The Basic Lifecycle of the Major Groups of Nematodes
The lifecycles of the parasitic species vary considerably, as would be expected from such a large and diverse group. There are however a number of common features. Firstly, the parasite undergoes a series of moults through larval stages (designated L1 to the adult L5 form). Secondly, in most (but not all) nematodes it is the L3 larvae that is the infective form, important exceptions to this being the Ascarids, such as Ascaris lumbricoides and the pinworms, where it is either the L1 larvae, or eggs containing L1 or L2 larvae that are infective. Thirdly the L3 form onwards in all species undergoes a migration within the body of the definitive host as it matures into the adult parasite, usually via the bloodstream or lymphatic system to the heart, lungs, trachea, and then to the intestine. Finally, in most cases the parasite leaves the definitive host as thin walled eggs in the faeces, important exceptions being the viviparous filarial worms (where L1 larvae infect intermediate hosts, usually in the blood meals of biting arthropods), Strongyloides stercoralis, (where the L1 larvae are found in the faeces), and the viviparous Trichinela spiralis, where the larvae do not leave the body as such, but develop to the L3 stage which then encysts in the muscles, infection being by ingestion of undercooked contaminated meat. Infection of the definitive host may be by a variety of routes, such as the oral route, where eggs are accidentally ingested, also many filarial worms are infective via the bite of flies, as previously decribed, and the L3 larvae of many nematodes such as the hookworms and other related nematodes are directly invasive.
In terms of complexity, the simplest lifecycles are those of the pinworms, where adults living in the colon mate and lay eggs which pass out in the faeces, infection being either by the oral route with eggs, or perianaly, where eggs hatch around the anus and L1 larvae migrate back through the anus.
The most diverse is probably that of S. stercoralis, where there are a number of alternative lifecycles which it may undergo, either as a completely free living soil nematode, or as the standard infective L3 larvae with tissue migration to the intestine, or even occasionally full completion of the lifecycle within the intestine, and finally in immunocompromised hosts a life-threatening disseminated infection can occur, with parasites found throughout the body.