Enteric nematodes are among the most common and widely distributed animal parasites of humans. In his classic address to the American Society of Parasitologists in 1946, entitled “This Wormy World,” Stoll estimated 2.3 billion helminthic infections in a human population of 2.2 billion. Since 1946, the world population has doubled and, by all indications, enteric nematode infections of humans have kept pace. The most common intestinal roundworms are those transmitted through contact with the soil (for example Ascaris lumbricoides, Trichuris trichiura, the hookworms, and Strongyloides stercoralis). In Stoll's estimate, these worms, with Enterobius vermicularis, accounted for three-quarters of all helminthic infections.

Most enteric nematodes have established a well-balanced host-parasite relationship with the human host; humans tolerate these parasites well. Little disease is associated with light infection, but when the worm load increases, a corresponding increase in disease usually occurs. The worms may irritate the intestinal mucosa, causing inflammation and ulceration. Some produce “toxic” substances. The larger worms may become entangled and block the intestinal tract. Larval worms that migrate through the tissue to complete their life cycle may lose their way, end up in the wrong organ, and cause severe disease. Nutritional problems occasionally are associated with the intestinal parasitosis, and persons with deficient diets often suffer from polyparasitism.

Diagnosis usually is based on microscopic examination of feces for eggs and larvae, except in the case of pinworm infections, which are diagnosed by examining samples taken with perianal swab. Many antihelmintics are available to treat patients with these infections. Control depends largely on proper disposal of human feces and on personal hygiene.

The enteric nematodes discussed in this chapter are A lumbricoides; the hookworms N americanus and A duodenale; S stercoralis; T trichiura; and E vermicularis.