Trichuris trichiura (Whipworm) Causative agent of trichuriosis
Trichuris trichiura occurs in humans and monkeys. Although this parasite has a worldwide distribution, it is found most frequently, like Ascaris lumbricoides, in moist, warm areas with low hygienic standards. The number of infected persons worldwide is estimated at one billion.
Parasite, life cycle, and epidemiology
The name whipworm characterizes the form of this 3–5 cm long nematode with a very thin anterior part reminiscent of a whiplash and a thicker posterior “handle.” The adult nematodes live in the large intestine, mainly in the cecum. The females lay 2000– 14 000 thick-shelled, yellow-brown eggs per day. The eggs are about 50– 55 lm long and are readily identified by their lemonlike shape and hyaline polar plugs (Fig. 10.1, p. 544). An infective larva develops in the egg within a few weeks. In moist surroundings, Trichuris eggs remain viable for months or even years.
Following peroral ingestion of infective eggs, the larvae hatch in the digestive tract, migrate into the mucosa, and return to the intestinal lumen after a histotropic phase lasting about 10 days. There the adult stages develop and remain with their slender anterior ends anchored in the mucosa. The prepatent period is two and a half to three months, the parasite can live for several years.
Pathogenesis and clinical manifestations
The whipworms, with their thin anterior ends anchored in the mucosa, ingest blood. Mild infections are asymptomatic. More severe infections, with hundreds or several thousand whipworms, cause catarrhal or hemorrhagic inflammations of the large intestine.
Ancylostoma and Necator (Hookworms)
Ancylostoma duodenale and Necator americanus are common parasites of the human small intestine, causing enteritis and anemia. Infection is mainly by the percutaneous route. The dog parasite Ancylostoma caninum has been identified as the cause of eosinophilic enteritis in humans. Larvae of various hookworm species from dogs and other carnivores can penetrate into human skin, causing the clinical picture of “cutaneous larva migrans”
Human hookworm infections are most frequent in the subtropics and tropics (for instance in southern Europe, Africa, Asia, southern US, Central and South America). The number of persons infected worldwide is estimated at about 1.25 billion (WHO, 1998). In central Europe, hookworm infections are seen mainly in travelers returning from the tropics or in guest workers from southern countries.
Morphology, life cycle, and epidemiology
The hookworms that parasitize humans are 0.7–1.8 cm long with the anterior end bent dorsally in a hooklike shape (ankylos: bent, stoma: mouth, necator: killer). The entrance to the large buccal capsule is armed with toothlike structures (Ancylostoma) or cutting plates (Necator). The thin-shelled, oval eggs (about 60 lm long) containing only a small number of blastomeres are shed with feces.
” Larvae in this stage are sensitive to dryness. In moist soil or water they remain viable for about one month. Higher temperatures (optimum: 20–30 8C) and sufficient moisture favor the development of the parasite stages outside of a host.
While penetrating the skin the larvae shed their sheaths and migrate into lymphatic and blood vessels. Once in the bloodstream, they migrate via the right ventricle of the heart and by tracheal migration (conf. Ascaris) into the small intestine, where they develop to sexual maturity. The prepatent period lasts five to seven weeks or longer (reason: arrested larval development). Following oral infection, immediate development in the intestine is probably possible. The parasites can survive in the human gut for one to 15 years.