Ancylostoma duodenale. Strongyloides stercoralis

Ancylostoma duodenale. Strongyloides stercoralis

Two small roundworms of the small intestine whose larvae go in through the skin. The hookworm holds on to the gut wall with its mouth and feeds on blood, a major cause of iron-deficiency anaemia in the tropics; Strongyloides, a roundworm that can multiply inside its host, can stay for decades and, in a patient given corticosteroids, overwhelm them.

Hookworms: Ancylostoma duodenale and Necator americanus
The hookworms, drawn as a parasitology textbook draws them. On the left, an Ancylostoma duodenale female and male seen from the side as if cleared, to one scale: each a slender cylinder with its front end bent back into the hook that gives the worm its name, a buccal capsule at the mouth, a short muscular oesophagus widening into a club, and the intestine running to the anus near the tail. The female's two ovaries and uteri coil forward and back from her vulva, which opens behind the middle of her body; her tail is pointed. The male's testis coils in his hind half and leads back to the cloaca, and his tail spreads into the copulatory bursa. At the top, the two species' mouths seen end-on, ventral side down: Ancylostoma's buccal capsule with two pairs of hooked teeth on its ventral rim, and Necator's with a pair of cutting plates on the ventral rim and a pair on the dorsal rim. Beside them the egg: oval, its shell thin and clear, with eight cells inside. In the middle, the male's tail enlarged: the bursa, an umbrella of cuticle held open by its rays, and the two long spicules. Below, the two larvae to one scale: the rhabditiform larva, with a long buccal canal and an oesophagus ending in a bulb, and the longer filariform larva, its oesophagus about half as long as its intestine, its tail pointed, in a loose sheath.

Point at a name or a part to light it.

Hookworms are small, about 10 to 15 mm (Ancylostoma) or 9 to 11 mm (Necator) for a female. They hold on to the lining of the small intestine with their buccal capsule and feed on blood. Their eggs hatch in warm, moist soil, and the larvae that grow there go through the skin.

Simplified: the female and the male drawn to one scale, and the two larvae to another; the mouths, the egg and the male's tail enlarged, each by a different amount; the ovaries, uteri and testis drawn as single coiling tubes; the bursa's rays drawn as a simple fan, without their branches; the excretory pore, the nerve ring and the cervical papillae left out; the mouths drawn end-on as rings; Ancylostoma's small pair of teeth, and Necator's small teeth, deep in the capsule left out; the teeth on the worms drawn whole only as two small hooks.

3 sources
  1. Centers for Disease Control and Prevention. DPDx: Hookworm (intestinal). Last reviewed 17 September 2019.
  2. Wikipedia. Hookworm; Ancylostoma duodenale; Necator americanus; Nematode.
  3. Liptáková M, Schreiberová A, Cellengová Z, Kožárová V, Štrkolcová G, and others. The canine hookworm Ancylostoma caninum: first confirmed evidence in a dog in Central Europe. Pathogens 2025;14(12):1241.

At a glance

  1. The hookworms, Ancylostoma duodenale and Necator americanus, and Strongyloides stercoralis are small roundworms of the small intestine. Their eggs or larvae leave in the stool, and their infective larvae go in through the skin, most often of bare feet.
  2. The larvae travel, classically in the blood, to the lungs, climb the airways, are swallowed and grow up in the small intestine.
  3. Adult hookworms hold on to the gut wall with their mouths and feed on blood. Heavy infections cause iron-deficiency anaemia. Eggs in the stool make the diagnosis; albendazole or mebendazole treats it.
  4. Strongyloides can multiply inside its host: larvae turn infective in the gut and go straight back in. Infection can last for decades, unnoticed, and corticosteroids can turn it into a hyperinfection that is often fatal.
  5. Its larvae, not eggs, are found in the stool, and few at a time, so serology helps. Ivermectin treats it. Screen anyone who has lived where it is found before corticosteroids or a transplant.

The hookworms

Ancylostoma duodenale and Necator americanus are the two hookworms of people, and with Ascaris and the whipworm they are the main soil-transmitted helminths. An estimated 406 to 480 million people carry them, in warm, moist places where stool reaches the soil. Both are found in Africa, Asia and the Americas; only A. duodenale is found in the Middle East, North Africa and southern Europe, and N. americanus predominates in the Americas. People are their main host.

