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

Overview

Onchocerciasis (river blindness) is caused by the parasitic worm Onchocerca volvulus, which is transmitted by Simulium species (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 L3 stage, at which time they can be transmitted to the next human host via subsequent bites. The parasite has no environmental reservoir or nonhuman hosts. Microfilariae cause severe itching and disfiguring skin disease, and may enter the eye, causing visual loss and blindness in some individuals. Ivermectin (Mectizan^®) is a safe and effective oral microfilaricide which has been donated by Merck (through the Mectizan Donation Program) since 1987 to control or eliminate onchocerciasis through community-wide 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 initially prevalent in 13 endemic foci (transmission zones) in 6 countries: the Bolivarian Republic of Venezuela, Brazil, Colombia, Ecuador, Guatemala and Mexico.