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 Chapter 5.
The Nematodes (Roundworms)

5.1  Infections Acquired Through the Gastrointestinal Tract

Nematodes (or round worms) are non-segmented helminths make up a large assemblage of relatively simple structured organisms. They possess bilateral symmetry and a complete digestive tract with oral and anal openings; they taper to a relative point at both ends. They are also found to have separate sexes, with the male being smaller than the female, ranging in size from a few millimeters to over a meter in length. Their cylindrical non-segmented bodies allow them to be easily distinguishable from other helminths.  

Nematode infections have a widespread distribution being found in both Temperate and Tropical climates.  They can be found in fresh water, in the sea and the soil, successfully invading both animals and plants. The nematodes found in man invade the body fluids such as the blood or lymph channels and also the intestine. The ones that successfully invade the intestine are generally larger but, the nematodes which invade the tissues can grow to relatively enormous lengths.   

Once hatched in the intestine they undergo an incredible migration. The larvae initially burrow into the mucosa, penetrate blood vessels and appear as second stage larvae in the liver within six hours post-infection. Here they remain for several days and develop into third stage larvae, L3. These larvae then migrate to the heart and are carried to the lungs via the pulmonary arteries, arriving within four to seven days. From there they break out of the capillaries into the alveoli and finally work their way up the trachea to the pharynx and reach the small intestine on the 8th or 10th day post-infection.

Within the intestine, the larvae begin their third molt and become fourth stage larvae by the tenth day. The pre-patent period of A. suum in pigs (40–53 days) is less than that of A. lumbricoides (54–61 days) in humans. Two to three months after ingestion of the eggs, the females lay eggs in the intestine.  

The fertilized female can lay about 200,000 eggs per day. Eggs require oxygen and moisture to embryonate and the worm is often found associated with Trichuris trichiura (see the Trichuris trichiura section). 





Stage of Development When Passed

Specific Features And Variations


Enterobius vermicularis

55 mm x 26 mm Range, 50-60 mm 20-32 mm.

Elongated, asymmetrical with one side flattened, other side convex.


Embryonated. Contains C shaped or tadpole-like embryo.

Smooth, thin eggshell with one flattened side.  Occasionally may contain a fully developed larva.  (More readily found on anal swabs than in feces).

Ascaris lumbricoides


fertile egg

60 mm x 45 mm. Range, 45-70 mm x 35-45 mm.

Round or ovoidal. with thick shell.

Brown or yellow brown.

1 cell, separated from the shell at both ends.

Mammillated albuminous coat or covering on outer shell.  Coat is sometimes lost and decorticated eggs have a colorless shell with gray or black internal material.  Eggs may be in 2, 4, or more cells, or contain a fully developed larva.

infertile egg

90 mm x 40 mm. Range, 85-95 mm x 35-45 mm.

Elongated, occasionally triangular, kidney shaped or other bizarre forms.  Shell often very thin.


Internal material is a mass of irregular globules and granules that fills shell.

Mammillated covering attenuated or missing in many cases.

Trichuris trichiura

54 mm x 22 mm. Range, 49-65 mm x 20-29 mm.

Elongated, barrel-shaped with a polar "plug" at each end.

Yellow to brown. "Plugs" are colorless.

1 cell or unsegmented.

Polar plugs are distinctive.  Eggs occasionally are oriented in a vertical or slanted position and may not be readily recognized.  A gentle tap on the coverslip will usually reorient the egg.  On rare occasions, atypical eggs lacking polar plugs may be seen.



Ancylostoma duodenale

60 mm x 40 mm. Range, 57-76 mm x 35-47 mm.

Oval or ellipsoidal with a thin shell.

Colorless with grayish cells.

4- to 8-cell stage.

Occasionally, eggs in advanced cleavage (16 or more cells) or even embryonated may be seen.  Rhabditiform larvae may be present if the specimens are old.  Species identification can not be made on eggs alone; therefore, eggs should be reported simply as hookworm.

Necator americanus

65 mm x 40 mm. Range, 57-76 mm x 35-47 mm.





Trichostrongylus species

90 mm x 40 mm. Range, 75-95 mm x 40-50 mm.

Elongated with one or both ends more pointed than hookworm.

Colorless with grayish cells.

May be in advanced cleavage or morula stage.

