Ascariasis

Information on Ascaris Round Worm from the Parasitic Roundworm Diseases Factsheet.

The name Ascaris lumbricoides reflects the resemblance of this intestinal roundworm to the common earthworm known as Lumbricus. Ranging in length from six to 13 inches, the female worm may grow to be as thick as a pencil. Ascaris infections are common throughout the world in both temperate and tropical areas. In areas of poor sanitation, an entire population may be harboring the parasite. The worm burden can reach staggering levels with up to a hundred worms infecting a given individual.

Almost more than any other parasitic disease, ascaris infection is a result of human carelessness. Human feces in streets, fields, and yards provides a major source of infective eggs in heavily populated areas. The eggs of ascarids are not infective for humans when first excreted. They are very resistant to extremes of temperature and humidity. They usually are transmitted by hand to mouth, although the use of human feces as fertilizer may also permit transmission of infective eggs by food that is grown in the soil and eaten without being thoroughly washed. The eggs require several weeks to embryonate and become infective.

When eggs are swallowed and pass into the intestine, they hatch into larvae. The larvae then begin their journey through the body. Once through the intestinal wall, they reach the lungs by means of the blood or lymphatic system. In the lungs, they pass through the air sacs, are carried up the bronchial tree, and are reswallowed to be returned to the small intestine where they grow, mature, and mate. The worms reach maturity in about two months.

A large number of larvae invading the lungs at one time may cause pneumonia. This stage of the disease precedes the intestinal phase by weeks, and the symptoms are difficult to diagnose. Once mature female worms are present in the intestine, however, a doctor can diagnose the infection by finding characteristic eggs in the stool.

A few worms in the intestine may cause no symptoms or may give rise only to vague or intermittent abdominal pain. Heavy infection may cause partial or complete blockage of the intestine resulting in severe abdominal pain, vomiting, restlessness, and disturbed sleep. The heavier or greater the worm infection, the more severe the symptoms are likely to be. Occasionally, the first sign of infection may be the presence of a worm in the vomitus or in the stool.

Other species of ascarids such as Toxocara, which infect dogs and cats, can under certain circumstances be picked up by humans. In their natural hosts, these ascarids have a migratory cycle similar to A. lumbricoides; however, in humans they fail to reach the intestine. Instead they remain active in other body tissue for some time. This state of larval migration is known as visceral larva migrans. Young puppies and kittens contribute most to contamination of soil by eggs that must incubate for some time in the soil. Almost all dogs are infected at birth; however, older dogs have usually become immune.

Ascariasis can be successfully treated with mebendazole or pyrantel pamoate.

National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health (NIH) Bethesda, MD 20892, October 1998.