Life Cycle
 
(6)
 
Animated Life Cycle
HUMAN PHASE (6, 7)
1. A female Simulium blackfly carrying the parasites takes a blood meal from the human host.  The host’s skin is punctured by the bite.  As the blood is pumped upward, the saliva of the fly carrying the parasite larvae passes into the wound.
  2.The larvae enter the host’s subcutaneous tissue, where they can migrate freely.
  3.The larvae take up residence in subcutaneous nodules.  Here they mature into adults of both sexes.  The females dwell in the nodules permanently while the smaller male worms migrate between nodules to mate.  The reproductive life of the adult is about 9-11 years.
  4.Within 10-12 months of the initial infection, eggs form inside the female worm and become unsheathed microfilariae (MF).  The female worm can produce up to 1000 microfilariae per day.  These thousands of MF migrate in the subcutaneous tissue and are often found in skin and in lymphatics of connective tissue.  They can sometimes be observed in peripheral blood, urine, and sputum.
 
BLACK FLY PHASE (6, 7)
  5.Another female blackfly takes a blood meal from an infected host, ingesting MF in the process.
6.Microfilariae penetrate the midgut and migrate to the thoracic muscles.
7.Here they develop into first-stage L1 larvae.
8.The L1 larvae molt and become L2 larvae.  The L2 larvae then molt and become infective L3 larvae.
9.These infective larvae migrate to the fly’s head and subsequently to the blackfly’s proboscis.  Here, they wait until they can be transferred to the human host when the fly takes a blood meal (step 1).