Enterobius vermicularis. Trichuris trichiura (Trichocephalus trichiurus)

Enterobius vermicularis. Trichuris trichiura (Trichocephalus trichiurus)

Two small roundworms of the large intestine that need no other host. The pinworm, the commonest worm infection in western Europe, whose eggs infect within hours and pass from the anus to the mouth on the fingers; and the whipworm, which threads its thin front into the lining of the caecum, and whose eggs must ripen in the soil.

Enterobius vermicularis
On the left, the female pinworm seen from the side as if cleared: a slender ivory worm, her head end with a small swelling of the cuticle on each side, a short oesophagus ending in a round bulb, and her body behind it packed with tiny eggs. Her tail is drawn out into a long, fine point, and the anus opens on her belly side where the tail begins. In the middle, at the top, her head end enlarged: the cuticle swollen into a wing on either side of the head, the cephalic alae; the coral oesophagus, a muscular tube that narrows into a short isthmus and ends in a round bulb; and behind the bulb the olive intestine. Below, the male, drawn at the female's scale: much smaller, with the same head and bulb, one blue testis, and a blunt tail curled toward his belly, with a single spicule. On the right, two eggs, long ovals flattened on one side with a clear, colourless shell: the first as it is laid, holding a young embryo; the second a few hours later, holding a larva folded back on itself.

Point at a name or a part to light it.

The pinworm lives in the caecum. At night the female crawls out of the anus to lay her eggs on the skin around it, and each egg can infect within 4 to 6 hours: unlike Ascaris's eggs, they need no time in the soil.

Simplified: the two worms drawn to one scale, but the head end about four times larger, and the eggs far larger still; the eggs inside the female drawn as one mass, without the ovaries and uteri that hold them; the lateral alae along the body, the excretory pore, the nerve ring and the vulva left out.

7 sources
  1. Centers for Disease Control and Prevention. DPDx: Enterobiasis. Last reviewed 5 August 2019.
  2. Marie C, Petri WA Jr. Pinworm infestation. MSD Manual Professional Edition, January 2025.
  3. Wikipedia. Pinworm (parasite).
  4. Mostafazadeh M, Mikaeili F, Teimouri A, Sadjjadi SM, Sharifi Y, et al. Morphological and molecular analysis of Enterobius vermicularis with Syphacia spp. Iranian Journal of Parasitology 2025;20(3).
  5. Wendt S, Trawinski H, Schubert S, Rodloff AC, Mössner J, Lübbert C. The diagnosis and treatment of pinworm infection. Deutsches Ärzteblatt International 2019;116:213–219.
  6. Issın G, Demır F, Simsek HA, et al. Seeds or parasites? Clinical and histopathological features of seeds and parasites in the appendix. Turkish Journal of Pathology 2023;39(1):42.
  7. Slavik T, Lauwers GY. Navigating the jungles of tropical infectious gastrointestinal pathology: a pattern-based approach to the endoscopic biopsy. Virchows Archiv 2018;472(1):135–147.

At a glance

  1. Both worms live in the large intestine, need only one host and do not multiply in the body.
  2. The pinworm, Enterobius vermicularis, is a small white worm. At night the female crawls out of the anus to lay her eggs on the skin round it, causing itching; the eggs infect within hours.
  3. It spreads from the anus to the mouth on the fingers, and through families and classrooms on bedding, clothes and dust. The tape test finds the eggs; mebendazole, repeated after 2 weeks, for the whole household, treats it.
  4. The whipworm, Trichuris trichiura, threads its thin front into the lining of the caecum. Its barrel-shaped eggs, with a plug at each end, must ripen in the soil for weeks.
  5. Most whipworm infections cause no symptoms; heavy ones in small children cause bloody diarrhoea, anaemia, slowed growth and rectal prolapse. One dose of the usual drugs often fails.

Enterobius vermicularis, the pinworm

Enterobius vermicularis, the pinworm, is named for the female's long, pointed tail; in Britain it is called the threadworm. It is found worldwide, in more than a billion people, and is the commonest worm infection in the United States, western Europe and Oceania, above all in school-aged and younger children: in Europe, estimates put it at up to about a fifth of kindergarten and primary-school children. Crowding helps it spread, and people are its only host.

