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Ascaris lumbrioides: One Big Worm

Ascaris lumbricoides is the large round worm of folk lore, and it still lives in the intestinal tract of an estimated quarter of the world’s population.  Ascaris lumbricoides can reach huge size, up to 49 cm or about 19 inches long.  They generally live in the human small intestine.  Their life cycle is both amazing and interesting, and explains why they are so ubiquitous in areas of the world where human waste is not disposed of in sanitary facilities.


The adult female Ascaris worm can lay up to 200,000 eggs daily and these eggs are passed in the stool.  The eggs are incredibly resistant to decay, and can remain infectious in the soil for up to 10 years.  When the eggs are ingested they hatch in the human duodenum, the first part of the small bowel.  There as a small larval worm they penetrate the wall of the small intestine and enter the blood stream.  It passes through the blood vessels of the gut, to the liver, into the venous circulation and through the heart. As the blood circulates to the lungs the larvae become stuck in the pulmonary capillaries.  There it breaks into the alveoli, the tiny lung air sacs where it grows and molts until it is coughed up and swallowed.


Seen on endoscopy

It passes into the small intestine where it lives to maturity within about 60 days.  Then the male and female worms mate, and the female begins to produce eggs and the process repeats itself.  These worms in small numbers are relatively harmless in most cases.  In large quantities especially in children they can divert enough intestinal nutrients to be a factor in nutrition.  In very large numbers they can ball up and cause intestinal obstruction.  Occasionally the juvenile worms get lost in their migration to the lungs and can accumulate in other organs of the body leading to inflammatory reactions.


Treatment of Ascaris lumbricoides infections is relatively simple and effective.  Mebendazole, the same treatment that is used for pinworms, or Enterobius vermicularis, is the drug of choice in most cases.  Prevention lies in community hygiene, especially avoiding use of human feces as fertilizer.


Many of the stories of Ascaris lumbricoides come from their occasional tendency to wander.  They will sometimes exit the human through any orifice they can get to from the small intestine, so have been known to crawl out of people through the anus, the mouth or even the nose.  This must be pretty startling to a person who sees a foot long or larger worm crawling out of their orifice.


As the intestinal parasitic infestations of the world go although this is incredibly common, the human morbidity burden is relatively low, and so it has not made the list of critical neglected tropical diseases that are the focus of the WHO and other international organizations.


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