Egg resembles hookworm egg but is larger and more pointed at the ends.


Table 5-1. Differential Morphology of the Diagnostic Stages of Helminths Found in Humans: Eggs (Nematodes) (SOURCE: CDC)


Ascaris lumbricoides


Ascaris lumbricoides is the largest of the intestinal nematodes found in man.  The male measures 15cm with a diameter of 3-4µm and has a curled tail with protruding spicules.  The female is 20-35cm long with a diameter of 5µm with a straight pointed posterior end.  The mouth has one dorsal and two ventral lips. Both are creamy white and the cuticle has fine circular striations.

A. lumbricoides adult female worm

Image 5-1.  Adult worm of Ascaris lumbricoides.  They appear creamy white with fine circular striations, for diagnosis they require to be distinguished from earthworms.  Females measure approximately 20-35cm long (SOURCE: CDC)

The ova can be unfertilized, fertilized or decorticated and can show considerable variation in shape and size. They measure 85-95µm by 43-47µm.  The fertilized ova are easily recognized, oval in shape with a thick wall showing an irregular bumpy surface. They measure 45-75µm by 35-50µm.  The outer covering has an albuminoid coat, stained golden brown by bile. The outer wall lies directly on top of a thick smooth shell, which is not easily distinguishable. Some have lost their albuminoid wall.  The unfertilized ova are longer and narrower than the fertile ova, measuring 75-85µm by 35-50µm.

The shell layers of the egg provide a very resistant structure which can withstand many chemicals which make them ideal parasites of the intestine.

Image 5-2.  Unfertilized (left) and fertilized (right) Ascaris lumbricoides eggs.  The unfertilized eggs are longer and narrower than fertilized eggs, measuring 75-85µm by 35-50µm wide. The fertilized eggs have a thick and bumpy outer wall which is stained golden brown with bile. (Saline wet prep) (SOURCE:  PHIL 411/4821 - CDC/Dr. Mae Melvin)

Clinical Disease

Small burdens of worms in the intestine may cause no symptoms. The patient may have symptoms of pneumonitis with cough and low grade fever during the migration of the larvae through the liver and lungs. This can be accompanied by wheezing, coughing and eosinophilia.  In heavy worm burdens the adult worms actively migrate in the intestine resulting in intestinal blockage, vomiting and abdominal pain but infections may also be asymptomatic.  The worms can penetrate through the wall of the intestine, or into the appendix, travel up the common bile duct, which may become blocked or they may then enter the gal bladder or liver. A heavy worm burden in children may lead to severe nutritional impairment and retardation in growth.  

Laboratory Diagnosis

The adults of A. lumbricoides may be expelled through the anus, mouth or nose.  It is important to distinguish the adult worms from earthworms which are segmented and are often collected as a contaminant from toilets. 

  The microscopic examination of stool deposits after concentration reveals the characteristic bile stained ova.  Eggs may be difficult to identify if an excess of iodine is added to the wet preparation as they retain the stain thus resembling debris.  Ova may also become decorticated.  In most symptomatic cases identification is easy due to the vast number of eggs, which can be found within a few seconds of starting to scan the slide.

Hookworm Species  


Hookworms infective to man comprise of two species, Necator americanus and Ancylostoma duodenale. They are classed as one of the most destructive of human parasitic helminths. There is no intermediate host, with man being the only definitive host.  

It is estimated that there are some 900 million cases of infection world wide (Crompton, 1989). The infection is serious where the worms insidiously undermine the health of their hosts.  

They occur in areas where sanitary and environmental conditions favor the development of the eggs and larval infections (e.g., warm, damp soil).  

The geographic distributions of the two species are remarkably divided into:

        Necator americanus which predominately is a New World hookworm, where it was introduced from Africa to the Western Hemisphere. It can also be found in the Far East, Asia, Africa, South America, and Oceania.  

        Ancylostoma duodenale is an Old World hookworm; it is the only species of Europe and areas bordering the Mediterranean. It can also be found in the Middle East, North China, Africa, Asia, and South America.  