The adults and the egg

  • The hook. The front end is bent back, the hook of the name.
  • Size. A. duodenale females are 10 to 15 mm long, the males 8 to 12 mm; N. americanus is a little smaller.
  • The mouth: a cup of thick cuticle, the buccal capsule, with which the worm grips the wall of the gut. Ancylostoma has two pairs of teeth on its rim, Necator cutting plates instead.
  • The male's tail spreads into the copulatory bursa, an umbrella of cuticle held open by rays, with two long spicules. In Necator the tips of the spicules are fused, in Ancylostoma separate.
  • The egg: 60 to 75 by 35 to 40 µm, oval, with a thin, clear shell and a few dividing cells inside. The eggs of the two species look the same, so the laboratory reports only "hookworm".

The hookworm's life cycle

Hookworms: the life cycle
A map of the life cycle of the hookworms, Ancylostoma duodenale and Necator americanus. Across the top, the patient's pale panel: the patient is the definitive host, and the worms need no other. Low on the left, infective larvae that have gone in through the skin. An arrow climbs from them to the top of the panel: they are carried in the blood vessels to the heart and then the lungs. From the lungs an arrow runs right and down: they break into the air spaces, climb the airways to the pharynx and are swallowed, and in the small intestine they grow into adult worms, attached to the wall and feeding on blood; most are gone in 1 to 2 years. From the adults, eggs are passed in the stool, down the right side onto the white page: thin-shelled eggs, not yet infective. An arrow runs down from them: in warm, moist, shaded soil they hatch in 1 to 2 days into rhabditiform larvae, which grow in the stool and soil. An arrow runs left: after 5 to 10 days they have moulted into filariform larvae, the infective stage, which survive 3 to 4 weeks. From them an arrow climbs the left side back to the larvae in the skin: they go in through the skin, most often of bare feet.

Point at a name to light its pathway.

Hookworms need only one host, but their eggs must hatch and the larvae grow infective in the soil. The infective larvae mostly do not wait to be swallowed: they go in through the skin, most often of a bare foot.

Simplified: a map, not anatomy: the host drawn as an icon and the parasite drawn small, nothing to scale; the two species drawn as one cycle; the larvae of the dog and cat hookworms, most of which wander in the skin but cannot grow up in people, left out; the larvae's way from the skin through the heart to the lungs drawn as one arrow; Ancylostoma duodenale's probable oral and breast-milk routes, and its larvae's rest in the tissues, left out.

3 sources
  1. Centers for Disease Control and Prevention. DPDx: Hookworm (intestinal). Last reviewed 17 September 2019.
  2. Marie C, Petri WA Jr. Hookworm Infection. MSD Manual Professional Edition, January 2025.
  3. Wikipedia. Ancylostoma duodenale; Necator americanus.
  1. In the soil. Eggs passed in the stool hatch in 1 to 2 days in warm, moist, shaded soil. The rhabditiform larvae grow in the stool and soil, and in 5 to 10 days become filariform larvae, the infective stage. These do not feed, and survive 3 to 4 weeks.
  2. Through the skin. When the skin touches the soil, most often a bare foot, the larvae go through it and are carried in the blood vessels to the heart and the lungs.
  3. Through the lungs. They break into the alveoli, climb the airways to the pharynx and are swallowed.
  4. In the gut. They grow into adults in the small intestine, where they hold on to the wall and feed on blood. Eggs appear in the stool 5 to 9 weeks after the larvae went in. Most adults are gone in 1 to 2 years, but some live several.

Like Ascaris and the whipworm, hookworms do not multiply in the body: every adult comes from a larva that got into the body. Some A. duodenale larvae rest dormant in the tissues and wake later, and A. duodenale can probably also infect when its larvae are swallowed, and through breast milk; N. americanus does not appear to infect by either route.

Hookworm disease

Most infected people have no symptoms; the harm grows with the number of worms.

  • Ground itch: an itchy rash where the larvae went in, usually on the feet.
  • In the lungs: many larvae passing through cause cough and wheeze with eosinophilia (Löffler syndrome).
  • In the gut: epigastric pain, poor appetite, nausea and diarrhoea.
  • Iron-deficiency anaemia from the chronic loss of blood, with pallor, breathlessness, weakness and swollen legs; in severe cases protein is lost too. Hookworms are a major cause of iron-deficiency anaemia where they are common. In children it slows growth and development and, when severe, causes heart failure; in pregnancy it slows the baby's growth.

Diagnosis, treatment and prevention

  • Stool microscopy finds the thin-shelled eggs; a concentration method is needed for light infections. If the stool stands, the eggs hatch, and the larvae must then be told from those of Strongyloides. In heavy infections the stool may hold occult blood. Before the eggs appear, eosinophilia may be the only finding. Check the blood count and iron stores.
  • Treatment: albendazole, given once, mebendazole or pyrantel pamoate, with iron for the anaemia. Ivermectin does not work against hookworms in the gut. In pregnancy, pyrantel pamoate is considered safe and benzimidazoles are best avoided, though the WHO's deworming programmes give them to pregnant women in the second and third trimesters.
  • Prevention: shoes, latrines and safe disposal of stool. Where hookworm is common, everyone at risk is dewormed regularly.

