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Life is dear to every living thing; the worm that crawls upon the ground will struggle for it.

— Solomon Northup


Nematodes are worms with bodies that are round in cross-section. They come in two broad categories: Intestinal nematodes (covered here) and tissue nematodes (covered in Chapter 55). The distinction between these groups may seem arbitrary because some intestinal nematodes migrate through tissue on their way to the gut, and some tissue nematodes spend part of their lives in the intestines! However, the difference between the groups will be clear if you focus on whether the adult form spends its time chiefly in the intestines or in other body tissues.


Six intestinal nematodes commonly infect humans: Enterobius vermicularis (pinworm), Trichuris trichiura (whipworm), Ascaris lumbricoides (large roundworm), Necator americanus and Ancylostoma duodenale (human hookworms), and Strongyloides stercoralis. Together, they infect more than 25% of all humans. Most people who carry a small number of any of these intestinal roundworms have no symptoms whatsoever. However, people with large numbers of adult worms may suffer from abdominal discomfort, malnutrition, anemia, and occasionally death. Other closely related nematodes of animals that occasionally infect humans are also listed in Table 54–1, but are not discussed here.

TABLE 54–1Intestinal Nematodes


All intestinal nematodes have cylindrical, tapered bodies covered with a tough, acellular cuticle. Sandwiched between this tegument and the body cavity are layers of muscle, longitudinal nerve trunks, and an excretory system. A tubular alimentary tract consisting of a mouth, esophagus, midgut, and anus runs from the anterior to the posterior extremity. Highly developed reproductive organs fill the remainder of the body cavity. The sexes are separate; the male worm is generally smaller than its mate, and may be distinguished by a more curled posterior end than the tapered end in females.

Life Cycles

Helminth life cycles have confused and frustrated generations of students. They may seem arcane, but they reveal how the pathogen will be transmitted to a new host. Therefore, physicians and public health experts who aim to develop strategies for prevention and control must understand life cycle fundamentals. The life cycles of the six main human intestinal nematodes are summarized in Table 54–2.

TABLE 54–2Life Cycles of Intestinal Nematodes

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