Life Cycle

The adult worms live in the small intestine, attached firmly to the mucous membrane of the gut lining, and feed on blood and tissue. The adult females deposit their eggs whilst in the gut (they can produce up to 20,000 eggs per day), the eggs are then passed out in the feces.  The rhabditiform larvae hatch in warm, damp soil (light sandy loam), feeding on bacteria. After about one week during which they have gone through two molts become infective and climb into a suitable position waiting for a suitable host to pass by. The larvae enter the host by penetrating unbroken skin (it is now recognized that A. duodenale can successfully enter man by oral ingestion, this may be more important for this species than skin penetration). The larvae then enter blood vessels and are carried to the heart, lungs and trachea. They are then swallowed and develop into adult worms in the small intestine.  Larvae that are initially swallowed may not show this migration.  


Life cycle of Hookworm

Illustration 5-1. Eggs are passed in the stool  , and under favorable conditions (moisture, warmth, shade), larvae hatch in 1 to 2 days.  The released rhabditiform larvae grow in the feces and/or the soil  , and after 5 to 10 days (and two molts) they become become filariform (third-stage) larvae that are infective  .  These infective larvae can survive 3 to 4 weeks in favorable environmental conditions.  On contact with the human host, the larvae penetrate the skin and are carried through the veins to the heart and then to the lungs.  They penetrate into the pulmonary alveoli, ascend the bronchial tree to the pharynx, and are swallowed  .  The larvae reach the small intestine, where they reside and mature into adults.  Adult worms live in the lumen of the small intestine, where they attach to the intestinal wall with resultant blood loss by the host  .  Most adult worms are eliminated in 1 to 2 years, but longevity records can reach several years.  Some A. duodenale larvae, following penetration of the host skin, can become dormant (in the intestine or muscle).  In addition, infection by A. duodenale may probably also occur by the oral and transmammary route.  N. americanus, however, requires a transpulmonary migration phase. (SOURCE: CDC)

Larvae live for an average of 3–6 weeks in the tropics (A. duodenale can live at lower temperatures than N. americanus can, and so is found in more temperate climates).

Image 5-3. Hookworm larva. Both Ancylostoma duodenale and Necator americanus larvae have similar general morphology and measuring approximately 10-13µm for females and 8-11µm for males.  (SOURCE:  PHIL 1513 – CDC/Dr. Mae Melvin)


Both species have similar general morphology and measure approximately, females 10–13µm and males 8–11µm.  The general morphology of the two species resemble those of Nippostrongylus brasiliensis, the rat hookworm, but they are approximately twice the size of the rat hookworm (Nippostrongylus brasiliensis a species not discussed here).   

The male species has a posterior copulate bursa which is absent from the female. The females though possess a vulva opening which is found almost one third of the body length from the posterior end, they also have two ovaries. Most of the female body is occupied with eggs.  

Image 5-4.  Hookworm egg, normally passed in feces during the 4 - 8 cell stage.  The ova of both species are similar with the shape being oval and transparent with a smooth thin shell and measuring 56-75µm by 36-40µm. (SOURCE:  PHIL 5220 – CDC)

The mouth (or buccal cavity) of the two species show a conspicuous pair of chitinous plates on the dorsal surface. Ancylostoma duodenale buccal cavity bears two hook like teeth on the top and two triangular cutting plates on the bottom.  While the mouth of N. americanus has four cutting plates, two on the ventral and two on the dorsal surfaces. The head is curved in both species but Necator adults it is finer but more pronounced forming a definite “hook” at the anterior end. The buccal cavity is used to attach the worms securely to the mucosa of the small intestine. With the teeth and cutting plates used to pierce the mucosa.   

A. duodenale adult, anterior end

N. americanus adult, anterior end



Illustration 5-2.  A: Adult worm of Ancylostoma duodenale.  Anterior end is depicted showing cutting teeth.  B: Adult worm of Necator americanus.  Anterior end showing mouth parts with cutting plates. (SOURCE: CDC)

The ova are oval and transparent with a smooth thin shell and measure 56-75µm by 36-40µ
m. They are usually passed in the 4-8 cell stage in feces and may be embryonated. The ova of both species of Hookworm are similar.

The bursa (the characteristic external feature which forms an umbrella-like extension surrounding the cloaca) of both male species is well developed, Necator americanus adults are distinguished from Ancylotstoma duodenale by the split dorsal rays and the close arrangement of the lateral rays.

