Onchocerciasis (River Blindness)

Description

Human onchocerciasis is caused by the prelarval (microfilaria) and adult stages of the filarial nematode Onchocerca volvulus. The disease is transmitted by the bite of certain species of female Simulium flies (black flies) that bite by day and are found near rapidly flowing rivers and streams.

Occurrence

Onchocerciasis is endemic in more than 25 nations located in a broad band across the central part of Africa. Small endemic foci are also present in the Arabian Peninsula (Yemen) and in the Americas (Brazil, Colombia, Ecuador, Guatemala, southern Mexico, and Venezuela).

Risk for Travellers

Persons travelling for short periods in onchocerciasis-endemic regions appear to be at very low risk for acquiring this condition. However, travellers who visit or live in endemic regions for >3 months and live or work near black fly habitats are at greater risk for infection. Infections tend to occur in expatriate groups such as missionaries, field scientists, and Peace Corps volunteers.

Clinical Presentation

Infection with O. volvulus can result in a highly pruritic, papular dermatitis; subcutaneous nodules; lymphadenitis; and ocular lesions, which can progress to visual loss and blindness. Symptoms in travellers are almost always dermatologic and may occur months to years after departure from endemic areas. Immigrants from endemic areas may present with skin and/or ocular disease. Diagnosis is made by finding the microfilariae in superficial skin shavings, adult worms in histologic sections of excised nodules, or characteristic eye lesions. Serologic tests are nonspecific outside the research setting.

Prevention

No vaccine and no effective chemoprophylaxis are available. Protective measures include avoidance of black fly habitats and the use of personal protection measures against biting insects (the vectors of onchocerciasis bite by day) such as those outlined in the section on Protection against Mosquitoes and Other Arthropods).

Treatment

Ivermectin (150-200 µg/kg orally once or twice per year) is the drug of choice for onchocerciasis. Repeated annual or semiannual doses may be required because the drug kills the microfilariae but not the adult worms, which can live for many years. If subcutaneous nodules are present, they should be excised. Travellers who have a diagnosis of onchocerciasis should be advised to consult with a specialist in infectious diseases or tropical medicine.

Bibliography
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  • Albiez EJ, Buttner DW, Duke BO. Diagnosis and extirpation of nodules in human onchocerciasis. Trop Med Parasitol. 1988;39 Suppl 4:331-46.
  • Brieger WR, Awedoba AK, Eneanya CI, et al. The effects of ivermectin on onchocercal skin disease and severe itching: results of a multicentre trial. Tropical Med Int Health. 1998;3:951-61.
  • Burnham, G. Onchocerciasis. Lancet. 1998;351:1341-6.
  • Drugs for parasitic infections. The Medical Letter on Drugs and Therapeutics. 2004 Apr:1-12.
  • Murdoch ME, Asuzu MC, Hagan M, et al. Onchocerciasis: the clinical and epidemiological burden of skin disease in Africa. Ann Trop Med Parasitol. 2002;96:283-96.
  • World Health Organization. Onchocerciasis and its control: report of a WHO expert committee on onchocerciasis control. World Health Organ Tech Rep Ser. 1995;852:1-104.

- Frank O. Richards, Jr.

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