Strongyloides stercoralis

Strongyloides stercoralis
On the left, the adults seen from the side as if cleared, drawn to one scale. The parasitic female, the only adult in the patient, is a long, slender ivory worm: her thread-like coral oesophagus runs a third of her length, then the olive intestine runs to the anus near the tip of her tail. Her vulva opens two-thirds of the way down; from it her two straight pink uteri run forward and back, each holding a single row of eggs, and her thin ovaries bend back toward it on the other side. Beside her, much smaller, the free-living female and male of the soil, each with a short oesophagus ending in a round bulb. The female's vulva opens at her middle, with eggs in the uteri either side, and her tail is pointed. The male has one straight blue testis and two amber spicules in his curved tail. At the top right, the middle of the parasitic female enlarged: the vulva on her belly side, the two uteri with their single row of eggs, the thin ovaries above, bent back toward the vulva. Below it, the front end of a rhabditiform larva enlarged, with its short buccal canal and its oesophagus in three parts ending in the bulb; and the tail of a filariform larva, its tip notched. Along the foot, the larvae to one scale: the rhabditiform larva, its oesophagus a third of its length and a pink genital primordium in its middle; and the filariform larva, twice as long, its oesophagus as long as its intestine, its tail notched.

Point at a name or a part to light it.

Strongyloides has two kinds of adult female. In the patient, a parasitic female 2 to 3 mm long lives in the lining of the small intestine and makes eggs without a male; in the soil, free-living females about 1 mm long and slightly smaller males mate. The larvae that pass in the stool are rhabditiform; the infective larvae are filariform, with a notched tail.

Simplified: the adults drawn to one scale but about twice as wide for their length as they are, and the larvae to another; the enlarged parts each by a different amount; the ovaries, uteri and testis drawn as simple tubes; the mouth's lips, the nerve ring and the excretory pore left out; the second-stage larva, and the autoinfective larvae, which look much like the filariform, left out.

2 sources
  1. Centers for Disease Control and Prevention. DPDx: Strongyloidiasis. Last reviewed 30 July 2019.
  2. Castelletto ML, Akimori D, Patel R, Schroeder NE, Hallem EA. Introduction to Strongyloides stercoralis anatomy. Journal of Nematology 2024;56(1):20240019.

Strongyloides stercoralis lives in the tropics and subtropics, where sanitation is poor, and in rural parts of the southern United States; some 600 million people carry it. It has also been caught in summer in temperate areas, and pockets have been reported in rural Italy. Unlike the other soil-transmitted roundworms, it can multiply inside its host, and it has a life cycle with two lives: a parasitic one in the patient and a free-living one in the soil.

Its forms

  • The parasitic female, the only adult in the patient, is long and slender, 2 to 3 mm long, with a long, thread-like oesophagus. She lives embedded in the lining of the small intestine and lays eggs without males, by parthenogenesis: there are no parasitic males.
  • The free-living adults of the soil are smaller, the female about 1 mm long and the male a little smaller, each with a short oesophagus ending in a bulb.
  • The rhabditiform larva, 180 to 380 µm long, is what the stool holds, as the eggs hatch in the gut. It has a short buccal canal and a prominent genital primordium, a cluster of cells from which the gonads grow. A hookworm's rhabditiform larva has a long buccal canal and a small primordium.
  • The filariform larva, the infective stage, is up to 600 µm long. Its oesophagus is as long as its intestine and its tail is notched; a hookworm's filariform larva has a short oesophagus and a pointed tail.

Strongyloides's life cycle

Strongyloides stercoralis: the life cycle
A map of the life cycle of Strongyloides stercoralis. Across the top, the patient's pale panel: the patient is the definitive host. Low on the left, filariform larvae that have gone in through the skin. An arrow climbs from them to the top of the panel: they are carried in the blood to the lungs. From the lungs an arrow runs right: they are coughed up and swallowed, and in the small intestine they grow into parasitic females, which live in the wall of the gut and lay eggs without males. An arrow runs down from them: the eggs hatch in the gut into rhabditiform larvae. Most of these pass in the stool, down the right side onto the white page. Some turn infective while still in the gut: an arrow runs left from the rhabditiform larvae to filariform larvae in the middle of the panel, and from those an arrow climbs back to the lungs: autoinfection, through the gut wall or the skin round the anus. On the white page, the soil: from the rhabditiform larvae passed in the stool one arrow runs left and down to infective filariform larvae, and another runs down to free-living adult males and females, which mate; an arrow runs from them to the filariform larvae too, since their young turn infective. From the filariform larvae an arrow climbs the left side back to the larvae in the skin.

