Helminth Taxonomy - Phylum Nematoda

These helminths are the roundworms, having elongated cylindrical unsegmented bodies, covered in a tough inert cuticle, which may, depending on the species, have striations, lateral spines, terminal expansions or other modifications. Internally the cuticle is formed from an underlying hypodermis, (which may either be a syncytium or made up of cells),which has four longitudinal thickenings, the two lateral thickenings containing excretory canals. Over this is a muscular layer, lining the fluid filled pseudocoelomate body cavity (i.e.. not lined with an epithelium as in a true coelom), within which the internal organs are held. For more details go to the Nematode Biology Pages within this site. In almost all cases there are separate sexes, sexual reproduction occurring in the definitive host in parasitic species.
Note: The classification and taxonomy of nematodes, as has been pointed out, is under continual discussion, and is thus rather confusing. For example, many older systems do not have a phylum Nematoda, the nematodes being grouped in the class Nematoda, part of a much larger grouping, the phylum Nemathelminthes, and including the Acanthocephala, (now also grouped in their own phyla). The system used here has been adapted from that of A. M. Dunn, in his book "Veterinary Helminthology" (William Heinemann Medical Books Ltd, 1978), an excellent introduction to veterinary helminthology.


The various aspects of nematode lifecycles are discussed in more detail on a seperate page in this site. 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 and 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, and the L3 larvae of many strongylid and other 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.

Class Adenophorea (Aphasmidea) - These lack phasmids, a pair of posterior bilateral cuticular organs. Mostly free living nematodes, some parasitic in plants (eelworms), and some in vertebrates.

Important parasitic groups include the Orders;

Order Mermithida

Family Mermithidae - Contains numerous 'Mermithid' nematodes, pathogenic parasites of insect. Of economic importance as they may be used to control insect pests.

e.g. Mermis nigrescens - Adults free living, larvae parasitise the body cavities of grasshoppers.

Order Enoplida

Superfamily Trichuroidea - Slender anterior and wider posterior regions of body. Oesophagus embedded in a single column of cells called a 'stichosome'. Eggs, if any (Trichinella sp. are viviparous), have opercular plugs at either end.

e.g. Trichuris trichiura - Human Whipworm. Adults in intestine. Infection by ingestion of eggs containing L2 larvae, (NB some authorities claim the infective eggs contain L1 larvae, but this is not now generally believed).

e.g. Trichinella spiralis - Trichinosis in man. Adults in intestine, L3 larvae encysted in muscles of the same host, (acting as a sort of 'intermediate host' tissue). Infection by ingestion of contaminated meat containing encysted L3 larvae.

Class Secernentea (Phasmidea) - These have phasmids. These include as well as free living nematodes, parasites of plants (eelworms), and of both invertebrates and vertebrates. Important parasitic groups include the following Orders;

Order Rhabditida

Superfamily Rhabitoidea - mainly free living forms, but some important parasites.

e.g. Caenorhabditis elegans - A free living nematode. Of particular interest as a model organism in genetic and other studies.

e.g. Strongyloides stercoralis- parasites of man and other animals. Interesting as it may adopt a completely free living lifecycle as well as a parasitic one. In the parasitic lifecycle it is the invasive L3 larvae that are infective and, in addition, rarely the whole lifecycle may be completed within the hosts intestine. This parasite is then an exception to the rule that Metazoan parasites do not replicate within their definitive hosts.

e.g.. Micronema (Halicephalobus) delatrix - usually free living, but accidental infections may occur by contamination of wounds (usually in horses, but has been reported in man). In these infections a fatal infection of the central nervous system takes place.

Order Strongylida

The Hookworms (feeding on blood in the intestine) and Lungworms

Superfamily Strongyloidea - The largest nematodes within this order, with large bucal capsules, often armed with cutting plates, teeth or other appendages. Includes the hookworms.

e.g. Necator americanus - hookworm in man. The L3 larvae invasive.

e.g. Ancylostoma duodenale - hookworm in man. L3 larvae invasive.

e.g. A. caninum - hookworms of dogs. L3 larvae are invasive and may penetrate human skin, giving rise to cutaneous larval migrans.

e.g. A. braziliense - hookworms of dogs. L3 larvae are invasive and may penetrate human skin, giving rise to cutaneous larval migrans.

