Onchocerciasis in the Americas: from arrival to (near) elimination

Onchocerciasis is a tropical disease caused by the parasite Onchocerca volvulus, which is transmitted by the black fly. The Simulium flies that transmit onchocerciasis breed in fast-flowing rivers, giving rise to the common name "river blindness" for the disease. Once transmitted to a human host, L3 infective-stage larvae molt twice and mature into adult worms within nodules underneath the skin, commonly in the midsection and around the head. The adult worms inside reproduce, creating millions of larvae (microfilariae) that move under the skin and cause intense itching, skin lesions and loss of pigmentation, as well as penetrating the eye causing sight impairment and eventual blindness.Worldwide there are more than 120 million people at risk of contracting the disease, with some 18 million people infected. More than 99% of the disease burden is in Africa. In the Americas, about 500,000 people have historically been at risk of infection in 13 foci throughout Brazil, Colombia, Ecuador, Guatemala, Mexico and Venezuela.Onchocerciasis arrived in the Americas through the slave trade. Starting in the early 16th century, slaves from the heavily endemic areas of West Africa were brought to Central and South America, bringing with them O. volvulus parasites. Slaves then migrated within the colonies, including between coffee plantations, carrying with them the parasites with which they were infected. Not being confined to the original African hosts, and as suitable Simulium species were present, the parasites were also transmitted to the indigenous American population and then spread further through migration, including among certain contiguous border countries like Guatemala and Mexico [1]. The spread of the disease through labor and other migration, and the presence of different vectors in the environment, explains the presence of onchocerciasis in Ecuador, Colombia, Guatemala, Mexico and Northern Venezuela [2], and genetic testing of parasites confirms this linkage between