Nematodes (also known as Nemathelminthes) are roundworms with a cylindrical body and a complete digestive tract, including a mouth and an anus. The body is covered with a noncellular, highly resistant coating called a cuticle. Nematodes have separate sexes; the female is usually larger than the male. The male typically has a coiled tail.
The medically important nematodes can be divided into two categories according to their primary location in the body, namely, intestinal and tissue nematodes.
The intestinal nematodes include Enterobius (pinworm), Trichuris (whipworm), Ascaris (giant roundworm), Necator and Ancylostoma (the two hookworms), Strongyloides (small roundworm), and Trichinella. Enterobius, Trichuris, and Ascaris are transmitted by ingestion of eggs; the others are transmitted as larvae. There are two larval forms: the first- and second-stage (rhabditiform) larvae are noninfectious, feeding forms; the third-stage (filariform) larvae are the infectious, nonfeeding forms. As adults, these nematodes live within the human body, except for Strongyloides, which can also exist in the soil.
The important tissue nematodes Wuchereria, Onchocerca, and Loa are called the “filarial worms,” because they produce motile embryos called microfilariae in blood and tissue fluids. These organisms are transmitted from person to person by bloodsucking mosquitoes or flies. A fourth species is the guinea worm, Dracunculus, whose larvae inhabit tiny crustaceans (copepods) and are ingested in drinking water.
The nematodes described above cause disease as a result of the presence of adult worms within the body. In addition, several species cannot mature to adults in human tissue, but their larvae can cause disease. The most serious of these diseases is visceral larva migrans, caused primarily by the larvae of the dog ascarid, Toxocara canis. Cutaneous larva migrans, caused mainly by the larvae of the dog and cat hookworm, Ancylostoma caninum, is less serious. A third disease, anisakiasis, is caused by the ingestion of Anisakis larvae in raw seafood.
In infections caused by certain nematodes that migrate through tissue (e.g., Strongyloides, Trichinella, Ascaris, and the two hookworms Ancylostoma and Necator), a striking increase in the number of eosinophils (eosinophilia) occurs. Eosinophils do not ingest the organisms; rather, they attach to the surface of the parasite via IgE and secrete cytotoxic enzymes contained within their eosinophilic granules. Host defenses against helminths are stimulated by interleukins synthesized by the Th-2 subset of helper T cells (e.g., the production of IgE is increased by interleukin-4, and the number of eosinophils is increased by interleukin-5 [IL-5]) (see Chapter 58). Cysteine proteases produced by the worms to facilitate their migration through tissue are the stimuli for IL-5 production.
Features of the medically important nematodes are summarized in Table 56–1. The medically important stages in the life cycle of the intestinal nematodes are described in Table 56–2, and those of the tissue nematodes are described in Table 56–3.