Your Guide to
Neglected Tropical Diseases

Onchocerciasis, commonly known as river blindness, is a parasitic disease caused by worms and transmitted through blackfly bites. Adult worms live under the skin in nodules and can survive up to 15 years, producing millions of microscopic larvae. These larvae move through the skin and into the eyes. When they die, they cause severe itching, skin damage including loss of color and elasticity, and eye damage that can lead to blindness.

Onchocerciasis is the second-leading infectious cause of blindness worldwide and has also been linked to epilepsy in children in some areas. While rarely fatal, it causes significant disability and can reduce life expectancy by 4-10 years for those who become blind.

Onchocerciasis spreads through the bites of infected blackflies that breed in fast-flowing rivers and streams. When an infected blackfly bites a person, it deposits larvae under the skin. These larvae grow into adult worms over 12-18 months and form nodules under the skin. The adult worms mate and produce microfilariae, which live in the skin. When another blackfly bites that person, it picks up the microfilariae, which can be passed to the next person it bites.

As it requires multiple bites of infected flies to develop the disease, people living or working near rivers and streams in affected areas face the highest risk. Long-term travelers can also become infected, but casual travelers rarely do.

Geographic scope:

• More than 99% of cases occur in sub-Saharan Africa and Yemen, with the remaining 1% along the Brazil-Venezuela border.
• The disease has been eliminated from Colombia, Ecuador, Mexico, Guatemala and Niger.
• Currently endemic in around 30 countries.
• More than 25 countries report active treatment programs.

Population affected:

• More than 250 million people require treatment each year, but less than 70% are actually treated.
• More than 20 million people are infected, with more than 1 million suffering vision loss or blindness.
• In some West African villages, up to 10% of residents have been blinded by the disease.

Communities in affected areas typically receive annual or biannual treatment with ivermectin (Mectizan), donated by Merck & Co., Inc. since 1987. Treatment must continue for 10–15 years to match the lifespan of adult worms.

Ivermectin kills the microscopic larvae and temporarily stops adult worms from producing new ones, reducing symptoms and transmission. In 2018, moxidectin was approved as an alternative treatment for people aged 12 and older, with longer-lasting effects than ivermectin. Both medications are generally safe, with side effects mainly from the body’s reaction to dying larvae (itching, rash, muscle pain, and fever). Dr. Satoshi Omura, who won the 2015 Nobel Prize in Physiology or Medicine, is the one who discovered ivermectin.

Prevention focuses on avoiding blackfly bites by using insect repellent, wearing long sleeves and pants during the day, and keeping screens on windows and doors. Some areas spray safe insecticides in rivers to reduce blackfly populations. No vaccine is currently available.