Intestinal Parasites

Worms (Helminths)

Intestinal worms range in size from only a few millimeters to several meters in length. The most important clinical and epidemiological differences between the species are related to the nature of their reproductive lifecycle and the way they interact or affect their human hosts. There are three types of helminthes: roundworms (nematodes), tapeworms (cestodes), and flukes (trematodes). Adult roundworms and tapeworms are found in the intestines, whereas adult flukes often live in other tissues such as the liver, blood vessels, or lungs depending on the species. The life cycles, diseases, and treatments associated with trematodes are somewhat more complex than for other helminths and will not be discussed here.

Most intestinal worms (with a few important exceptions) cannot complete their life cycle inside a human host—that is, they cannot multiply and reproduce without exiting the human body and developing further in the soil or another host organism. For this reason, intestinal worms rarely rapidly overwhelm or kill their human hosts. Intestinal parasites are most often transmitted by the fecal-oral route; persons accidentally orally ingest worm eggs present in soil that has been contaminated by human or other feces.

The life cycle of many species involves ingestion as a cyst or egg that hatches in the host stomach or intestines. The larvae then migrate through the body, to the lungs, and back to the intestines. The diseases these parasites cause are usually related to their migration [Page 945]through the lungs, or more often via their effects on the intestines, including preventing absorption of nutrients and obstruction. Most helminths only cause serious disease when they are present in large numbers. These effects may include anemia, retardation of growth and development, malnutrition, decreased work capacity, or increased susceptibility to other diseases. Because they are transmitted by contaminated soil or foods, they are usually more common in children who may be less careful with hygienic practices, may eat or play in contaminated dirt, or be more susceptible to the effects of helminth infections.

Specific types of nematode intestinal worms include the following:

• Ascaris roundworm—the largest worm, which may obstruct the bowels
• Hookworm—a small worm that enters the body through the skin, attaches to the intestinal wall, and lives off of blood, causing anemia
• Pinworm—a small worm common in North America and the rest of the world. It causes an itchy anus and is primarily found in children
• Whipworm—has a whip-like tail and can rarely cause rectal prolapse (rectum falls out through the anus)
• Trichinella—a relatively common worm in North America acquired by eating undercooked pig or game meat. Worm larvae spread throughout the muscles of the body and cause inflammation and illness which is usually treated with

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