Onchocerciasis, also known as river blindness, is a disease caused by infection with the parasitic worm Onchocerca volvulus. Symptoms include severe itching, bumps under the skin, and blindness. It is the second-most common cause of blindness due to infection, after trachoma.
The parasitic worm is spread by the bites of a black fly of the Simulium genus. Usually, many bites are required before infection occurs. These flies live near rivers, hence the common name of the disease, river blindness. Once inside a person, the worms create larvae that make their way out to the skin, where they can infect the next black fly that bites the person. There are a number of ways to make the diagnosis, including placing a biopsy of the skin in normal saline and watching for the larva to come out, looking in the eye for larvae, and looking within the bumps under the skin for adult worms.
A vaccine against the disease does not exist. Prevention is by avoiding being bitten by flies. This may include the use of insect repellent and proper clothing. Other efforts include those to decrease the fly population by spraying insecticides. Efforts to eradicate the disease by treating entire groups of people twice a year are ongoing in a number of areas of the world. Treatment of those infected is with the medication ivermectin every six to twelve months. This treatment kills the larvae but not the adult worms. The antibiotic doxycycline weakens the worms by killing an associated bacterium, Wolbachia, and is recommended by some as well. The lumps under the skin may also be removed by surgery.
According to the Centers for Disease Control and Prevention, as of 2017, about 20.9 million people were infected with onchocerciasis, and an estimated 1.15 million have some vision loss from the infection. Most infections occur in sub-Saharan Africa, although cases have also been reported in Yemen and isolated areas of Central and South America. In 1915, the physician Rodolfo Robles first linked the worm to eye disease. It is listed by the World Health Organization (WHO) as a neglected tropical disease. In 2013 Colombia became the first country to eradicate the disease.
Cause Onchocerciasis is a parasitic infection caused by the roundworm species Onchocerca volvulus. The larvae of O. volvulus enter a human host when an infected female adult fly from the genus Simulium bites them. After that, it can take up to three months for the worms to mature under the skin of their host. The worms mainly get nutrients for growth in humans from blood, but they have also been seen to rely on other bodily fluids such as cerebrospinal fluid, and urine. It is common to see nodules formed in the skin where the adult worms reside and mate. However, these worms will often travel throughout the body using blood vessels in connective tissues and even settle behind the cornea.
Life cycle The life of the parasite can be traced through the black fly and the human hosts in the following steps:
A Simulium female black fly takes a blood meal on an infected human host and ingests microfilariae. The microfilariae enter the black fly's gut and thoracic flight muscles, progressing into the first larval stage (J1.). The larvae mature into the second larval stage (J2.) and move to the proboscis and into the saliva in its third larval stage (J3.). Maturation takes about seven days. The black fly takes another blood meal, passing the larvae into the next human host's blood. The larvae migrate to the subcutaneous tissue and undergo two more molts. They form nodules as they mature into adult worms over six to 12 months. After maturing, adult male worms mate with female worms in the subcutaneous tissue to produce between 700 and 1,500 microfilariae daily. The microfilariae migrate to the skin during the day, and the black flies only feed during the day, so the parasite is in a prime position for the female fly to ingest it. Black flies take blood meals, ingest these microfilariae and restart the cycle.
Signs and symptoms The larvae can move through the body without triggering a response from the host's immune system, so some people who are infected with the parasite experience no symptoms; the Global Burden of Disease Study estimated that in 2017 there were at least 20.9 million people infected worldwide, of which 14.6 million had skin disease symptoms and 1.15 million experienced symptoms that impacted vision. After a blackfly bite, it can take 12–18 months for the larvae to develop into mature adult worms that will produce their larvae, which is what leads to the development of symptoms. Almost all the clinical manifestations of onchocerciasis are due to localized host inflammatory responses to dead or dying microfilariae (larvae). The signs and symptoms of onchocerciasis are usually divided into two categories, skin and eye symptoms.
Skin symptoms will develop years before any vision problems. These symptoms include:
Intense itching Swelling Inflammation Depigmentation Hyperpigmentation Rash Nodules under the skin Skin atrophy Hanging groin (folds of inelastic atrophic skin in the groin associated with enlarged lymph nodes)
Eye symptoms include:
Vision impairment, low vision, or permanent blindness. Clouding of the cornea Light sensitivity Lesions on eyes Glaucoma Eye pain Eye redness
Eye symptoms provide the common name associated with onchocerciasis, river blindness, and may involve any part of the eye from conjunctiva and cornea to uvea and posterior segment, including the retina and optic nerve. The microfilariae migrate to the surface of the cornea. Punctate keratitis occurs in the infected area. This clears up as the inflammation subsides. However, if the infection is chronic, sclerosing keratitis can occur, making the affected area become opaque. Over time, the entire cornea may become opaque, thus leading to blindness. Some evidence suggests an immune response to bacteria present in the worms causes the effect on the cornea.
