Onchocerciasis ( /ˈɒŋkɵsɜrˈsaɪ.əsɨs/ or /ˈɒŋkɵsɜrˈkaɪ.əsɨs/), also known as river blindness and Robles' disease, is a parasitic disease caused by infection by Onchocerca volvulus, a nematode (roundworm). Onchocerciasis is the world's second-leading infectious cause of blindness. It is not the nematode, but its endosymbiont, Wolbachia pipientis, that causes the severe inflammatory response that leaves many blind. The parasite is transmitted to humans through the bite of a black fly of the genus Simulium. The larval nematodes spread throughout the body. When the worms die, their Wolbachia symbionts are released, triggering a host immune system response that can cause severe itching, and can destroy optical tissue in the eye.
The vast majority of infections occur in sub-Saharan Africa, although cases have also been reported in Yemen and isolated areas of Central and South America. An estimated 18 million people suffer from onchocerciasis, with approximately 270,000 cases of blindness related to the infection.
In 1915, Dr. Rodolfo Robles Valverde's study on patients with river blindness in Guatemala led to the discovery that the disease is caused by filaria of O. volvulus, and sheds light on the life cycle and transmission of the parasite. Using case studies of coffee plantation workers in Guatemala, Robles hypothesized the vector of the disease is a day-biting insect, and more specifically, two anthropophilic species of Simulium flies found in the endemic areas. He published his findings on a “new disease” from Guatemala associated with subcutaneous nodules, anterior ocular (eye) lesions, dermatitis, and microfilariae in 1917.
Treatment may involve the use of the drug ivermectin. For best effect, entire communities are treated at the same time. A single dose may kill first-stage larvae (microfilariae) in infected people, and it prevents transmission for many months in the remaining population. Other drugs are also available, including the tetracycline-class antibiotic doxycycline, which kills the Wolbachia and renders the female nematodes sterile. The removal of the palpable nodules is common in Guatemala, Ecuador, and Mexico.