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

Demographics

• 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

Diagnosis

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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