Introduction to the Nematodes
Basic Life Cycle
Nematode Examples

Nematode Examples

This page provides information on some common nematodes, the list is not exhaustive.

1) Ascaris lumbricoides (Large Roundworm of Man)

ascaris egg 1ascaris egg 2
Ascaris eggs

The egg of this nematode has a relatively thick shell wall, and is highly resistant to the external environment. In addition to the unembryonated eggs, as illustrated on the left here, embryonated eggs containing the L1 larval worms may also be seen, as on the right here.

Infection with this roundworm is extremely common, with estimates of the annual incidence of infection being greater than 1500 million cases, or around one quarter of the worlds population. In addition to the species in man, Ascaris lumbricoides, a morphologically indistinguishable species Ascaris suum is found in the pig.

Other related Genera include Parascaris in equines, and Toxascaris in a variety of domesticated animals.

Morphology

The adult Ascaris lumbricoides are large white, or pinkish-white, cylindrical roundworms, slightly narrower at the head. The more slender males measure between 10 to 30cm long and have a curved tail with two spicules, but no copulatory bursa. The females are very similar, being slightly larger at between 20 to 35cm long, a vulva approximately a third of the length of the body down from the head, and have a blunt tail. They are both characterised by having a smooth, finely striated, cuticle, and a mouth, which is characteristic of all of the Ascarids (e.g. Toxocara ), having three lips each equipped with small papillae. Internally they follow the generalised body plan of all nematodes, and have a cylindrical oesophagus opening into a flattened ribbon like intestine.
The eggs are highly characteristic, with, for nematodes, thick shells consisting of a thick transparent inner shell which is covered in a thick, warty, albuminous coat.

Lifecycle

These parasites have a direct lifecycle, with no intermediate hosts. The adult parasite lives in the lumen of the small intestine of man, usually only feeding on the semi-digested contents of the gut, although there is some evidence that they can bite the intestinal mucous membrane and feed on blood and tissue fluids. The female parasite is highly prolific, laying an estimated 2 million eggs daily. In the intestine these only contain an unembryonated mass of cells, differentiation occurring outside the host. This requires a temperature less than 30°C, moisture and oxygen, before the development of the young L1 larvae after approximately 14 days. Eggs containing the L2 larvae take another week to develop, before they are infective to man, and may remain viable in the soil for many years if conditions are optimal. Infection occurs on ingestion of raw food, such as fruit or vegetables, that a contaminated with these infective eggs. The eggs then hatch in the small intestine, to release the L2 rhabditiform larvae (measuring approximately 250 by 15µm in size. These do not simply grow into the adult forms in the intestine, but must then undergo a migration through the body of their host. These L2 larvae penetrate the intestinal wall, entering the portal blood stream, and then migrate to the liver, then heart, then after between 1 to 7 days, the lungs. Here they moult twice on the way to form the L4 larvae, (measuring approximately 1.5mm long), then burrow out of the blood vessels, entering the bronchioles. From here they migrate up through the air passages of the lungs, to the trachea. They then enter the throat and are swallowed, finally ending up in the small intestine where they mature and mate, to complete their lifecycle.

Pathology of Infection.

The majority of infections (~85%) appear to be asymptomatic, in that there is no gross pathology seen. However the presence of these parasites appears to be associated with the same general failure to thrive in their hosts seen with many of these intestinal nematodes.

In terms of more easily identified pathology, this may be divided into three areas;

- Pathology Associated with the Ingestion and Migration of Larvae

Severe symptoms of Ascaris infection may be associated with the migrating larvae, particularly in the lungs. If large numbers of these larvae are migrating through the lungs simultaneously this may give rise to a severe haemorrhagic pneumonia. More commonly, as is the case with most infections, the haemorrhages are smaller in scale, but still may lead to breathing difficulties, pneumonia and/or fever. A complication here is that many of the parasites proteins are highly allergenic. Because of this the presence of the migrating larvae in the lungs is often associated with allergic hypersensitivity reactions such as asthmatic attacks, pulmonary infiltration and urticaria and oedema of the lips.

