Trevor A. Crowell, M.D., Paul G. Auwaerter, M.D.


  • Onchocerciasis is caused by filarial worm Onchocerca volvulus.
  • Epidemiology: the world’s second-leading cause of blindness.
    • Approximately 18 million people are infected, and nearly one million have been permanently blinded or suffer severe visual impairment.
    • 99% of onchocerciasis cases occur in Africa.
    • Disease is endemic in 31 countries in sub-Saharan Africa, plus Venezuela, Brazil, and Yemen; WHO certified Columbia (2013), Ecuador (2014), Mexico (2015), and Guatemala (2016) to be free of human onchocerciasis.
  • O. volvulus transmitted to humans by blackflies (Simulium damnosum), which breed in fast-flowing rivers and streams.
  • Life cycle:
    • Infected blackfly transmits third-stage filarial larvae onto the skin of the human host during a blood meal.
    • Larvae penetrate the subcutaneous tissues and develop into adult microfilariae over 6-12 months.
    • Adults persist in subcutaneous nodules for about 15 years, with females producing 1,000-3,000 microfilariae per day for up to 9 years.
    • Blackfly ingests microfilariae during a blood meal.
    • Microfilariae penetrate the blackfly midgut and migrate to thoracic muscles, where they develop into first-stage larvae and then third-stage larvae.
    • Third-stage larvae migrate to the blackfly proboscis and can infect another human host during a blood meal.

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Last updated: January 12, 2020