It is one of the 20 neglected tropical diseases listed by the World Health Organization, with elimination from certain countries expected by 2025.

[4] Onchocerca volvulus is primarily found in sub-Saharan Africa, and there is also disease transmission in some South American nations, as well as Yemen (see global map bottom right).

[5] Onchocerca volvulus parasites obtain nutrients from the human host by ingesting blood or by diffusion through their cuticle.

The normal microfilarial lifespan is 1.0 to 1.5 years; however, their presence in the bloodstream causes little to no immune response until death or degradation of the microfilariae or adult worms.

[13] Onchocerca volvulus has been proposed as one of the causative agents of nodding syndrome, a condition that affects children aged 5 to 15 and is currently only observed in South Sudan, Tanzania, and northern Uganda.

Artificial water systems, such as hydroelectric power plants, built in Africa, provide ideal conditions all year for blackfly development and make controlling its spread difficult.

[17] About 99% of cases of onchocerciasis are found in 31 countries in sub-Saharan Africa, although areas of limited transmission occur in Brazil, Venezuela, and Yemen.

[18] Onchocerciasis was eliminated from Colombia in 2013, Ecuador in 2014, Mexico in 2015, and Guatemala in 2016[16] due to control programs that used mass drug administration with ivermectin.

This pattern of low genetic variation and high haplotype diversity suggests fast population expansion after a bottleneck and has led to the theory that a host shift event from cattle allowed O. volvulus to infect humans.

[23] The immune response involves raising antibodies (IgG, IgM and IgE type) that can react with soluble antigens released by Onchocerca volvulus.

[24] Opsonising antibodies that tag cells for destruction are also found against the infective J3 stage and microfilariae, but there is not enough evidence at the moment to say whether this is protective.

However, after the age of 40, the number of parasites carried (the intensity of infection) decreases, suggesting that over time, some sort of protective immune response develops.