MICROBIOLOGY

• 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.