What is Onchocerciasis?

Onchocerciasis, also known as river blindness, is a parasitic infection caused by the filarial nematode Onchocerca volvulus. It is one of the most common and widespread of the neglected tropical diseases, and is transmitted by the blackfly Simulium spp. It causes intense itching, disfiguring skin disease, and irreversible blindness in millions of people in Africa, South America, and Yemen. The life cycle of this parasite depends on two hosts in different environments; the human host and the fly or other insect vector. In the host, adult parasites live in nodules formed in the dermis. This parasite is passed to a biting fly vector, where it develops in the gut, from which infective larvae are released. The larvae travel through the circulation into the skin and eyes of the host, precipitating pathology.

Signs & Symptoms

Symptoms vary according to the stage of the infection but include:

  • Skin disease, with intense itching
  • Skin discoloration, nodules, and thickening of the skin
  • Eyesight problems, blurred vision, or even blindness
  • Chronic fatigue

Transmission & Risk Factors

Onchocerciasis is transmitted by infected blackflies of the genus Simulium, which breed in fast-flowing rivers and streams. The flies carry larvae from infected humans to non-infected humans, potentially introducing infection. Various activities that increase the chances of being bitten by an infected fly, such as fishing, washing in infected rivers, and occupations such as gold-panning, increase exposure to Onchocerciasis. The female larvae migrate through the body, and eventually mature into adult worms in nodules in the subcutaneous layer of skin. The adult worms produce thousands of microfilariae, which migrate through the skin, and can be ingested by the fly vector, infecting it and completing the life cycle.

Diagnosis & Treatment

Diagnosis is made by visual inspection of the skin lesions, and confirmed by the presence of microfilariae in a skin snip or biopsy. In cases of suspected ocular infection, biopsy or slit-lamp examination for ocular onchocerciasis is performed.

Treatment of onchocerciasis is with the antiparasitic drug ivermectin, administered in high doses, and repeated every 6–12 months. Antibiotic treatment is also recommended.