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Onchocerciasis

River blindness, a disease caused by a parasitic worm (Onchocerca volvulus) which is transmitted to persons by biting blackflies (buffalo gnats) that breed in fast-flowing rivers. The adult worms can live for up to 15 years in nodules beneath the skin and in the muscles of infected persons, where they produce millions of worm embryos (microfilariae) that invade the skin and other tissues including the eyes. About 18 million persons are affected, mostly in Africa and also in Yemen and Latin America. Both living and dead microfilariae cause severe itching in the skin and sometimes blindness after many years. Until the 1980s, the main control measure was to use larvicides to kill immature blackflies in rivers. This method was used effectively to reduce the incidence of the disease in part of West Africa but is expensive. Since 1987, the drug ivermectin (brand name: Stromectol) has been provided by the manufacturer (Merck) free of charge to control programs for treating persons with onchocerciasis. This treatment, effective in a single oral dose administered once a year, prevents the accumulation of microfilariae in persons at risk. No drug suitable for mass treatment can kill the adult worms in the body and onchocerciasis cannot be eradicated without such means. The blindness, however, can be eliminated.

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Infection with Onchocerca (especially O. volvulus, a filarial nematode transmitted from person to person by black flies of the genus Simulium), marked by nodular swellings forming a fibrous cyst enveloping the coiled parasites (onchocercoma); microfilariae move freely out of the nodule and escape into the intercellular lymph in the dermis. Dermatologic changes often develop, especially in Africa, resulting in intense pruritus, scaly or lichenoid skin, depigmentation, and destruction of elastic fibers. Most important are the ocular complications that may develop after a long chronic course, with blindness frequently occurring in advanced cases, caused by the presence of living or dead microfilariae seen by slitlamp biomicroscopy. SYN: blinding disease, onchocercosis, volvulosis.

- ocular o. ocular complications, such as keratitis, iridocyclitis, or retrobulbar neuritis, caused by the microfilariae of Onchocerca volvulus. SYN: river blindness.

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on·cho·cer·ci·a·sis .äŋ-kō-.sər-'kī-ə-səs n, pl -a·ses -.sēz infestation with or disease caused by filarial worms of the genus Onchocerca esp a human disease that is marked by subcutaneous nodules, dermatitis, and visual impairment and is caused by a worm (O. volvulus) which is found in Africa and tropical America and transmitted by the bite of a female blackfly called also onchocercosis, river blindness see CRAW-CRAW

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a tropical disease of the skin and underlying connective tissue caused by the parasitic worm Onchocerca. Fibrous nodular tumours grow around the adult worms in the skin; these may take several months to appear, and if secondary bacterial infection occurs they may degenerate into abscesses. The skin also becomes inflamed and itches. The migration of the microfilaria into the eye can cause total or partial blindness - called river blindness in Africa. Onchocerciasis occurs in Africa and Central and South America. ivermectin is used in treatment; if possible, the nodules are removed as and when they appear.

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on·cho·cer·ci·a·sis (ong″ko-sər-kiґə-sis) [Onchocerca + -iasis] infection with nematodes of the genus Onchocerca. Human infection is caused by O. volvulus, with heavy infestations usually characterized by the firm subcutaneous nodules called onchocercomas; persistent dermatitis with a pruritic papular rash and sometimes edema, lichenification, thickening, wrinkling, skin atrophy, and areas of leukoderma; lymphadenitis; and ocular lesions. The latter are caused by invasion by microfilariae (eye worms), which die and cause irritation with punctate or sclerosing keratitis that may progress to opacification of the cornea and blindness. Called also onchocercosis, volvulosis, and numerous local names.

Medical dictionary. 2011.