections and prevent 600 000 cases of associated blindness through the use of large-scale insecticide spraying and distribution of medication to kill the parasites in infected people. Subsequent region-specific onchocerciasis control programs were launched to specifically target the various endemic regions and those programs have helped interrupt transmission, to the point where some control programs are not being turned into elimination programs. Some countries have managed to eliminate river blindness altogether, including Mexico (2015), Colombia (2013), Ecuador (2014) and Guatemala (2016).

Geographic distribution of river blindness, 2013. Courtesy WHO.

Geographic distribution of river blindness, 2013. Courtesy WHO.

Many endemic countries utilize community-based treatment programs to deliver medication to community members on a regular (annually, bi-annually, etc) schedule. Ghana has had success in pairing insecticide applications with regular drug treatment to kill the parasites in infected people and prevent the disease from progressing to blindness. The Carter Center is working with Brazil, Ethiopia, Nigeria, Sudan, Uganda, and Venezuela to eliminate river blindness in those countries as well. Eliminating river blindness is possible, but as a NTD that doesn’t cause death, onchocerciasis can fall low on the priority list as countries grapple with such killers as HIV/AIDS, tuberculosis, and malaria. But I guess if river blindness was getting the same level of attention and funding as the previously mentioned “Big 3”, it wouldn’t be a NTD at all.

Leave a Reply

Your email address will not be published. Required fields are marked *