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 (hookworms), and Strongyloides stercoralis. Together, they infect more than 25% of all humans, producing embarrassment, discomfort, malnutrition, anemia, and occasionally death. Other closely related nematodes of animals that may occasionally infect humans are also listed in Table 54–1, but are not discussed here.
TABLE 54–1Intestinal Nematodes ||Download (.pdf) TABLE 54–1 Intestinal Nematodes
|HUMAN PARASITE ||ANIMAL PARASITE ||HUMAN DISEASE |
|Enterobius vermicularis (pinworm) || ||Enterobiasis |
|Trichuris trichiura (whipworm) || ||Trichuriasis |
| ||Capillaria philippinensis ||Intestinal capillariasis |
|Ascaris lumbricoides (large roundworm) || ||Ascariasis |
| ||Ascaris suum ||Ascariasis |
| ||Anisakis spp. ||Anisakiasis |
|Necator americanus (hookworm) || ||Hookworm disease |
|Ancylostoma duodenale (hookworm) || |
| ||Ancylostoma braziliense ||Cutaneous larva migrans |
|Strongyloides stercoralis || ||Strongyloidiasis |
All intestinal nematodes have cylindrical, tapered bodies covered with a tough, acellular cuticle. Sandwiched between this integument 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.
Helminth life cycles 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 ||Download (.pdf) TABLE 54–2 Life Cycles of Intestinal Nematodes
|PARASITE ||ROUTE OF INFECTION ||MIGRATION IN BODY ||DIAGNOSTIC FORM ||SITE OF EMBRYONATION ||INFECTIVE FORM ||FREE-LIVING CYCLE |
|Enterobius vermicularis (pinworm) ||Mouth ||Intestinal ||Egg ||Perineum ||Egg ||No |
|Trichuris trichiura (whipworm) ||Mouth ||Intestinal ||Egg ||Soil ||Egg ||No |
|Ascaris lumbricoides (giantworm) ||Mouth ||Pulmonary ||Egg ||Soil ||Egg ||No |
|Necator americanusa (hookworm) ||Skin ||Pulmonary ||Egg ||Soil ||Filariform larvae ||No |
|Strongyloides stercoralis ||Skin ||Pulmonary ||Rhabditiform larvae ||Soil; intestineb ||Filariform larvae ||Yes |