Ascaris lumbricoides – Life Cycle
Natural History of the Large Intestinal Roundworm of Humans
Ascaris lumbricoides is the large intestinal worm familiar to people all over the world. This roundworm infects people when they swallow soil containing infective eggs.
An adult Ascaris lumbricoides worm is larger than the average earthworm. Though it is not an earthworm, A. lumbricoides has a close relationship with the soil: that is where infective eggs of the roundworm parasite lie until they are swallowed by a human.
Ascaris Roundworm Facts and Basics
- Eggs are found in warm moist soil where other infected humans have defecated. They are swallowed on unwashed raw fruits and vegetables, transferred to the mouth on soiled hands, even swallowed in untreated, unfiltered water that has washed off contaminated soils. In some cases, they may be inhaled.
- Eggs pass through the stomach unharmed and arrive in the duodenum, the first section of the small intestine. Worm larvae, which have lain quiet inside the eggs, emerge.
- Larvae burrow through the intestinal lining into the tissue underneath and continue on until they push through blood vessel walls and are carried off in the bloodstream.
- Larvae are carried through the liver and heart, eventually arriving in the lungs, where they break out into the airspace there.
- In the lungs, the larvae grow, passing through several stages of development. By the time they are ready to leave the lung, about nine days have passed since the eggs were swallowed.
- Larvae travel up the airways or are coughed up. Those that are subsequently swallowed survive their second trip through the stomach only if they are sufficiently mature to withstand the acid conditions there.
- Larvae remain in the small intestine and continue to mature. Females will eventually grow to 45cm in length (males are smaller).
- Sexually mature females begin producing eggs that are mixed with intestinal contents and passed in the stool.
- When an infected human defecates outside, eggs are introduced into the soil. In warm moist conditions the eggs will be infective to another person in a couple of weeks. When conditions are cold, dry, or otherwise hostile, eggs can remain dormant for long periods of time, then mature when the environment is more favorable.
Ascaris lumbricoides is virtually exclusively a human parasite, meaning that it does not infect dogs, cats, or other domestic animals, with the possible exception of pigs. The large intestinal roundworm of pigs, Ascaris suum, is very similar to A. lumbricoides and there is considerable evidence that A. suum occasionally infects humans and vice versa.
The whole process from swallowing the egg to having mature worms in the intestine takes between two and three months. The worms live, on average, about a year and a female can produce up to six million eggs in her lifetime. No wonder this is one of the most common intestinal worms of humans.
Read more about Ascaris lumbricoides:
Other intestinal parasites of humans:
Giardia lamblia and Giardiasis
Cryptosporidium parvum - Parasite
Other interesting topics in microbiology:
Sources:
Clinical Parasitology 9th ed. Beaver, Paul Chester, Rodney Clifton Jung, and Eddie Wayne Cupp. Philadelphia: Lea and Febiger, 1984.
Diagnostic Medical Parasitology 3rd ed. Garcia, Lynn S. and David A. Bruckner. Washington: ASM Press, 1997.
Foundations of Parasitology 6th Ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
