Phylum Nematoda. Ascaris lumbricoides

Phylum Nematoda. Ascaris lumbricoides

The roundworms, the round, unsegmented worms with a complete gut and a false body cavity, how their body is built and how they grow; then Ascaris lumbricoides, the large roundworm, the commonest worm of people, whose larvae travel through the blood and the lungs before they settle in the gut.

Ascaris lumbricoides
On the left, the female and the male Ascaris lumbricoides seen from the side as if cleared: long, round, ivory worms tapering to both ends. The female is the longer and thicker, her tail straight. A short muscular oesophagus runs back from the mouth into the olive intestine, which runs the length of the body to the anus near the tail. About a third of the way down, on her belly side, the vulva opens, and behind it her pink ovaries and uteri loop through the rest of her body. The male is smaller, his tail curled toward his belly, with one blue testis looping through his body and two spicules at the tail. In the middle, at the top, the head seen end-on: three rounded lips round a three-cornered mouth, two papillae on the upper, dorsal lip and one on each of the others. Below it a female cut across: the outer cuticle; the hypodermis under it, thickened into four cords, those at the sides with their excretory canals and those at the top and bottom with the dorsal and ventral nerve cords; the longitudinal muscle cells between the cords, bulging inward; and inside them the open body cavity, holding the flattened intestine, the two large round uteri full of eggs and the small cut loops of the ovaries. On the right, a fertile egg, round, with a thick shell under a bumpy brown coat and a single cell inside; below it an infertile egg, longer and narrower, its coat uneven, full of granules.

Point at a name or a part to light it.

Ascaris lumbricoides, the large roundworm, is the commonest parasitic worm of people. Females are 20 to 35 cm long, males 15 to 31 cm. A female lays about 200,000 eggs a day, and each must lie in the soil for 18 days to several weeks before it can infect.

Simplified: not to scale: the worms are drawn far shorter for their width than they are, and the head, the section and the eggs are enlarged, each by a different amount; the ovaries, uteri and testis drawn as single looping lines with far fewer loops than they have; the excretory pore, the nerve ring and the male's papillae left out, and the excretory canals shown only in the cross-section; the amphids beside the ventrolateral papillae left out, and each double papilla drawn as one; the cross-section simplified, its muscle cells, cords and organs drawn as at mid-body of a female; the muscle cells' arms to the nerve cords left out.

6 sources
  1. Centers for Disease Control and Prevention. DPDx: Ascariasis. Last reviewed 19 July 2019.
  2. Wikipedia. Ascaris lumbricoides; Ascaris suum.
  3. Wikipedia. Nematode: anatomy and reproduction; Spicule (nematode anatomy).
  4. Castelletto ML, Akimori D, Patel R, Schroeder NE, Hallem EA. Introduction to Strongyloides stercoralis anatomy. Journal of Nematology 2024;56:e20240019.
  5. Wikipedia. Coelom: pseudocoelomates; Ascaris.
  6. Civáňová Křížová K, Seifertová M, Baruš V, et al. First study of Ascaris lumbricoides from the semiwild population of the Sumatran orangutan Pongo abelii in the context of morphological description and molecular phylogeny. Life 2023;13:1016.

At a glance

  1. Roundworms (phylum Nematoda) are round, unsegmented worms covered by a tough cuticle, which they shed as they grow. Their gut runs from a mouth to an anus, and their organs lie in a fluid-filled false body cavity, the pseudocoelom.
  2. Their body wall has only longitudinal muscle, in four fields between four cords of the hypodermis. In most the sexes are separate, and the male is smaller, his tail often curved, with spicules for mating.
  3. Ascaris lumbricoides, the large roundworm, is the commonest worm of people: females 20 to 35 cm long, males smaller, living in the small intestine for 1 to 2 years.
  4. Its eggs leave in the stool, usually as a single cell, and must ripen in the soil before they infect. Swallowed, they hatch, and the larvae travel through the blood and the lungs before they settle in the gut as adults.
  5. Most infections cause no symptoms. The larvae can cause a cough with eosinophilia; masses of adults can block the bowel, and a single worm the bile or pancreatic duct. Albendazole or mebendazole usually cures it.

The roundworms

The Nematoda, the roundworms, are one of the largest phyla of animals: more than 25,000 species have been described, most of them free-living in soil and water, many parasites of plants, and some of animals and people. With the arthropods they belong to the Ecdysozoa, the animals that grow by shedding their outer layer.