Simplistically the life cycle has three phases:

The parasitic adult females lay eggs while they are in the duodenum where they hatch producing rhabditiform (non-infective) larvae.

1.   The larvae can have two fates in life, one where they are passed out in the feces to continue down the free-living path or they develop into infective filariform larvae whilst traveling down the small intestine.

2. The larvae which, develop in the environment can also undergo different development. Some larvae undergo direct development (homogonic) or indirect development (heterogonic).

The non-infective first stage (rhabditiform) larvae develop into free living adults in the soil within 2–5 days and produce infective third stage or filariform larvae which can penetrate exposed skin (heterogonic development). This phase is common in moist, warm tropical countries.

The non-infective rhabditiform larvae which are excreted in the feces develop into infective filariform larvae in the soil (homogonic development). These infective larvae penetrate exposed skin. There is no development of free living adult worms and this phase is common in temperate zones. The larvae never undergo sexual maturity.

Both types of larvae can become established in the host by penetrating the skin or by oral ingestion.

 The larvae which infect the host by penetrating the skin undergo a migration through the dermal tissues and into the circulation to the heart and lungs, then up the bronchi and trachea, where they are eventually swallowed and pass down into the intestine. On reaching the mucosa of the duodenum the females develop and produce eggs. Adult males are unable to attach themselves to the mucosa, therefore, for any copulation to take place they must mate in the lumen of the intestine.

3. The non-infective rhabditiform larvae develop into infective filariform larvae while passing down the small intestine. Autoinfection occurs when the larvae reinfect the host by penetrating the intestinal mucosa or the perianal or perineal skin. The larvae migrate to the lungs via the circulatory system and then return to the intestine.

From initial infection to maturity usually takes less than four weeks.

Hyperinfection Syndrome

The autoinfective capability of larvae may be responsible for long term infections which persist for many years. The parasite and host reach an equilibrium state where neither host nor parasite suffers any adverse reactions. If this equilibrium is disturbed e.g. immunosuppression, the infection proliferates with immense numbers of larvae migrating to every tissue in the body, especially the lungs. This condition is referred to as disseminated strongyloidiasis. This results in tissue damage, pneumonitis, brain damage or respiratory failure.

Laboratory Diagnosis


Laboratory diagnosis depends on finding larvae in stool, sputum or duodenal aspirates.  

Strongyloides larvae may be present in the stool in very small numbers and culture methods may be needed to encourage the rhabditiform larvae to develop into filariform larvae and migrate from the sample.  The Enterotest or string test can be used to recover larvae from duodenal aspirates. 

Larvae must be distinguished from hookworm larvae especially if it is an older sample. Rhabditiform larvae are most commonly seen. 

A good concentration technique is essential to increase the chances of seeing larvae, though they are easily killed making diagnosis more difficult. 


Serological tests are of value in the diagnosis of strongyloidiasis when larvae cannot be found. An enzyme linked immunosorbent assay (ELISA) using larva antigen, is usually employed.

Little can be used to distinguish between the two species, but using the curved shape of the head is a good indicator. 

Clinical Disease

Larval penetration of the skin may lead to pruritis, often termed as ‘ground itch’ at the site of penetration. Respiratory symptoms may arise during the larval migration.

The adult worm in the intestine may cause intestinal necrosis and blood loss as a result of the attachment of the adult to the intestinal mucosa.  Patients with acute infections may experience nausea, vomiting, abdominal pain, diarrhea and eosinophilia.   

Chronic infections may lead to iron deficiency and anemia resulting from the excessive loss of iron.  Heavy worm burden in children may have serious consequences including death.

Laboratory Diagnosis

Adults of Hookworm species may be passed out spontaneously in feces.   The microscopic examination of stool deposits after an iodine stained, formol-ether concentration method concentration reveals the characteristic ova.  

Cutaneous Larva Migrans

If man comes in contact with hookworm larva of the dog (or cat), A. braziliense or A. caninum, penetration of the skin may take place. The larvae are unable to complete the migration to the small intestine and become trapped. Trapped larvae may survive for weeks or even months, migrating through the subcutaneous tissues.  

Trapped larvae have been known to produce severe reaction, forming tunnels through the tissues, causing intense itchy skin eruption, producing a red, track under the skin which demonstrates accurately the wanderings of the larvae.  