The adults and the egg

  • The female is a white thread 8 to 13 mm long, about half a millimetre thick, her tail drawn out to a fine point. A gravid female is almost filled with eggs, 11,000 to 16,000 of them.
  • The male is much smaller, 2 to 5 mm, his blunt tail curled toward his belly, with a single spicule.
  • The head: in both sexes the cuticle swells into a wing on each side, the cephalic alae, and the oesophagus ends in a round bulb. These two marks name a pinworm found on the skin. Along the body the cuticle also forms a narrow ridge on each side, the lateral alae; cut across in a tissue section, as of an appendix, they stand out as two small spines, and these mark a pinworm there.
  • The egg: 50 to 60 by 20 to 30 µm, a long oval flattened on one side, with a clear, colourless shell and a sticky outer coat. It is laid with an embryo already inside, and a larva has grown within 4 to 6 hours.

The pinworm's life cycle

Enterobius vermicularis: the life cycle
A map of the life cycle of the pinworm. Across the top, the patient's pale panel: the patient is the definitive host, and the only one. Low on the left, larvae hatch from swallowed eggs in the small intestine. An arrow climbs from them to the top of the panel: the larvae grow into adults on the way to the large intestine, where the adults live, usually in the caecum; the females lay eggs about 1 month after the eggs were swallowed, and the adults live about 2 months. From the adults an arrow runs right and down: at night the females crawl out of the anus and lay their eggs on the skin around it, where each egg can infect within 4 to 6 hours; a few larvae may hatch there and crawl back into the rectum. From the eggs on the skin an arrow runs straight back left to the larvae: scratching carries the eggs on the fingers to the mouth. A second arrow runs down the right side onto the white page: the eggs are shed onto bedding, clothes, toys and into dust, where they survive up to 3 weeks. From there an arrow climbs the left side back to the larvae: the eggs are swallowed by the same person or another, from the fingers or food, and rarely breathed in with dust.

Point at a name to light its pathway.

The pinworm needs one host and no time outside it: its eggs can infect within hours of being laid, so it spreads from the anus to the mouth on the fingers, and through a household on bedding, clothes and dust.

Simplified: a map, not anatomy: the host drawn as an icon and the parasite drawn small, nothing to scale; the larvae's moults, and the mating of the adults, left out; the larvae that crawl back from the skin into the rectum noted, not drawn.

3 sources
  1. Centers for Disease Control and Prevention. DPDx: Enterobiasis. Last reviewed 5 August 2019.
  2. Marie C, Petri WA Jr. Pinworm infestation. MSD Manual Professional Edition, January 2025.
  3. Wikipedia. Pinworm (parasite): life cycle.

The whole cycle happens in or on one person, and the eggs need no time in the soil.

  1. Swallowed. Eggs are swallowed from fingers that have scratched round the anus, or picked up from bedding, clothes, toys, toilet seats and dust, where they survive up to 3 weeks; rarely they are breathed in with dust and swallowed.
  2. Hatching. The larvae hatch in the small intestine and grow into adults on the way to the large intestine. The adults live in the caecum and nearby colon.
  3. Laying. About a month after the eggs were swallowed, the gravid female crawls out of the anus at night and lays her eggs on the skin round it, in a sticky film. She then dies. Adults live about 2 months.
  4. Back to the mouth. Within hours the eggs can infect. Scratching carries them on the fingers back to the mouth (autoinfection), and some larvae may hatch on the skin and crawl back into the rectum (retroinfection).

Enterobiasis

Most infected people have no symptoms.

  • Itching round the anus, worst at night, from the moving females and the sticky film of eggs. Scratching can break the skin, which may become infected.
  • In girls, worms can wander into the vagina and cause vulvovaginitis, itching and discharge. Rarely they go further up the genital tract and leave granulomas in the pelvis or on the peritoneum.
  • The appendix: pinworms are often found in removed appendices, but whether they cause appendicitis is doubtful.
  • The itch at night can disturb sleep and make a child irritable. Teeth grinding, bed-wetting, abdominal pain and other complaints are often blamed on pinworms, but whether they cause them is uncertain.