How a roundworm is built

The roundworm's way
Shapelong, round in cross-section and unsegmented, tapering to both ends
Coveringa tough cuticle of collagen, with no cells, shed at each moult
Body wallthe hypodermis under the cuticle, thickened into four cords; then a layer of longitudinal muscle only
Body cavitya pseudocoelom, a fluid-filled cavity round the gut
Gutcomplete: mouth with lips, a muscular pharynx (oesophagus), an intestine with no muscle of its own, an anus near the tail; no stomach
Nervesa nerve ring round the pharynx, and a dorsal and a ventral nerve cord in their cords of hypodermis
Excretionin Ascaris and its relatives, excretory canals in the two lateral cords; some roundworms have only a single gland cell
Sexesseparate in most; the male smaller, his tail often curved, with one or two spicules
  • The cuticle and the moults. The cuticle covers the whole worm. A roundworm grows through four larval stages, shedding its cuticle at each moult, before it becomes an adult.
  • The muscle. The muscle cells all run along the body, in four fields between the cords. Roundworms have no circular muscle, unlike the flatworms and the earthworm.
  • The pseudocoelom. A true coelom is a cavity lined by mesoderm; flatworms have no body cavity at all. The roundworm's cavity is a "false" one, only partly lined by tissue from the mesoderm, and its fluid bathes the organs.
  • Mating. In the male, the gut and the reproductive tract open together at a cloaca. In mating his spicules come out of the cloaca and hold the female's vulva open, and his sperm, which crawl rather than swim, pass into her.

The roundworms of medicine

Most roundworms that infect people live in the gut as adults, and reach a new host as eggs swallowed from soil or from hands (Ascaris, the whipworm, the pinworm) or as larvae that go through the skin (the hookworms, Strongyloides). Others come with food, as Trichinella in meat, or, like the filariae, which live in the tissues, with a biting fly. Ascaris, the whipworm and the hookworms are the soil-transmitted helminths, which together infect about 1.5 billion people, a quarter of the world.

Ascaris lumbricoides

Ascaris lumbricoides, the large roundworm, is the commonest worm of people, with about 819 million infected, most of them in the tropics and subtropics where sanitation is poor; school-aged children carry the most. A near twin, Ascaris suum, lives in pigs and can also infect people who handle pigs or eat vegetables grown with pig manure.

The adult

The adults are pale, round worms that live free in the lumen of the small intestine. They live 1 to 2 years.

  • The female is 20 to 35 cm long, her tail straight. Her vulva opens about a third of the way along her body; behind it two long ovaries, oviducts and uteri loop through the rest of her body. She lays about 200,000 eggs a day.
  • The male is smaller, 15 to 31 cm, and his tail is curled toward his belly, with two spicules. He has one long testis.
  • The head: a three-cornered mouth between three lips, one dorsal and two ventrolateral, with small papillae. Behind it the muscular oesophagus leads to the intestine.

The eggs

Fertile eggInfertile egg
Shape and sizeround to oval, 45 to 75 µm longlonger, up to 90 µm
Shellthick, under a bumpy (mammillated) coat stained brown by bile; some eggs lose the coat (decorticated)thinner, its coat uneven, with large bumps or almost none
Insideusually a single cella mass of granules
Infective?after weeks in the soilnever

Both kinds turn up in the stool, and a student must know both.

The life cycle

Ascaris lumbricoides: the life cycle
A map of the life cycle of the large roundworm. Across the top, the patient's pale panel: the patient is the definitive host, and the parasite needs no other. Low on the left, larvae hatch from swallowed eggs in the small intestine and penetrate the gut wall. An arrow climbs from them to the top of the panel: the larvae are carried in the portal, then the systemic blood to the lungs, where they mature further. From the lungs an arrow runs right and down: within about two weeks of infection the larvae break into the air spaces, climb the airways to the throat and are swallowed, and in the small intestine they grow into adult worms, which start laying eggs 2 to 3 months after the eggs were swallowed and live 1 to 2 years. From the adults, eggs are passed in the stool, down the right side onto the white page: fertile eggs, usually still a single cell and not yet infective. An arrow runs left from them: in the soil a larva grows inside each egg in 18 days to several weeks, best in moist, warm, shaded soil. From the infective eggs an arrow climbs the left side back to the larvae in the gut: a person swallows them on vegetables, in water or from soiled hands.

Point at a name to light its pathway.

Ascaris needs only one host, but its eggs must ripen in the soil before they can infect: there is no spread straight from person to person. Inside the patient, the larvae take a long way round, through the blood and the lungs, before they settle in the gut.