- Pathology Associated with Adult Parasites in the Intestine

The most common symptoms of infection are due to the adult parasite, and consist of rather generalised digestive disorders, such as a vague abdominal discomfort, nausea, colic. These symptoms are dependent to some extent of the parasite burden of the host, which in severe cases may consist of many hundreds or even thousands of parasites, although these are extreme cases. In the case of these heavy infections the presence of many of these large parasites may contribute to malnutrition in the host, especially if the hosts (often children) are undernourished anyway. A more serious, and potentially fatal, condition may arise in these more heavy infections, where the mass of worms may block the intestine and need to be surgically removed. This may also occur sometimes on treatment for other intestinal nematodes such as hookworms, where the curative drug dose for these parasites irritates the ascarids.

- Pathology due to "Wandering" Adults outside of the Intestine

Adult parasites often leave the small intestine to enter other organs, (sometimes in response to anti-helminthic drugs used to treat other intestinal nematode infections), where they may cause various types of pathology, sometimes with severe consequences. For example adult Ascaris worms may migrate to the bile duct, which may then become blocked causing jaundice and a general interference in fat metabolism. Adult parasites may also migrate to the appendix, or through the intestinal wall, both conditions which may cause a fatal peritonitis as they may well carry intestinal bacteria to these sites. They may, alarmingly, sometimes migrate forward through the intestinal tract, to be either vomited up or emerging through the nose. More seriously, if they enter the trachea they may cause suffocation.

2) Enterobius vermicularis - (The Human Pinworm)

enterobius
The adult stage

The human pinworm Enterobius vermicularis is a ubiquitous parasite of man, it being estimated that over 200 million people are infected annually. It is more common in the temperate regions of Western Europe and North America, (it being relatively rare in the tropics) and is found particularly in children. Samples of Caucasian children in the U.S.A. and Canada have shown incidences of infection of between 30% to 80%,with similar levels in Europe, and although these regions are the parasites strongholds, it may be found throughout the world, again often with high degrees of incidence. For example in parts of South America the incidence in schoolchildren may be as high as 60%. Interestingly non-Caucasians appear to be relatively resistant to infection with this nematode. As a species, and contrary to popular belief, E. vermicularis is entirely restricted to man, other animals harbouring related but distinct species that are non-infective to humans, although their fur may be contaminated by eggs from the human species if stroked by someone with eggs on their hands. In man anywhere where there are large numbers of children gathered together, (such as nurseries, play groups, orphanages etc.), especially if conditions are insanitary, are ready sources of infection, as one child may rapidly transmit the parasite his or her fellows.

Morphology

These creamy white coloured nematodes are relatively small, with the female measuring only approximately 10mm by 0.4mm wide. These females have a cuticular expansion at their anterior ends, with a long pointed tail. The male parasites, which are much less numerous than the females, are much smaller, measuring only up to 5mm long, and have a curved tail, with a small bursa like expansion, and a single spicule. The head has a mouth with three small lips

Lifecycle

The adult parasites live predominantly in the caecum. This illustration shows a transverse section of the adult parasite, in-situ in the intestine. The male and females mate, and the uteri of the females become filled with eggs. The gravid females (each containing up to 15 000 eggs) then migrate down the digestive tract to the anus. From here they make regular nocturnal migrations out of the anus, to the perianal region, where air contact stimulates them to lay their eggs, before retreating back into the rectum. Eventually the female die, their bodies disintegrating to release any remaining eggs. These eggs, which are clear and measure ~55 by 30µm, then mature to the infectious stage (containing an L1 larvae) over 4 to 6. To infect the host, typically these eggs must then be ingested, the eggs hatching in the duodenum.

