Life cycle and modes of transmission of infection

• route of threadworm infection is generally via human ingestion of an an ova
• the eggs contain a partially developed juvenile that can develop to infectivity within 6 hours at body temperature, are swallowed and hatch in the duodenum
• once ingested the immature worms move slowly down the small intestine and are adults by the time of their arrival at the ileocecal junction
  • total time from ingestion to sexual maturity is 15–43 days (1)
• the adult worms generally congregate in the ileocecal region; however may wander throughout the gastrointestinal tract
• adult worms attach to the mucosa and feed upon epithelial cells and bacteria
• gravid females begin migrating within the lumen and passing out of the anus onto the perianal skin
  • as the females crawl, they leave a trail of eggs
  • a single female worm may deposit 4,600–16,000 eggs
  • female worms may literally explode and liberate masses of eggs
  • female worms die after oviposition
  • male worms die after copulation
• eggs are infective within 6 hours. In ideal conditions the eggs may remain infective for up to 20 days
• human infection occurs via either:
  • faecal-oral transmission by contaminated fingers or fomites
  • airborne infection via through airborne eggs that are dislodged from bed linens and clothes
  • retroinfection - in this form of infection threadworm larvae that hatch on the anus then crawl back into the rectum and bowel to mature and mate
  • reinfection by any of these methods can result in an extremely heavy parasite load within the host

Notes:

• humans are the only host for threadworms - threadworm infection cannot be transmitted by dogs and cats because these animals are free of threadworm infection. However the fur of animals can pick up eggs from the environment and be a source of infection
• sources of reinfection include clothing, bedding, curtains, walls, and carpets become sources of reinfection. There have been reports of eggs having been found in the dust of schoolrooms and school cafeteria (1)

Reference:

1. L.S. Roberts , Nematodes: Oxyurida, the pinworms. In: L.S. Roberts and J. Janovy, Editors, Foundations of parasitology, McGraw Hill, Boston (2000), pages 433–437.