Often intense pruritis and scratching may lead to secondary bacterial invasion, known as ‘creeping eruption’ or ‘cutaneous larval migrans’.

Trichuris trichiura


Trichuris trichiura, more commonly known as the Whip Worm, due to the whip-like form of the body. These nematodes are most commonly seen in tropical climates and in areas where sanitation is poor. They seem to occur in areas particularly where Ascaris and Hookworms are found due to the eggs requiring the same conditions to allow for embryonation.  Both species can be found in humans together. There are several species within this genus each infecting specific hosts, but only T. trichiura infects man. Causing human trichuriasis. It is a parasite that infects many more people than is generally appreciated, up to 800 million people throughout the tropics and temperate regions.

Life cycle eggs require a warm, moist environment with plenty of oxygen to ensure embryonation, but once they have embryonated they are extremely resistant to environmental conditions.

Adult worms are found in the cecum and upper part of the colon of man. In heavy infection they can be found in the colon and the terminal ileum. They attach to the mucosa by the anterior end or by embedding the anterior portion of the body in the superficial tissues, obtaining nutrition from the host tissues.

Once fertilized the female worms lay several thousands of eggs, which are unsegmented at the oviposition and are passed out in the feces. Once they have been passed out they require an embryonation period in the soil which may last from two weeks to several months, after which they become infective.  

When embryonated eggs are swallowed by human hosts larvae are released into the upper duodenum. They then attach themselves to the villi lower down the small intestine or invade the intestinal walls. After a few days the juveniles migrate slowly down towards the cecum attaching themselves to the mucosa, reaching their final attachment site simultaneously.  

The larvae reach maturity within three weeks to a month after infection, during which they undergo four molts. There is no lung migration and the time from ingestion of infective eggs to the development of adult worms is about three months.

Infection is achieved by swallowing soil that contains embryonated eggs. Therefore, children are most commonly seen to possess the infections, as they are more likely to swallow soil.


The adult worms of T. trichuria are characterized by the enormously elongated capillary-like esophagus (anterior end); with the anus situated in the extreme tip. 

The thin anterior portion of the worm is found embedded in the mucosa.  There are no lips and the vulva is at the junction of the thread-like and thickened regions of the body. The posterior end is much thicker and lies free in the lumen of the large intestine.   

The female measures 35-50µm long and the male 30-45µm long.

The ova are characteristically barrel shaped, bile stained with bipolar plugs.  They measure 50-54µm by 20-23µm.  

Life cycle of Trichuris trichiura

Illustration 5-3.  The unembryonated eggs are passed with the stool  .  In the soil, the eggs develop into a 2-cell stage  , an advanced cleavage stage  , and then they embryonate  ; eggs become infective in 15 to 30 days.  After ingestion (soil-contaminated hands or food), the eggs hatch in the small intestine, and release larvae  that mature and establish themselves as adults in the colon  .  The adult worms (approximately 4 cm in length) live in the cecum and ascending colon.  The adult worms are fixed in that location, with the anterior portions threaded into the mucosa.  The females begin to oviposit 60 to 70 days after infection.  Female worms in the cecum shed between 3,000 and 20,000 eggs per day.  The life span of the adults is about 1 year. (SOURCE: CDC)


Image 5-5. Trichuris trichuria ovum with its opercular plugs, shown as white gaps at either end of the egg. (Saline wet prep) (SOURCE:  PHIL 652 – CDC/Dr. Mae Melvin)

Clinical Disease

Most infections due to this nematode are light to moderate with minimal or no symptoms. However, a heavy worm burden may result in mechanical damage to the intestinal mucosa due to the adult worm being threaded into the epithelium of the cecum.  Abdominal cramps, tenesmus, dysentery and prolapsed rectum may occur in these cases.

If a prolapsed rectum is observed, many worms may be seen adhering to the mucosa of the rectum.

Symptomatic infections are usually only seen in children. The majority of infections are chronic and mild, with nonspecific symptoms like diarrhea, anemia, growth retardation, eosinophilia.

Laboratory Diagnosis

The adult worms of T. trichiura are rarely seen in the feces. The microscopic examination of stool deposits after an iodine stained, formol-ether concentration method concentration reveals the characteristic barrel shaped ova. In symptomatic infections numerous numbers of eggs can be seen due to the prolific nature of the female worms, even in light infections many eggs can be seen in the smear.

