helminths

Parasitic worms are highly differentiated multicellular organisms. Their life cycles are complex; most alternate between sexual reproduction in the definitive host and asexual multiplication in an intermediary host or vector.

Thus, depending on parasite species, humans may harbor either adult worms (e.g., Ascarus lumbricoides) or immature stages (e.g., Toxocara canis) or asexual larval forms (e.g., Echinococcus species).

Once adult worms take up residence in humans, they do not multiply but generate eggs or larvae destined for the next phase of the cycle. An exception is Strongyloides stercoralis, the larvae of which can become infectious in the gut and cause overwhelming autoinfection in immunosuppressed persons.

There are two important consequences of the lack of replication of adult worms:
 (1) Disease is often caused by inflammatory responses to the eggs or larvae rather than to the adults (e.g., schistosomiasis)
 (2) disease is in proportion to the number of organisms that have infected the individual (e.g., 10 hookworms cause little disease, whereas 1000 hookworms cause severe anemia by consuming 100 mL of blood per day).

Pathology

 helminth infections (helminthiases)

See also

 infectious agents

• parasites

Videos

 Trichuris trichiura in the caecum

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See also

 helminthiases

• digestive helminthiasis