Mazzotti reaction
The Mazzotti reaction, first described in 1948, is a symptom complex seen in patients after undergoing treatment of onchocerciasis with the medication diethylcarbamazine (DEC). Mazzotti reactions can be life-threatening, and are characterized by fever, urticaria, swollen and tender lymph nodes, tachycardia, hypotension, arthralgias, oedema, and abdominal pain that occur within seven days of treatment of microfilariasis.
The phenomenon is so common when DEC is used that this drug is the basis of a skin patch test used to confirm that diagnosis. The drug patch is placed on the skin, and if the patient is infected with O. volvulus microfilaria, localized pruritus and urticaria are seen at the application site.
Nodding disease
This is an unusual form of epidemic epilepsy associated with onchocerciasis although a definitive link has not been established. This syndrome was first described in Tanzania by Louise Jilek-Aall, a Norwegian psychiatric doctor in Tanzanian practice, during the 1960s. It occurs most commonly in Uganda and South Sudan. It manifests itself in previously healthy 5–15-year-old children, is often triggered by eating or low temperatures, and is accompanied by cognitive impairment. Seizures occur frequently and may be difficult to control. The electroencephalogram is abnormal but cerebrospinal fluid (CSF) and magnetic resonance imaging (MRI) are normal or show non-specific changes. If there are abnormalities on the MRI, they are usually present in the hippocampus.
Diagnosis
When a clinical diagnosis of onchocerciasis is obtained, doctors take small snips of skin containing 3–5 mg of skin tissue. The skin samples taken are only from the upper dermis. These samples will then be soaked in saline and examined underneath a microscope to check for the presence of microfilaria. If microfilariae are not detected in the samples, the Mazzotti test is used. In this test, 6 mg of diethylcarbamazine is administered to the affected area. If the patient experiences intense inflammation or itching in the affected area Microfilaria is present. Slit-lamp eye exams are used to identify signs of the parasites in and around the eyes of patients whose eyes are affected. Antibody tests when available can aid in the diagnosis of Onchocerciasis.
Classification Onchocerciasis causes different kinds of skin changes, which vary in different geographic regions; it may be divided into the following phases or types:
Erisipela de la costa An acute phase, it is characterized by swelling of the face, with erythema and itching. This skin change, erisípela de la costa, of acute onchocerciasis is most commonly seen among victims in Central and South America. Mal morando This cutaneous condition is characterized by inflammation accompanied by hyperpigmentation. Sowda A cutaneous condition, it is a localized type of onchocerciasis.
Additionally, the various skin changes associated with onchocerciasis may be described as follows:
Leopard skin The spotted depigmentation of the skin that may occur with onchocerciasis Elephant skin The thickening of human skin that may be associated with onchocerciasis Lizard skin The thickened, wrinkled skin changes that may result with onchocerciasis
Prevention Various control programs aim to stop onchocerciasis from being a public health problem. The Onchocerciasis Control Programme (OCP) launched in 1974, and at its peak, covered 30 million people in the following countries: Benin, Burkina Faso, Côte d'Ivoire, Ghana, Togo, Mali, and Niger. The OCP utilized the following initiatives: the use of larvicide spraying into fast-flowing rivers to control black fly populations, and from 1988 onwards, the use of ivermectin to treat infected people as a core treatment therapy. Alongside the OCP, a joint effort of the World Health Organization, the World Bank, the United Nations Development Programme, and the UN Food and Agriculture Organization, was considered to be a success in controlling onchocerciasis, and in 2002 shifted from control of onchocerciasis to elimination. According to the World Health Organization, five countries have eradicated onchocerciasis: Colombia (2013), Ecuador (2014), Mexico (2015), Guatemala (2016), and Niger (2025). Continued monitoring ensures onchocerciasis cannot reinvade the area through the OCP. Other effective prevention efforts include personal protection from black fly bites. Recommended protection measures from the CDC include using insect repellents and wearing long sleeves and pants to eliminate exposed skin. Using insect repellent that contains N,N-Diethyl-meta-toluamide (DEET) as well as clothing treated with permethrin.
Elimination In 1995, the African Programme for Onchocerciasis Control (APOC) was initiated to eliminate onchocerciasis in African countries where the disease was endemic. The initiative relied primarily on the use of the antiparasitic drug ivermectin. The initiative was to set up community-directed treatment with ivermectin for those at risk of infection. Overall transmission has declined. The APOC ended in 2015 and aspects of its work has been taken over by the WHO Expanded Special Programme for the Elimination of Neglected Tropical Diseases (ESPEN). As in the Americas, the objective of ESPEN has been to work with Government Health Ministries and partner non-governmental organizations, to eliminate the transmission of onchocerciasis. This requires consistent annual treatment of 80% of the population in endemic areas for at least 10–12 years, the life span of the adult worm. In 2025, Niger was announced by the World Health Organization to be the first African country to eradicate onchocerciasis. Treatment has stopped in some areas (e.g. Nigeria), following epidemiological and entomological assessments that demonstrated no ongoing transmission could be detected.