Intestinal Nematodes
Microbiology · Parasitology · lean revision notes
Intestinal Nematodes
Intestinal nematodes (roundworms) are non-segmented, cylindrical, bilaterally symmetrical helminths that inhabit the human gut. For NEET PG they are an easy, high-return cluster in Parasitology: a handful of life-cycle quirks, stool ova morphology, and classic clinical buzzwords (Löffler syndrome, scotch-tape test, rectal prolapse, autoinfection) repeatedly convert into single-best-answer questions. Master the egg pictures and the infective stage and you bank these marks.
Classification & the "big five"
Soil-transmitted helminths (STH) dominate this group. A useful way to organise them is by route of entry and infective stage.
| Nematode | Common name | Infective stage / route | Diagnostic stage in stool | Buzzword |
|---|---|---|---|---|
| Ascaris lumbricoides | Giant roundworm | Embryonated egg (ingested) | Bile-stained, mammillated egg | Löffler syndrome, biliary obstruction |
| Ancylostoma duodenale / Necator americanus | Hookworm | Filariform (L3) larva (skin penetration) | Oval, thin-shelled, segmented egg | Iron-deficiency anaemia, ground itch |
| Enterobius vermicularis | Pinworm / threadworm | Embryonated egg (ingested, autoinfection) | Egg on perianal skin (D-shaped) | Perianal pruritus, scotch-tape test |
| Trichuris trichiura | Whipworm | Embryonated egg (ingested) | Barrel-shaped egg with bipolar plugs | Rectal prolapse, "coconut cake" mucosa |
| Strongyloides stercoralis | Threadworm (US) | Filariform (L3) larva (skin / autoinfection) | Rhabditiform larva (not egg) | Autoinfection, hyperinfection syndrome |
High-yield: Strongyloides is the only intestinal nematode in which the rhabditiform LARVA (not the egg) is the usual diagnostic stage in stool — because eggs hatch within the gut mucosa. Ascaris lays the most eggs per day (~200,000), making egg detection extremely easy.
A quick mnemonic for nematodes whose larvae penetrate skin from soil: "S.A.N." (Strongyloides, Ancylostoma, Necator) — these also undergo a lung migration (Loeffler-type) phase, as does Ascaris. Enterobius and Trichuris are acquired by simple egg ingestion with no lung migration.
Ascaris lumbricoides
The largest intestinal nematode (female 20–35 cm). It is the commonest helminthic infection worldwide.
Life cycle & pathophysiology
Embryonated eggs are ingested → larvae hatch in the small intestine → penetrate the gut wall → portal circulation → liver → right heart → lungs → break into alveoli → ascend the bronchial tree → swallowed → mature in the jejunum. This obligatory heart-lung migration is the basis of pulmonary symptoms.
Ingest egg → small bowel → portal vein → liver → lung → trachea → swallowed → adult in jejunum
- Larval (lung) phase: Loeffler syndrome — transient migratory pulmonary infiltrates with peripheral eosinophilia, cough, wheeze, low-grade fever. Larvae may be found in sputum.
- Adult (intestinal) phase: often asymptomatic; heavy worm loads (especially in children) cause malnutrition, vitamin A deficiency, and growth retardation.
- Migration of adults: a hallmark of Ascaris is the tendency of adults to migrate when irritated (fever, anaesthesia, antihelminthics given alone) → bolus intestinal obstruction (commonest at ileocaecal junction in children), biliary ascariasis, cholangitis, pancreatitis, or worms emerging from mouth/nose.
High-yield: Recurrent pyogenic cholangitis and a worm in the common bile duct on ultrasound = biliary ascariasis. ERCP can both diagnose and extract. The "pipe-stem" / "railway-track" sign on contrast studies and ultrasound reflects the worm's gut.
Diagnosis & treatment
- Stool microscopy for the characteristic bile-stained, mammillated (corticated) egg; decorticated eggs lack the albuminous coat. Adult worms may be passed in stool or vomit.
- Eosinophilia is marked in the larval phase, minimal in the established intestinal phase.