The eggs themselves are sticky, and have a characteristic shape, shared with all members of the group Oxyuridea, with an asymettrical form, flattened on one side, (see below);

enterobius egg

The larvae then undergo a series of moults, as they migrate down the digestive tract. The adult worms then mature in the caecum, before copulating to complete the cycle (typically 6 weeks). Occasionally the eggs hatch in the perianal region itself, the resulting L1 larvae being fully infective, crawling back through the anus, then migrating up the intestine to the caecum (retroinfection).

Pathology of Infection.

The majority of infections with this nematode are asymptomatic, although in some cases the emerging females and the sticky masses of eggs that they lay may causes irritation of the perianal region, which in some cases may be severe. As the females emerge at night this may give rise to sleep disturbances, and scratching of the affected perianal area transfers eggs to the fingers and under the finger nails. This in turn aids the transmission of the eggs, both back to the original host (autoinfection), and to other hosts.

3)Trichinella spiralis

trichinella
The larval nematode, encysted in muscle

Trichinosis, caused by infection with this nematode, is a very cosmopoloitan disease, more common in temperate than tropical regions. The epidemiology of trichinosis is highly complex, due to the very low host specificity exhibited by the parasite, resulting in many zoonotic infections. In fact man is usually regarded as an accidental host for the parasite as under normal conditions the parasite reaches a dead end here. This is because to complete the lifecycle the flesh of the host containing the infective larvae must be ingested by another host. In addition rates of infection within populations may be difficult to estimate as the females are viviparous, are found within the intestinal mucosa, and the larvae produced are immediately carried via the circulatory and lymphatic systems to the muscle fibres within which they quickly encyst. Helminth infections are usually detected by the presence of eggs in the faeces, or in the case of the filarial nematodes migrating larvae in the skin or blood. Neither are seen in Truchinella infections, and unless the intensity of the infection is high enough to result in clinical disease the infection may be undetected. In fact estimates of rates of infection within populations are often based on autopsy surveys. These surveys have indicated a marked decrease in rates of infection over the last 40 years, when estimates of world wide infections were over 27 million. For example surveys carried out in the United States indicated prevalences of between 15 to 25% in the 1940's, whilst now the rates have been reduced to ~2%. Similar decreases have been reported in Europe.

4) Necator americanus and Ancylostoma duodenale (The Human Hookworms)

The hookworms belong to the Order Strongylida, a very large order, of great interest as it contains many important pathogens of man and domesticated animals. This order is further subdivided into three Superfamilies, the Strongyloidea (the hookworms in man, discussed below in this page), and two related groups, the Superfamily Trichostrongyloidea, intestinal nematodes important in many domesticated animals (e.g. Haemonchus contortus   in cattle and Nippostrongylus brasiliensis  in rodents) and members of the Superfamily Metastrongyloidea (the lungworms, in domesticated animals)

ancylostoma
necator

The Bucal Cavity of
Ancylostoma duodenale

The Bucal Cavity of
Necator americanus

In man there are two species capable of causing intestinal infections, Ancylostoma duodenale  native to parts of Southern Europe, North Africa and Northern Asia parts of Western South America, and Necator americanus   in Central and Southern Africa, Southern Asia, Australia anf the Pacific Islands. These are very important human pathogens, it being estimated that there are 1200 million cases of hookworm infection in man annualy, of which ~100 million of which are symptomatic infections with accompanying anaemia (see below). In addition the larvae of several species of hookworms infecting domesticated animals may penetrate human skin, causing pathology even though they do not develop the adult parasites in man (see below).

