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For further information, see CMDT Part 35-33: Onchocerciasis

Key Features

Essentials of Diagnosis

  • Conjunctivitis progressing to blindness

  • Severe pruritus; skin excoriations, thickening, and depigmentation; and subcutaneous nodules

  • Microfilariae in skin snips and on slit-lamp examination; adult worms in subcutaneous nodules

General Considerations

  • Caused by Onchocerca volvulus

  • Infection transmitted by simulium flies (blackflies), which breed in fast-flowing streams and bite during the day

  • After the bite of an infected blackfly, larvae are deposited in skin, where adult worms develop over 6–12 months

  • Adult worms live in subcutaneous connective tissue or muscle nodules for a decade or more

  • Microfilariae are released from nodules and migrate through subcutaneous and ocular tissues

  • Disease is due to responses to worms and to intracellular Wolbachia bacteria


  • An estimated 37 million persons are infected, of whom

    • 3–4 million have skin disease

    • 300,000 are blinded

    • 500,000 are severely visually impaired

  • Over 99% of infections are in sub-Saharan Africa, especially the West African savanna

  • Half of the cases are in Nigeria and Congo

  • In hyperendemic African villages,

    • Close to 100% of persons are infected

    • 10% or more of the population is blind

  • Disease also prevalent in

    • Southwestern Arabian peninsula

    • Latin America (eg, Mexico, Guatemala, Venezuela, Colombia, Ecuador, Northwestern Brazil)

Clinical Findings

Symptoms and Signs

  • Incubation period of up to 1–3 years

  • Inguinal and femoral lymphadenopathy

  • Systemic symptoms include weight loss and musculoskeletal pain

  • Skin

    • Erythematous, papular, pruritic rash, which may progress to chronic skin thickening and depigmentation

    • Itching may be severe and unresponsive to medications

    • Numerous firm, nontender, movable subcutaneous nodules of about 0.5–3 cm contain adult worms

  • Eye

    • Microfilariae migrating through the eyes elicit host responses that lead to pathology

    • Findings include punctate keratitis and corneal opacities

Differential Diagnosis

  • Glaucoma

  • Loiasis (Loa loa infection)

  • Gnathostomiasis

  • Cysticercosis (with ophthalmic involvement)

  • Sporotrichosis

  • Coccidioidomycosis


Laboratory Tests

  • Complete blood count; eosinophilia is a common but inconsistent finding

  • Serologic tests

    • Poorly standardized

    • Nonspecific

    • Do not distinguish current from past infection

Imaging Studies

  • Ultrasound may identify characteristic findings suggestive of adult worms in skin nodules

Diagnostic Procedures

  • Diagnosis is made by identifying microfilariae in

    • Skin snips from the iliac crest in persons from Africa or from the scapula in persons from the Americas

    • Cornea or anterior chamber by slit-lamp examination

    • Urine (rare)

  • Skin snips stand in saline for 2–4 h or longer before examination

  • Identifying adult worms in biopsy or aspirate of a nodule also confirms diagnosis

  • Deep punch biopsies not needed

  • The Mazzotti test uses 50 mg of diethylcarbamazine

    • Should be used only after other tests are negative


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