The life cycles of the intestinal nematodes are summarized in Table 55 – 2. E. vermicu-laris (pinworm), the best adapted of the intestinal nematodes, has the simplest life cycle. It feeds, grows, and copulates within the gut of its host before transiting the anus to deposit its eggs on the perineal skin. The eggs embryonate within hours and are subse-quently transported to the same, or a new, host via fingers or dust. Following their inhalation or ingestion, the eggs are swallowed and hatch in the bowel lumen, completing the cycle. The only significant difference between this and the life cycle of T. trichiura (whipworm) is that the eggs of the latter are passed in the stool and must incubate on soil before becoming infectious. This relatively minor difference has profound epidemiologic ramifications, because Trichuris can be passed only in populations that practice indis-criminate defecation and live in climates suitable for the maturation of eggs in the soil.
A. lumbricoides is transmitted in a manner similar to T. trichiura. However, afterhatching from the egg in the gut lumen, ascarid larvae penetrate the bowel wall and mi-grate through the host’s liver and lung before returning, older and more sedentary, to the protective environment of the gut lumen. This maladaptive sojourn of juvenile worms through the host tissue is also seen in the life cycles of the hookworms and S. stercoralis. In contrast to Ascaris, however, the eggs of the latter two nematodes hatch shortly before or after they are passed in the stool of the original host, resulting in the seeding of the external environment with larval forms capable of penetrating human skin. Transmission is effected when a new host comes into physical contact with the contaminated soil. The adaptation of S. stercoralis is the least satisfactory of the intestinal nematodes and, in an evolutionary sense, appears to have occurred quite recently. In addition to the hookworm-like cycle described above, it has the twin capacities to complete its life cycle entirely within the body of the host or to survive in the external environment as a free-living soil organism.