Diagnosis, treatment and prevention

  • The tape test: clear adhesive tape pressed on the skin round the anus first thing in the morning, before washing or the toilet, then stuck on a slide and looked at under the microscope. It may need repeating on three mornings. Eggs are found in the stool only now and then. An adult female may be seen on the skin an hour or two after the child goes to bed, and in girls with itching the same tape test can be done at the vaginal opening.
  • Treatment: a single dose of mebendazole, albendazole or pyrantel pamoate, repeated after 2 weeks: the drugs kill the worms but not the eggs. Treat the whole household, everyone aged 2 and over, as others are often infected too. Mebendazole, the usual first choice in Europe, is licensed from 2 years of age; for younger children, and in pregnancy and breastfeeding, a doctor decides, and hygiene measures alone, kept up for 6 weeks, are an alternative.
  • Prevention: washing hands with soap and warm water after the toilet and before food, which matters most; washing clothes, bedding and toys; a morning shower to wash off the night's eggs; vacuuming. Eggs can be passed for a week after treatment, and eggs already laid survive 3 weeks, which is why reinfection is so common.

Trichuris trichiura, the whipworm

Trichuris trichiura
On the left, the female and the male whipworm seen from the side as if cleared. Each is a thread-thin front, about two thirds of the worm, then a short, thick hind part, like a whip and its handle. Along the thin front runs the coral oesophagus among a single row of gland cells, the stichosome. Where the thin part ends, the olive intestine begins and runs through the thick part to the anus at the tip of the tail. The female's hind part is straight and blunt; her vulva opens where the oesophagus meets the intestine, and her pink ovary and uterus full of eggs fill the hind part. The male's hind part is coiled toward his belly, with one blue testis looping through it and a single long spicule at the tail. In the middle, at the top, the wall of the caecum cut across: a band of pink tissue with test-tube crypts, under a row of surface cells. A worm's thin front runs just under the surface, inside the lining, and its thick hind part rises free into the gut. Below, the front end enlarged: the mouth, a short muscular oesophagus, then the narrow tube running past a row of large, round gland cells. On the right, two barrel-shaped eggs with a clear plug at each end: the first as it is passed, a single cell; the second with a folded larva inside.

Point at a name or a part to light it.

The whipworm threads its thin front into the lining of the caecum and the ascending colon, where it lives about 1 year. A female sheds 3,000 to 20,000 eggs a day, and each needs 15 to 30 days in the soil before it can infect.

Simplified: not to scale: the worms are drawn far shorter for their width than they are, and the wall, the front end and the eggs are enlarged, each by a different amount; the stichosome in the whole worms drawn small; the bacillary band along the thin front, the mouth's stylet, the male's spicule sheath and the vagina left out; the ovary, uterus and testis drawn as single looping lines; the wall of the caecum drawn as a band of crypts under a row of surface cells; in life the worm's front lies in a tunnel of the lining's cells.

4 sources
  1. Centers for Disease Control and Prevention. DPDx: Trichuriasis. Last reviewed 10 June 2024.
  2. Marie C, Petri WA Jr. Trichuriasis. MSD Manual Professional Edition, last updated September 2026.
  3. Rivero J, Cutillas C, Callejón R. Trichuris trichiura (Linnaeus, 1771) from human and non-human primates: morphology, biometry, host specificity, molecular characterization, and phylogeny. Frontiers in Veterinary Science 2021;7:626120.
  4. Wikipedia. Trichuris trichiura.

Trichuris trichiura, the whipworm (its genus was once called Trichocephalus), is one of the three soil-transmitted helminths, with Ascaris and the hookworms. About 800 million people carry it, most in the tropics where sanitation is poor, and children carry the most.

The adults and the egg

  • A whip and its handle. Each worm is a thread-thin front, about two thirds of its length, and a short, thick hind part. The female is 35 to 50 mm long, her hind part straight; the male is 30 to 45 mm, his hind part coiled, with a single long spicule in a sheath.
  • The front holds the long oesophagus: a short muscular part, then a narrow tube running past a single row of large gland cells, the stichosome. The intestine begins where the front ends, and the female's vulva opens at that point.
  • Where it lives. The adults live in the caecum and ascending colon, their thin front threaded into the lining and their thick hind part free in the gut. They feed on the cells and fluid of the lining; in heavy infections the damaged lining bleeds, which with the diarrhoea causes the anaemia.
  • The egg: 50 to 55 by 20 to 25 µm, barrel-shaped (often called lemon-shaped), with a thick brown shell and a clear plug at each end. It is passed unembryonated.

