An order of nematodes in which the cephalic region may be adorned with three or six labia or the labia may be replaced by a corona radiata. All strongylid nematodes are parasitic. The order embraces eight superfamilies: Strongyloidea, Diaphanocephaloidea, Ancylostomatoidea, Trichostrongyloidea, Metastrongyloidea, Cosmocercoidea, Oxyuroidea, and Heterakoidea.
Strongyloidea
The Strongyloidea contain important parasites of reptiles, birds, and mammals. The early larval stages may be free-living microbivores, but the adults are always parasitic. Three species are important parasites of horses, Strongylus vulgaris, S. equinus, and S. edentatus. All three undergo direct life cycles; that is, infestations are acquired by ingestion of contaminated food.
Trichostrongyloidea
The Trichostrongyloidea comprise obligate parasites of all vertebrates but fishes. Normally they are intestinal parasites, but some are found in the lungs. The species are important parasites of sheep, cattle, and goats. The adult females lay eggs in the intestinal tract, which are passed out with the feces. In the presence of oxygen the eggs hatch in a few days. When the larvae are ingested by an appropriate host, their protective sheath is lost, and they proceed through the fourth larval stage to adulthood in the intestinal tract, where they may enter the mucosa. No migration takes place outside the gastrointestinal tract.
Metastrongyloidea
The Metastrongyloidea comprise obligate parasites of terrestrial and marine mammals, found commonly in the respiratory tract. In their life cycle they utilize both paratenic and intermediate hosts, among them a variety of invertebrates, including earthworms and mollusks. Two important species are Metastrongylus apri (swine lungworm) and Angiostrongylus cantonensis (rodent lungworm).
Heterakoidea
The Heterakoidea are capable of parasitizing almost any warm-blooded vertebrate as well as reptiles and amphibians. The species Ascaridia galli is the largest known nematode parasite of poultry; males are 2–3 in. (50–76 mm) long, and females 3–4.5 in. (75–116 mm).
Oxyuroidea
The Oxyuroidea constitute a large group of the phylum Nemata. Hosts include terrestrial mammals, birds, reptiles, amphibians, fishes, insects, and other arthropods.
The species are small to medium sized and thin bodied. With one exception, known life cycles are direct. Typically the eggs pass out of the host's alimentary tract onto the ground, where they become fully embryonated and infective. Normally the infective egg does not hatch until a susceptible animal ingests it. The cecum and colon of the host are the typical locations of these parasites. Larvae in all stages of development and adults occur in the gut.
The human pinworm, Enterobius vermicularis, is probably the most contagious of all helminthic diseases. It is estimated that 10% of the world's population suffer from this parasite, the majority being children. Indeed, incidence among schoolchildren in the cool regions of the world often approaches 100%. Infection occurs when eggs are inhaled or ingested. The most common method of transmission is from anus to mouth. Because of the aerial transmission, this disease is highly contagious. Though the infection is seldom serious, the behavioral symptoms are disturbing: nail biting, teeth grinding, anal scratching, insomnia, nightmares, and even convulsions. Several medical treatments are available, but there is often the danger of reinfestation from contaminated objects within the household or institution. See also Nemata.