Point at a name to light its pathway.

Strongyloides is the roundworm that can multiply inside the patient. Its larvae can turn infective while still in the gut and go straight back in, so an infection can last for decades, and in a patient on corticosteroids it can grow into a deadly hyperinfection.

Simplified: a map, not anatomy: the host drawn as an icon and the parasite drawn small, nothing to scale; the free-living adults drawn as one chip; their eggs, and the larvae they hatch into, left out; Strongyloides fuelleborni left out; the larvae's way to the gut drawn through the lungs, as long taught, though they may also reach it through the tissues of the abdomen; the larvae spread through the body in disseminated infection left out.

2 sources
  1. Centers for Disease Control and Prevention. DPDx: Strongyloidiasis. Last reviewed 30 July 2019.
  2. Castelletto ML, Akimori D, Patel R, Schroeder NE, Hallem EA. Introduction to Strongyloides stercoralis anatomy. Journal of Nematology 2024;56(1):20240019.
  1. Through the skin. Filariform larvae in the soil go through the skin, most often of bare feet, and reach the small intestine, by the classic account through the lungs, coughed up and swallowed; some may travel through the tissues of the abdomen instead.
  2. In the gut. They grow into parasitic females, which live in the wall of the gut and lay eggs. The eggs hatch in the gut into rhabditiform larvae, and most of these are passed in the stool.
  3. In the soil. The larvae either turn straight into infective filariform larvae, or grow into free-living males and females, which mate. Their young turn into filariform larvae too, which must find a host.
  4. Autoinfection. Some rhabditiform larvae turn infective while still in the gut and go back in, through the wall of the bowel or the skin round the anus, and the cycle starts again in the same patient.

Strongyloidiasis

Most chronic infections cause no symptoms, or only vague ones, and up to three quarters of patients have a mild eosinophilia or a raised IgE.

  • Where the larvae went in, an itchy red rash; then a dry cough as they pass through the lungs.
  • In the gut: abdominal pain, bouts of diarrhoea and constipation, poor appetite; in heavy infections malabsorption.
  • Larva currens: larvae of autoinfection moving under the skin of the buttocks, perineum and thighs leave an itchy, raised, winding weal that moves fast, up to 10 cm an hour, and comes back again and again. It is the mark of Strongyloides.
  • Hyperinfection: in a patient on corticosteroids, after a transplant, or infected with the virus HTLV-1, autoinfection runs out of control. Huge numbers of larvae pour through the bowel wall into the lungs, and in disseminated disease into the brain, liver and other organs, carrying gut bacteria with them: Gram-negative bacteraemia and meningitis are common. Untreated, the mortality can approach 90%. Advanced HIV, by contrast, raises the risk less than might be expected.

Diagnosis, treatment and prevention

  • Stool microscopy for larvae: one sample finds only about a quarter of infections, so 3 to 7 are examined, ideally with special methods such as agar plate culture. In hyperinfection, larvae are found in the sputum, bronchial washings and duodenal fluid too.
  • Serology: an antibody test (EIA) is the most sensitive. It cannot tell a past infection from a present one, can cross-react with filariae and other worms, and a negative test does not rule out infection in the immunocompromised. Antibody levels fall within about 6 months of cure, so it can follow treatment. Eosinophilia helps, but corticosteroids can hide it.
  • Screening: anyone who has lived where Strongyloides is found, even long ago, and who has unexplained eosinophilia, HTLV-1, or is about to receive corticosteroids or a transplant, is screened and treated first; so is an organ donor from such a place. When corticosteroids cannot wait, ivermectin may be given without waiting for the tests.
  • Treatment: ivermectin is the drug of choice; albendazole is a weaker alternative, and the single doses of mass deworming do not clear it. Before ivermectin, ask about west or central Africa, where Loa loa is found. Stool is examined again 2 to 4 weeks later to show cure. Hyperinfection is an emergency: ivermectin every day until the stool and sputum have been free of larvae for 2 weeks, broad-spectrum antibiotics, and the immunosuppression reduced where possible.
  • Prevention: latrines and shoes, as for the hookworm.

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

1Why does a heavy hookworm infection cause anaemia?

2A stool sample left on the bench overnight shows rhabditiform larvae. Which feature says they are Strongyloides and not hookworm?

3Why can Strongyloides infection last for decades after a person has left the tropics, when hookworm infection cannot?

4A man who grew up in rural South-East Asia is given high-dose prednisolone. Two weeks later he has diarrhoea, cough, breathlessness and a Gram-negative bacteraemia. What should have been done first?

5Which drug is best for strongyloidiasis?