Tiny worms can make people sick.
Small flies carry them. These flies live near rivers. The worms can hurt your eyes. You can wear long clothes to stay safe.
Can you stay away from flies?
Small worms can make people sick.
Black flies carry these worms. These flies live near rivers. When a fly bites, the worms enter the body. They can grow under the skin. They can also move to the eyes. This can make people go blind.
People can stay safe by wearing long clothes. Using bug spray also helps. Some lands have worked hard to stop the sickness. One land even cleared it away for good.
Some people call onchocerciasis "river blindness." This is a sickness caused by tiny worms. These worms are called Onchocerca volvulus.
Black flies carry the worms. These flies live near fast rivers. When an infected fly bites a person, the worms enter the body. They grow under the skin. They can also move to the eyes. This can make a person go blind.
The worms can cause many skin problems. They may cause itching or bumps. Some people get spots on their skin. Doctors can find the worms in a tiny piece of skin. They can also look in a person's eye.
There is no vaccine to stop this sickness. People can stay safe by wearing long clothes. Bug spray also helps. Doctors use a medicine called ivermectin to treat people. It kills the tiny baby worms. Many countries work hard to stop the disease. Colombia was the first country to get rid of it in 2013.
In 2025, Niger became the first African country to clear the disease.
Onchocerciasis is a disease caused by a tiny worm called Onchocerca volvulus. Many people know it by its other name, river blindness. This sickness is a major cause of vision loss from infections. It is the second most common cause of blindness after a disease called trachoma.
The disease can cause many hard jobs for the body. People might feel intense itching or see bumps under their skin. In some cases, it can lead to permanent blindness. This happens when the tiny worms move to the eyes.
The way this disease works involves a black fly from the genus Simulium. These flies live near fast-moving rivers. When an infected female fly bites a person, tiny larvae enter the body. These larvae move into the skin to grow. It can take up to 18 months for them to become adult worms.
Once they are adults, they mate under the skin. They produce many tiny larvae called microfilariae every single day. These larvae move to the skin during the day. This is when the black flies usually feed. This allows the cycle to start all over again.
Doctors have worked to understand this worm for a long time. In 1915, a physician named Rodolfo Robles first linked the worm to eye disease. Later, in 1948, a symptom called the Mazzotti reaction was described. This reaction happens after a person takes a specific medicine. A doctor named Louise Jilek-Aall also studied a related condition in Tanzania during the 1960s. She described a syndrome called nodding disease. This is an unusual type of epilepsy linked to the infection. It mostly affects children in places like Uganda and South Sudan.
There are many important facts about how many people are affected. In 2017, about 20.9 million people were infected with this disease. Of those people, 1.15 million had vision loss. Most cases happen in sub-Saharan Africa. There are also cases in Yemen and parts of Central and South America.
Some countries have worked very hard to stop the disease. Colombia was the first country to eradicate it in 2013. In 2025, Niger became the first African country to clear the disease. This shows that big efforts can work.
We can protect ourselves by using things we already know about. You can use insect repellent to keep flies away. Wearing long sleeves and pants also helps cover your skin. Doctors use a medicine called ivermectin to treat people. This medicine kills the tiny larvae but not the adults. Some doctors also use an antibiotic called doxycycline. This medicine weakens the worms by killing a bacterium they live with. By treating groups of people, we can help stop the disease from spreading.
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.
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