Superfamily Trichostrongyloidea - includes many species of economic importance, causing pathology in domesticated animals. Small nematodes, with small bucal capsules.

e.g. Haemonchus contortus - parasites in ruminants. L3 larvae invasive.

e.g. Ostertagia sp. - parasites in ruminants. L3 larvae invasive.

e.g. Nematodirus sp. - parasites in ruminants. L3 larvae invasive.

e.g. Nippostrongylus brasiliensis - parasitic in rats and mice. L3 larvae invasive.

e.g. Heligmosomoides polygyrus - parasitic in rats and mice. L3 larvae ingested.

many others....

Superfamily Metastrongyloidea - Slender, long and threadlike. Adult worms live in the lungs, (or occasionally blood vessels associated with the lungs) of their definitive hosts.

e.g. Dictyocaulus viviparus - Lungworm in cattle. L3 larvae ingested.

e.g. Angiostrongylus cantonensis - Adults in pulmonary artery of rats. Has molluscan intermediate hosts, and man or other animals that feed on these molluscs may be accidentally infected as paratenic hosts.

Order Ascaridida

Large intestinal round worms, feeding on gut contents. They have three conspicuous lips around the mouth, and the eggs usually have thick shells.

Family Ascarididae - All parasites of the small intestine.

e.g. Ascaris lumbricoides - Roundworm in man. L2 larvae in eggs ingested. Eggs have relatively thick shells.

e.g. Toxocara canis - Roundworm in dogs. L2 larvae in eggs ingested. Eggs have relatively thick shells. May accidentally infect humans, but is not able to complete its lifecycle, causing a larval migrans type of condition.

Family Anisakidae - Parasites of marine mammals, with crustacean then fish intermediate hosts.

e.g. Anisakis sp. - Fish intermediate hosts (e.g. cod), are normally eaten by seals, the definitive hosts. Man may act as an accidental host on eating raw infected fish.

Order Oxyurida

Small nematodes living in the large intestine of their hosts. Have a high degree of host specificity.

Family Oxyuridae

e.g. Enterobius sp. - Pinworms in man and many other animals. L1 larvae in eggs ingested, or hatched L1 larvae invade perianaly.

Order Spirurida

These have intermediate hosts as part of their lifecycles, (usually insects). For more information go to the University of Missouri Veterinary Parasitology pages.

Superfamily Spiruroidea - Infection by inoculation by biting flies.

e.g. Thelazia sp. - Parasites of domesticated animals and occasionally man. Intermediate hosts are biting flys.

Superfamily Filarioidea - Filarial nematodes. Intermediate hosts are biting flies such as mosquitoes, blackflies and others, and infection of the definitive host is by inoculation of L3 larvae from these flies when they are feeding. Adult worms are very elongated, the males are also often very short lived.

e.g. Wuchereria bancrofti - Lymphatic Filariasis (elephantiasis) in man. L1 larvae (microfilaria) in blood infect the intermediate hosts with their blood meals. The intermediate hosts are night feeding mosquitoes (Culex, Aedes and Anopheles sp.).

e.g. Brugia malayi - Lymphatic Filariasis (elephantitis) in man. L1 larvae (microfilaria) in blood infect the intermediate hosts with their blood meals. The intermediate hosts are night feeding mosquitoes (Mansonia and Anopheles sp.).

e.g. Loa loa - The African Eye Worm. L1 larvae (microfilaria) in blood infect the intermediate hosts with their blood meals. The intermediate hosts are deerflies (Chrysops sp.).

e.g. Onchocercar volvulus - River blindness. L1 larvae (microfilaria) in skin infect the intermediate hosts whilst feeding. The intermediate hosts are blackflies (Simulium sp.).

Superfamily Dracunculoidea

e.g. Dracunculus medinensis - Guinea worm in man. Viviparous adult females release L1 larvae into fresh water. Intermediate host is the crustacean Cyclops. Infection is by accidental ingestion of Cyclops in drinking water.