Onchocerciasis (river blindness) is caused by the parasitic worm Onchocerca volvulus, which is transmitted by Simulium black flies that breed in fast-flowing rivers and streams. In the human host, adult male and female O. volvulus worms become encapsulated in subcutaneous fibrous "nodules", and fertilized females produce embryonic microfilariae that migrate to the skin, where they are ingested by the black fly vectors during a blood-meal. In the vector, the microfilariae develop into the infectious third larval stage, at which time they can be transmitted to the next human host via subsequent bites. The parasite has no environmental reservoir or natural nonhuman hosts. Microfilariae cause severe itching and disfiguring skin disease, and they may enter the eye, causing vision loss and blindness in some individuals. Ivermectin (Mectizan[R]) is a safe, effective oral microfilaricide donated by Merck Sharp and Dohme (1) since 1987 to control or eliminate onchocerciasis through repeated community-wide mass drug administration (MDA). The drug rapidly kills microfilariae, and repeated rounds of treatment with high coverage can stop transmission and increase the mortality of adult worms.
The Onchocerciasis Elimination Program for the Americas (OEPA) (2) was created at the end of 1993 by a resolution of the Directing Council of the Pan American Health Organization in 1991 to eliminate onchocerciasis from the Americas. Its objectives are to provide technical and supplemental financial assistance to the elimination programmes of the 6 endemic countries: Brazil, Colombia, Ecuador, Guatemala, Mexico and Venezuela (Bolivarian Republic of). OEPA was originally supported by the River Blindness Foundation and later (in 1996) by The Carter Center. The regional initiative has been successful, and WHO has since verified elimination of onchocerciasis transmission in 4 countries: Colombia (2013), Ecuador (2014), Mexico (2015) and Guatemala (2016). The Ministry of the Popular Power for Health of Venezuela announced elimination of transmission of the parasite in 2 of the 3 transmission zones (foci) in that country. The 538 517 individuals no longer at risk for onchocerciasis in these formerly endemic areas represent 94% of the original regional at-risk population. The remaining 6% (35 518 individuals) inhabit the last active area of transmission, known as the Yanomami focus area (YFA), named for the nomadic indigenous people who live in 654 communities scattered over approximately 230 000 [km.sup.2] of savannah and Amazon rainforest. The YFA is a cross-border transmission zone consisting of the contiguous Brazilian Amazonas focus (273 communities with a population at risk of 18 000 individuals) and the Venezuelan South focus (381 communities with a population at risk of 17 518 individuals). According to an analysis in 2020 of administrative units that had achieved at least 20 cumulative rounds of ivermectin treatment and MDA coverage of at least 85% of the eligible population, transmission is "suspected" of having been interrupted in 75% of the YFA. (3)
Ivermectin treatment for onchocerciasis in the Yanomami focus area in 2020
The strategy for the YFA is to provide ivermectin MDA in treatment rounds at least every 6 months. Children who are [greater than or equal...