Onchocerciasis

Onchocerciasis is a parasitic infection caused by the roundworm species Onchocerca volvulus. It is often called river blindness because the flies that spread it live near fast-flowing rivers. This disease is a major global health concern. It is the second most common cause of blindness due to infection, following only trachoma.

The World Health Organization classifies it as a neglected tropical disease. This means it affects many people but often receives less attention than other medical issues.

The life cycle of the parasite involves a complex relationship between a human host and a black fly of the genus Simulium. The cycle begins when an infected female black fly takes a blood meal from a human. During this meal, she passes larvae into the person's blood. These larvae migrate to the subcutaneous tissue, which is the layer of tissue just under the skin. Over 6 to 12 months, the larvae undergo molts to become adult worms. These adults often live in nodules, which are lumps formed under the skin.

Once mature, male and female worms mate to produce between 700 and 1,500 microfilariae, or tiny larvae, every single day.

These microfilariae move through the body in a specific way. They often travel through blood vessels in connective tissues. During the day, they migrate to the surface of the skin. This timing is important because black flies typically feed during daylight hours. When a fly bites an infected person, it ingests the microfilariae. Once inside the fly's gut and thoracic flight muscles, the larvae progress through three developmental stages. After about seven days of maturation, they move to the fly's proboscis and saliva. This prepares them to infect the next human host.

Symptoms of onchocerciasis are usually divided into skin and eye categories. Skin symptoms can appear years before vision problems. These include intense itching, swelling, and rashes. Some people develop skin changes like "leopard skin," which is spotted depigmentation. Others may experience "elephant skin," where the skin becomes thick, or "lizard skin," which is wrinkled and thick. In some regions, an acute phase called Erisipela de la costa causes facial swelling.

Severe skin issues can also include skin atrophy or a condition called hanging groin.

Eye symptoms are what lead to the name river blindness. The microfilariae can migrate to the surface of the cornea, the clear front part of the eye. This causes inflammation known as punctate keratitis. If the infection becomes chronic, it can lead to sclerosing keratitis, where the cornea becomes opaque or cloudy. This process can eventually cause permanent blindness. The infection can affect many parts of the eye, including the conjunctiva, the uvea, and the retina.

Some evidence suggests that the body's own immune response to bacteria inside the worms causes this eye damage.

Medical history shows how much we have learned about this parasite. In 1915, the physician Rodolfo Robles first linked the worm to eye disease. In 1948, doctors described the Mazzotti reaction. This is a complex of symptoms, such as fever and abdominal pain, that can occur after treatment with the drug diethylcarbamazine. In the 1960s, Dr. Louise Jilek-Aall described "nodding disease" in Tanzania. This is an unusual form of epilepsy linked to onchocerciasis that mostly affects children in Uganda and South Sudan.

The scale of the disease is significant. According to the Centers for Disease Control and Prevention, about 20.9 million people were infected as of 2017. Of those, an estimated 1.15 million people experienced vision loss. Most infections occur in sub-Saharan Africa, but cases also exist in Yemen and parts of Central and South America.

To fight this, doctors use the medication ivermectin. This drug kills the larvae but not the adult worms. Some also use doxycycline, an antibiotic that kills Wolbachia, a bacterium associated with the worms.

Global efforts have led to incredible successes in disease elimination. Colombia became the first country to eradicate the disease in 2013. Since then, Mexico, Ecuador, and Guatemala have also achieved eradication. In 2025, Niger was announced as the first African country to eradicate onchocerciasis.

Prevention focuses on avoiding fly bites through insect repellents like DEET or wearing protective clothing. Programs also use insecticides to decrease the black fly population in rivers to break the cycle of transmission.