Simplified: a map, not anatomy: the host drawn as an icon and the parasite drawn small, nothing to scale; the pig roundworm, Ascaris suum, which can also infect people, left out; the larvae's way from the gut wall to the lungs, through the liver and the heart, drawn as one arrow; the larvae's moults left out.

3 sources
  1. Centers for Disease Control and Prevention. DPDx: Ascariasis. Last reviewed 19 July 2019.
  2. Musimbi ZD, Laubschat A, Oser L, et al. Larval ascariasis elicits a prominent IgA and IgG1/2 antibody response to adult Ascaris excretory/secretory antigens in pigs. Frontiers in Immunology 2025;16:1606128.
  3. World Health Organization. Soil-transmitted helminth infections, fact sheet. 18 January 2023.

Ascaris needs only one host, but its eggs must spend time in the soil. There is no spread straight from person to person, and fresh stool is not infective.

  1. In the soil. The fertile eggs, passed usually as a single cell, need 18 days to several weeks to grow a larva inside, best in moist, warm, shaded soil.
  2. Swallowed. People swallow the infective eggs on vegetables that were not cooked, washed or peeled, in water, or from their hands, above all children who play in the soil.
  3. Hatching. The larvae hatch in the small intestine and penetrate the gut wall.
  4. Through the blood to the lungs. They are carried in the portal blood through the liver, then in the systemic blood through the heart to the lungs, where they mature further.
  5. Up and down again. Within about two weeks of the eggs being swallowed, they break through the alveolar walls into the air spaces, climb the bronchial tree to the throat and are swallowed.
  6. Adults. Back in the small intestine they grow into adults. Eggs appear in the stool 2 to 3 months after the eggs were swallowed.

The worms do not multiply in the body: every adult comes from one swallowed egg, so the number of worms a person carries depends on how many eggs they swallowed.

Ascariasis

Ascariasis causes no symptoms in most people. What it does depends on where the worms are:

  • The lungs, as the larvae pass: cough, wheeze and sometimes blood in the sputum, with patchy infiltrates on the chest radiograph and marked eosinophilia, Löffler syndrome. It happens above all in people meeting the worm for the first time.
  • The gut, the adults: a few worms rarely cause symptoms, though a worm passed by mouth or rectum may bring the patient to a doctor. A tangled mass of worms can block the bowel, above all in children, most often at the ileocaecal valve.
  • Wandering adults: a single worm can enter and block the bile or pancreatic duct, causing biliary colic, cholecystitis, cholangitis or pancreatitis, or the appendix. Fever and some anthelmintics can set the worms moving.
  • In children, even moderate infections add to undernutrition and slow growth. Most deaths come from bowel or biliary obstruction in children.

Diagnosis, treatment and prevention

  • Stool microscopy: the eggs, fertile or infertile. Adult worms may be passed or come out of the nose or mouth, and may be seen on imaging of the gut. In the lung phase there are no eggs yet: larvae are occasionally found in the sputum, and eosinophilia is marked.
  • Treatment: albendazole 400 mg or mebendazole 500 mg as a single dose; ivermectin also works. In pregnancy, the MSD Manual recommends pyrantel pamoate, as the benzimidazoles are potentially teratogenic; the WHO's deworming programmes give albendazole or mebendazole in the second and third trimesters. Every intestinal infection should be treated.
  • Before ivermectin, ask whether the patient has lived in central Africa where Loa loa is found: in heavy loiasis it can cause severe reactions.
  • Obstruction usually settles with nothing by mouth, a nasogastric tube and fluids, and the anthelmintic is given once it has: given during the obstruction, it can set the mass of worms moving and tighten it. Some patients need the worms removed endoscopically or by surgery. The lung phase is treated for its symptoms.
  • Prevention: latrines and safe disposal of stool; washing hands before food and after touching soil; washing, peeling or cooking vegetables and fruit, above all where human or pig faeces is used as fertiliser. Where infection is common, the WHO advises deworming everyone at risk regularly, without testing each person first.

Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.

1What kind of body cavity does a roundworm have?

2Which muscle does the body wall of a roundworm have?

3Where do the larvae of Ascaris go after they hatch from a swallowed egg?

4Why can Ascaris not pass straight from one person to the next?

5A child in an endemic area has a cough and wheeze, patchy shadows on the chest radiograph and marked eosinophilia, but no eggs in the stool. What is the likeliest explanation?