Strongyloides stercoralis


Strongyloides stercoralis is an intestinal nematode commonly found in warm areas, although it is known to survive in the sub-tropics (hot and humid conditions).  The geographic range of Strongyloides infections tend to overlap with that of Hookworm due to the eggs requiring the same environmental conditions to induce embryonation.

This parasite is interesting in that it contains a free-living stage (exogenous) and a parasitic stage (endogenous) where the larvae undergo development in both stages.

Life cycle

The life cycle of S. stercoralis is a complex one as demonstrated in the diagram below.  


Illustration 5-4. Diagram of the life cycle of Strongyloides stercoralis. (SOURCE:  PHIL 3419 – CDC/ Alexander J. da Silva, PhD/Melanie Moser)


The first stage rhabditiform larvae measure approximately 250µm long by 20µm wide. They have a bulbed esophagus and a short buccal cavity. In an old specimen, rhabditiform larvae of S. stercoralis must be differentiated from those of hookworm which have a longer buccal cavity.  The third stage or filariform larva is approximately 500µm long and has a notched tail (see below) compared with that of hookworm which is sheathed and has a long slender tail.

Adults are slender and possess and extremely long esophagus which in the female extends1/3 to1/2 of the body. The anal opening is ventral and the tail is pointed.


RHABDITIFORM LARVA (First Stage.  Has bulbed esophagus.)

FILARIFORM LARVA (Third Stage.  Lacks prominent bulb in esophagus.)



Genital Primordium

Buccal Cavity


Length of Esophagus

Tip of Tail

Strongyloides stercoralis

225 mm × 16 mm.  Range, 200- 300 mm × 16-20 mm.

Prominent.  Is an elongate, tapered, or pointed structure located along ventral wall about the body length.

Short, about 1/3-1/2 as long as the width of the anterior end of the body.

550 mm × 20 mm.  Range, 500-550 mm × 20-24 mm.

Extends approximately 1/2 length of body.



250 mm × 17 mm.  Range, 200- 300 mm × 14-17 mm.

Inconspicuous.  Rarely distinct. When seen, is small, located nearer the tail than that of Strongyloides.

Long.  Approximately as long as the width of the body.

500 mm.  Range, 500-700 mm × 20-24 mm.

Extends about 1/4 1ength of body.


 Table 5-1. Differential Morphology of the Diagnostic Stages of Helminths Found in Humans: Larvae (SOURCE: CDC)


Eggs are rarely found in the stool as they hatch in the intestine. They are oval and thin shelled, resembling those of hookworm but are smaller measuring 50-58µm by 30-34µm.

Image 5-6.  Thin shelled ova of Strongyloides stercoralis. Measuring 50µm by 30µm,  they are smaller than Hookworm eggs.  (SOURCE: CDC)

Clinical Disease

Disease associated with infections due to S. stercoralis is varied, ranging from some patients being totally asymptomatic to the hyperinfection syndrome. There are three areas of involvement in Strongyloides infections; skin, lungs and intestine. 

1.  Initial skin penetration of the filariform larvae usually causes very little reaction, however with repeated infections the patient may mount a hypersensitive reaction thus preventing the larvae from completing its life cycle. The term larva currens is used when there is a rapidly progressing urticarial track. 

2.  The migration of larvae through the lungs may stimulate an immune response which can result in a cough, wheezing and fever. 

3.  Symptoms associated with intestinal strongyloidiasis may mimic a peptic ulcer due to ulceration of the intestinal mucosa. In heavy infections the intestinal mucosa may be severely damaged resulting in malabsorption. There may also be lower gastrointestinal bleeding.  Eosinophila may be high.

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Ch 1. The Ameba
Ch 2. The Ciliates, Coccidia, and Microsporidia
Ch 3. The Flagellates
Ch 4. The Cestodes
Ch 5. The Nematodes
Ch 6. The Trematodes
Ch 7. Tissue Dwelling Nematodes
Ch 8. Larval Cestodes and Nematodes
Ch 9. Malaria
Ch 10. The Blood Nematodes
Ch 11. Babesia, Trypanosomes, and Leishmania
Ch 12. Arthropod Vectors
Ch 13. Artifacts and Confounders