The whipworm's life cycle

Trichuris trichiura: the life cycle
A map of the life cycle of the whipworm. Across the top, the patient's pale panel: the patient is the definitive host, and the parasite needs no other. On the left, larvae hatch from swallowed eggs in the small intestine. An arrow runs right: the larvae mature and settle as adults in the caecum and the ascending colon, their thin front threaded into the lining; the females start laying eggs 60 to 70 days after infection, and the adults live about 1 year. From the adults, eggs are passed in the stool, down the right side onto the white page: unembryonated, not yet infective. An arrow runs left from them: in the soil the egg divides and a larva grows inside it in 15 to 30 days. From the infective eggs an arrow climbs the left side back to the larvae in the gut: a person swallows them from soiled hands or food.

Point at a name to light its pathway.

Like Ascaris, the whipworm needs one host, and its eggs must ripen in the soil: there is no spread straight from person to person. Unlike Ascaris, its larvae never leave the gut.

Simplified: a map, not anatomy: the host drawn as an icon and the parasite drawn small, nothing to scale; the egg's stages in the soil, from two cells to the larva, drawn as one arrow; the larvae's moults left out.

3 sources
  1. Centers for Disease Control and Prevention. DPDx: Trichuriasis. Last reviewed 10 June 2024.
  2. Marie C, Petri WA Jr. Trichuriasis. MSD Manual Professional Edition, last updated September 2026.
  3. World Health Organization. Soil-transmitted helminth infections, fact sheet. 18 January 2023.
  1. In the soil. The eggs passed in the stool divide and grow a larva in 15 to 30 days. Until then they cannot infect.
  2. Swallowed. People swallow the infective eggs from soiled hands or food, or vegetables not washed, peeled or cooked.
  3. In the gut. The larvae hatch in the small intestine, mature and settle as adults in the caecum and ascending colon.
  4. Eggs. The females begin to lay 60 to 70 days after infection, each shedding 3,000 to 20,000 eggs a day. The adults live about a year.

As with Ascaris, the worms do not multiply in the body: the number a person carries depends on how many eggs they swallowed, and the harm depends on that number.

Trichuriasis

  • Light infections usually cause no symptoms.
  • Heavy infections, above all in small children, inflame the lining of the large bowel: abdominal pain, poor appetite and diarrhoea, sometimes bloody (dysentery), with anaemia, weight loss and slowed growth.
  • Rectal prolapse: in a heavily infected child, the rectum can slip out through the anus, and the worms can be seen on its lining.
  • Whipworm often comes with Ascaris and the hookworms, as they spread the same way.

Diagnosis, treatment and prevention

  • Stool microscopy: the barrel-shaped eggs with their two plugs are easy to recognise. A concentration method helps in light infections, and counting the eggs (the Kato-Katz method) measures the worm burden. Adult worms may be seen at proctoscopy or colonoscopy, or on a prolapsed rectum. A blood count looks for anaemia.
  • Treatment: mebendazole or albendazole; a single dose often fails against the whipworm, so a 3-day course is used, and ivermectin added to albendazole raises the cure rate. Before ivermectin, ask whether the patient has lived in central Africa, where Loa loa is found. Mebendazole and albendazole are avoided in the first trimester of pregnancy; where whipworm is common, the WHO treats pregnant women in the second and third trimesters. Ivermectin is not given in pregnancy.
  • Prevention: latrines and safe disposal of stool, hand washing, and washing, peeling or cooking food. Where infection is common, the WHO advises regular deworming of everyone at risk, above all children.

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

1A mother says her 5-year-old scratches his bottom at night. How do you confirm pinworm?

2Why does a child with pinworm reinfect themselves so easily?

3Why is a single dose of mebendazole for pinworm repeated after 2 weeks?

4Where and how does the adult whipworm live?

5A 4-year-old from a rural tropical village has chronic bloody diarrhoea, anaemia and rectal prolapse. Stool shows barrel-shaped eggs with a clear plug at each end. What is the cause?