Progress towards eliminating onchocerciasis in the WHO Region of the Americas: verification by WHO of elimination of transmission in Colombia

Overview

 Onchocerciasis (river blindness) is caused by Onchocerca volvulus, a parasitic worm which is transmitted by certain Simulium species (black flies) that breed in fast flowing rivers and streams. Adult male and female O. volvulus worms become encapsulated in fibrous tissue (nodules) and fertilized females produce embryonic microfilariae which migrate to the skin where they are ingested by the black fly vectors during a blood-meal. Microfilariae then develop into the infectious L3 stage in the fly and are transmitted to the next human host via subsequent bites. There are no environmental reservoirs or significant nonhuman hosts. Microfilariae cause severe itching, disfiguring skin disease and may enter the eye, causing visual loss and blindness over time. Ivermectin (Mectizan®) is a safe and effective oral microfilaricide which has been donated by Merck & Co. Inc. (through the Mectizan Donation Programme) since 1987 to control onchocerciasis through communitywide mass drug administration (MDA) programmes. The drug rapidly kills the microfilariae and, through repeated rounds of treatment with good coverage, can stop transmission and increase mortality in adult worms. In the Americas, the infection was formerly prevalent in 13 foci in 6 countries: the Bolivarian Republic of Venezuela, Brazil, Colombia, Ecuador, Guatemala and Mexico.

The Onchocerciasis Elimination Programme for the Americas (OEPA) is a regional partnership whose goal (under the Pan American Health Organization [PAHO] Directing Council resolutions CD48.R12 and CD49.R19) is to interrupt onchocerciasis transmission in the Region of the Americas by 2015. Its strategy is the provision of MDA with ivermectin tablets ≥2 times each year to all communities in endemic areas, reaching ≥85% treatment coverage of eligible populations. The partnership includes the governments of countries where the disease is endemic, the Carter Center, PAHO, the United States Agency for International Development (USAID), Lions Clubs International and local Lions Clubs, the United States Centers for Disease Control and Prevention, the Bill & Melinda Gates Foundation, several universities, institutes, and the Mectizan Donation Programme.

WHO certification guidelines for onchocerciasis elimination were published in 2001. They stipulate that in areas where transmission has been interrupted and MDA has been stopped, a post-treatment surveillance (PTS) period of at least 3 years is needed. If no recrudescence of infection is detected, then O. volvulus can be declared to have been eliminated and the resident population no longer at risk. Of the 563 544 people originally at risk in 2000, in 2013, 184 310 are no longer at risk of infection because they reside in areas where PTS has been successfully completed. Of the 379 234 persons at risk, 354 207 are under PTS (not being offered MDA) and only 25 027 are included in MDA programmes. Those eligible for treatment in 2013 (20 495) are now only in the Bolivarian Republic of Venezuela and Brazil.