Onchocerciasis is caused by infection with the filarial parasite Onchocerca volvulus, which is transmitted by the blackfly species. It mainly affects communities living near rivers where the blackfly live and breed.
The filarial worm is transmitted through the bites of infected blackflies of the species Simulium. These flies carry the immature larval forms of the parasite and transmit it from human to human. In the human body, the larvae form nodules in the subcutaneous tissue, where they mature to adult worms. After mating, the female adult worm can release up to 1000 microfilariae a day. These move through the body, and when they die they cause a variety of conditions, including blindness, skin rashes, lesions, intense itching and skin depigmentation.
Onchocerciasis is endemic in 30 countries in Africa, Brazil, Venezuela and in Yemen; the vast majority of the infected people live in West, Central and East Africa.
Treatment and successes
Control measures entail larvicide spraying of blackfly breeding sites and treatment of endemic communities with the microfilaricide Mectizan® (ivermectin). Mectizan kills the larvae of the parasite which are found in the skin and has almost no side effects, relieving the agonizing itching that accompanies the disease and halts progression toward blindness. Administered once annually, Mectizan is well suited for mass distribution in remote areas by community health workers. It is the only well-tolerated drug known to halt the development of river blindness.
Trends and challenges
The African Programme for Onchocerciasis Control (APOC) was launched in 1995 and was so successful that in 2009 APOC shifted from control to elimination of onchocerciasis in most countries of Africa by 2025.
Recent successes have been the elimination of onchocerciasis in Guatemala in 2016 – which is now the fourth country to eliminate onchocerciasis, the others being, Mexico, Colombia, and Ecuador.