Conditions · Infectious and tropical diseases
Onchocerciasis (river blindness) !
A worm spread by blackflies near fast rivers that causes severe itching, skin damage and, over years, blindness. Colombia was the first country to eliminate it.
Updated 2026-11-01
Structures involved
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Microorganisms that cause it
Why it happens
What it is
A filarial infection by Onchocerca volvulus transmitted by blackflies that breed in fast-flowing rivers. Adult worms live in subcutaneous nodules and release microfilariae that migrate through the skin and eyes; the inflammation around dying microfilariae causes chronic itchy dermatitis, skin depigmentation and atrophy, and eye disease that can end in blindness.
Who gets it
More than 99% of infected people live in sub-Saharan Africa. In the Americas, a regional elimination programme launched in 1992 covered six countries. Colombia, whose only focus was on the Pacific coast in Cauca, was in 2013 the first country in the world verified by WHO as free of onchocerciasis, followed by Ecuador, Mexico and Guatemala. Transmission in the Americas persists only in the Yanomami area on the border of Venezuela and Brazil.
Causes
- Onchocerca volvulus, a filarial nematode that carries Wolbachia endosymbiotic bacteria
- Transmitted by repeated bites of Simulium blackflies, which breed in fast-flowing rivers and streams
- Adult worms live up to about 15 years; microfilariae live in the skin, not mainly in the blood
Risk factors
- Living or working near fast-flowing rivers in endemic areas (farming, fishing, mining)
- Years of repeated exposure; brief travel rarely causes disease
- Communities without regular ivermectin distribution
Pathophysiology
Larvae injected by the blackfly mature into adults that coil inside fibrous subcutaneous nodules (onchocercomas), often over bony prominences such as the hips, ribs, knees and, in the Americas, the head. Millions of microfilariae migrate through the dermis and into the eye. Living microfilariae cause little reaction, but dying ones and their released Wolbachia trigger inflammation. In the skin this leads to itching, papules, thickening, then atrophy and patchy depigmentation. In the eye, microfilariae in the cornea cause punctate keratitis that becomes sclerosing keratitis, and posterior-segment inflammation causes chorioretinitis and optic atrophy.
Classification
- Skin disease: acute and chronic papular onchodermatitis, lichenified onchodermatitis (sowda), atrophy ('lizard skin'), depigmentation ('leopard skin'), hanging groin
- Nodules (onchocercomas)
- Eye disease: punctate keratitis, sclerosing keratitis, iridocyclitis, chorioretinitis, optic atrophy
- Systemic: weight loss, and onchocerciasis-associated epilepsy in children in some African foci
Symptoms and signs
- Severe, persistent itching that disturbs sleep
- Papular rash, thickened skin, and later thin, wrinkled or patchily depigmented skin, often on the shins
- Firm, painless, mobile nodules under the skin over bones
- Eye irritation, photophobia and blurred vision progressing to visual field loss and blindness
- Enlarged groin nodes with loose hanging skin
Diagnosis
- Clinical
- Suspected in people with long residence near rivers in endemic areas who have chronic itching, typical skin changes, nodules or eye lesions.
- Tests
- Skin snips (small superficial skin biopsies incubated in saline) to see emerging microfilariae; skin-snip PCR is more sensitive. Excision of a nodule shows adult worms. Serology (antibodies against the Ov16 antigen) is used for surveillance and to confirm interruption of transmission. Eosinophilia is common. Test for Loa loa in patients from Central Africa before treatment.
- Imaging
- Slit-lamp examination of the eye shows moving microfilariae in the anterior chamber and cornea; fundoscopy shows chorioretinitis or optic atrophy. Ultrasound can locate deep nodules.
Differential diagnosis
- Scabies and other causes of chronic itching
- Atopic or contact dermatitis, insect bite reactions
- Leprosy and vitiligo (depigmented skin)
- Lymphatic filariasis and loiasis
- Other causes of keratitis, uveitis and optic atrophy
Goal of treatment
Kill microfilariae to relieve symptoms and stop eye damage, shorten the life of adult worms, and interrupt transmission through repeated community treatment.
Treatment
- Medical
- Ivermectin kills microfilariae and is repeated every 6–12 months for years because it does not kill adult worms; it is the drug of mass drug administration programmes. Moxidectin is a newer longer-acting option in some settings.
- Medical
- Doxycycline for several weeks targets Wolbachia and sterilises and slowly kills adult worms. Diethylcarbamazine is avoided because it can provoke severe reactions and worsen eye disease.
- Surgical
- Removal of nodules (nodulectomy), especially those on the head, which are close to the eyes.
- Follow-up
- Watching for inflammatory reactions in the days after treatment, regular eye examinations, and care for the visually impaired.
Complications
- Irreversible blindness and visual field loss
- Disfiguring skin atrophy and depigmentation
- Chronic itching with sleep loss and social stigma
- Hanging groin with predisposition to hernias
- Onchocerciasis-associated epilepsy (in some African foci)
Prognosis
With early and repeated treatment, itching and skin lesions improve and eye damage stops progressing, but established blindness does not recover. Community programmes have eliminated transmission in several countries of the Americas, including Colombia.
Prevention
- Community-directed ivermectin distribution at high coverage for 10–15 years
- Blackfly vector control in rivers where feasible
- Long clothing and repellents near rivers
- Post-elimination entomological and serological surveillance
Test yourself
Think of the answer, then tap the question to see it.
Which vector transmits onchocerciasis and where does it breed?
Simulium blackflies, which breed in fast-flowing rivers and streams.
What is the classic diagnostic test?
Skin snips showing microfilariae emerging from small skin biopsies.
Why is diethylcarbamazine avoided in onchocerciasis?
Rapid killing of microfilariae causes severe inflammatory (Mazzotti) reactions and can worsen eye disease.
Which country was the first verified by WHO as free of onchocerciasis, and when?
Colombia, in 2013.
What is the target of doxycycline in onchocerciasis?
The Wolbachia endosymbiotic bacteria, whose loss sterilises and kills adult worms.
Related conditions
- Lymphatic filariasis
- Uveitis
- Cataract
- Keratitis and corneal ulcer
- Scabies
- Atopic dermatitis
- Leprosy (Hansen disease)
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Onchocerciasis (World Health Organization, 2025).
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