Morphology

The adult parasites are small cylindrical worms, 0.5 - 1.5mm long (Ancylostoma duodenale  being slightly larger than Necator americanus  ). The posterior end of the male worm is equiped with a characteristic copulatory bursa, used to catch and hold the female nematode during mating. The females themselves have a vulva situated near the center of the body, slightly anterior in Necator  and slightly posterior in Ancylostoma. The anterior end of the parasites are formed into a buccal capsule, absent in members of the other Strongylida superfamilies, by which the different genera and species within the group may be differentiated. For example members of the genus Necator  have capsules equiped with cutting plates on the ventral margins, and within the capsule itself small dorsal teeth. In contrast members of the genus Ancylostoma  have pairs of teeth on the ventral margin of the capsule. The number of teeth will vary between different species of Ancylostoma , but is usually between one and four pairs.
The eggs are bluntly rounded, thin shelled, and are almost indistinguishable between the different species, measuring approximately 60 by 40 µm, the eggs of Ancylostoma  being slightly larger than those of Necator.

Lifecycle

The lifecycles of all the hookworms are very similar. The eggs are passed in the faeces, once exposed to air they mature rapidly if conditions are right, with both moisture and warmth essential for development. When mature they hatch to liberate a rhabditiform (i.e. having an oesophagus where a thick anterior region is connected via a neckline region with a posterior bulb) L1 larvae after a few days.

These larval nematodes feed on bacteria and organic material in the soil, where they live and grow for about two days before undergoing the first moult. After about five days more growth they moult again, to produce a much more slender L3 larvae. The L3 larvae has a much shorter oesophagus, is a non-feeding form, and is the infective form of the parasite. Infection takes place by penetration of the skin, for example when walking with bare feet over contaminated damp soil, followed by entry into the circulatory system. Here they are carried to the heart, and then lungs. Once in the lungs, they are too large to pass through the capillary bed therer. Instead they become trapped, and the burrow through the capillary epithelium, entering the air spaces. They then migrate up through bronchi and trachae, and are then swallowed. Once swallowed they pass into the intestine and bury themselves between the intestinal villi. Here they moult to form the L4 larvae, equiped with a buccal capsule allowing adherence to the gut wall. After about thirteen days post-infection they moult for the final time, producing immature adult worms. These the mature over three to four weeks (i.e. five to six weeks after infection), then mate and commense egg laying to complete the lifecycle. These parasites show a very high fecundity, female Necator americanus   producing up to 10 000 eggs daily, whilst female Ancylostoma duodenale   procuse up to 20 000 eggs daily.

Pathology of Infection.

The Pathology associated with hookworm infections may be divided roughly into two areas. Firstly the pathology associated with the presence of the adult parasite in the intestine, and secondly the pathology associated with the penetration of, and migration of the larval worms within, the skin.

The Pathology Associated with the Parasite

The adult hookworms attach themselves to the intestinal wall using their buccal capsules. Their prefered site of infestation is in the upper small intestine, but in very heavy infections (where many thousands of worms may be present) the parasites may spread down as far as the lower ileum. Once attached to the intestinal wall, the hookworm mouthparts penetrate blood vessels, and the parasites obtain nutrition by sucking blood. A single Necator americanus  will take approximately 30 µl of blood daily, whilst the larger Ancylostoma duodenale   will take up to 260 µl. The gross pathology of the disease is very dependant on the intensity of infection. Light infections appear asymptomatic, but in heavy infections, the continuous loss of blood leads to a chronic anaemia, with down to 2gm of haemoglobin per 100ml of blood in extreme cases. Experiments carried out in the 1930's showed that in dogs infected with 500 Ancylostoma caninum  a similar species to the human parasite, nearly a pint of blood a day was lost. This leads to permenant loss of iron and many blood proteins as well as blood cells. This in turn has consequences for further production of erythrocytes, which have been shown to contain less haemoglobin, as well as being reduced in size and smaller in numbers. This form of anaemia may be directly fatal, but more often it induces more non-specific symptoms, the most noticable being the severe retardation in growth and development, both physical and  mental, in infected children, and a general weakness and lassitude, often wrongly interpreted as "laziness".

hookworm insituhookworm larvae
1, Adult parasite attached to intestinal wall. 2, Migrating Larvae