Progress towards eliminating onchocerciasis in the WHO Region of the Americas in 2011: interruption of transmission in Guatemala and Mexico

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, can stop transmission and increase mortality among the adult worms. Ivermectin tablets are delivered through community-wide mass drug administration (MDA) programmes.

The infection was originally 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.The goal of the partnership is to eliminate onchocerciasis from the Region of the Americas by providing MDA with ivermectin ≥2 times each year to all communities in endemic areas. MDA aims at reaching ≥85% coverage of the population eligible for treatment (people aged ≥5 years of age living in affected communities, excluding those who are chronically ill and women who are pregnant or breastfeeding infants in the first week of life). The programme operates under PAHO Directing Council resolution CD48.R12, which calls for interruption of transmission of the parasite.

The total number of people eligible for treatment in the Region in 2011 was 322 980; this number is known as the ultimate treatment goal (UTG) and represents the total number of persons who should be reached in each treatment round. In those areas where ivermectin treatment is provided twice a year (every 6 months), the annual treatment goal is the UTG multiplied by 2; designed by the notation UTG(2). Treatment with ivermectin at 3-month intervals increases death of worms and decreases the proportion of inseminated females. In areas where ivermectin treatment is being distributed 4 times a year (quarterly), the eligible population is multiplied by 4, designated UTG(4). Annual treatment coverage is calculated by dividing the total number of treatments given in a year by either the UTG(2) or the UTG(4). In 2011, 490 442 treatments were given semiannually, which was 94% coverage of the regional UTG(2) of 521 120 and 231 746 quarterly treatments were given, 93.6% of the regional UTG(4) of 247 698.