- Drug of choice: albendazole 400 mg single dose (or mebendazole). Pyrantel pamoate is an alternative. In obstruction, treat conservatively first (nasogastric suction, fluids) and give piperazine, which paralyses worms (avoids stimulating migration); surgery if conservative measures fail.
Hookworm (Ancylostoma duodenale & Necator americanus)
The major cause of helminth-associated iron-deficiency anaemia in the tropics.
Life cycle & pathophysiology
Filariform (L3) larvae in soil penetrate intact skin (usually feet) → "ground itch" → bloodstream → lungs → trachea → swallowed → attach to small intestinal mucosa with cutting plates (Ancylostoma) or cutting plates that are actually buccal plates vs. Necator's cutting plates. Adults suck blood; each worm causes chronic occult GI blood loss.
| Feature | Ancylostoma duodenale | Necator americanus |
|---|---|---|
| Mouth parts | Two pairs of cutting teeth | A pair of cutting plates |
| Blood loss / worm / day | ~0.15–0.26 mL (more) | ~0.03 mL (less) |
| Shape of adult | "C"-shaped | "S"-shaped |
| Oral infection possible? | Yes (also vertical/transmammary) | No (skin only) |
| Geography | "Old World" — Europe, Asia | "New World" — Americas, also tropics |
High-yield: Ancylostoma duodenale causes more blood loss per worm and can be acquired by the oral route and transmammary route, whereas Necator infects only via skin penetration. Both produce identical, indistinguishable thin-shelled segmented eggs in fresh stool (4–8 cell stage).
Clinical picture: microcytic hypochromic anaemia, hypoproteinaemia/oedema, fatigue; in children, physical and cognitive impairment. Larval skin entry causes pruritic dermatitis ("ground itch"); lung migration may cause mild Loeffler-like symptoms.
Diagnosis & treatment
- Stool microscopy — oval, thin transparent shell, clear space between shell and segmented ovum. If stool is left standing, eggs hatch into rhabditiform larvae (must distinguish from Strongyloides).
- Drug of choice: albendazole 400 mg single dose (or mebendazole). Correct iron deficiency with oral iron.
Enterobius vermicularis (Pinworm)
Commonest helminth infection in temperate/developed countries and among children; spreads readily within families and institutions.
Life cycle & clinical features
Ingested embryonated eggs hatch in the small bowel; adults live in the caecum/colon. The gravid female migrates to the perianal skin at night to lay eggs → intense perianal/perineal pruritus (pruritus ani), disturbed sleep, irritability. Scratching → finger contamination → autoinfection and retroinfection. Eggs become infective within hours and survive on fomites, bedding and under nails. Aberrant migration in girls can cause vulvovaginitis; ectopic worms are implicated in some cases of appendicitis.
Diagnosis & treatment
- NIH swab / cellophane (scotch) tape test applied to the perianal skin early morning before defecation or bathing — picks up the characteristic D-shaped egg (flattened on one side). Stool examination is usually negative (eggs are deposited on skin, not in faeces).
- Drug of choice: albendazole or mebendazole, single dose REPEATED after 2 weeks to kill worms from re-ingested eggs. Pyrantel pamoate is an alternative. Treat the whole family/household and wash linen.
High-yield: Three things tie pinworm to exam stems — scotch-tape test, D-shaped (planoconvex) egg, and the need to repeat the dose at 2 weeks and treat all contacts because of autoinfection.
Trichuris trichiura (Whipworm)
Named for its whip-like shape (thin anterior threaded through mucosa, thick posterior). Frequently co-exists with Ascaris (same faeco-oral, soil-mediated transmission).
Clinical features, diagnosis & treatment
- Light infections are asymptomatic. Heavy infection (Trichuris dysentery syndrome) in children → chronic mucoid/bloody diarrhoea, tenesmus, anaemia, growth retardation, and classically rectal prolapse with visible worms studding the prolapsed mucosa.
- Colonoscopy shows worms threaded into the mucosa; mucosa may show a "coconut cake" appearance.
- Stool microscopy — the unmistakable barrel-shaped (lemon-shaped) egg with transparent bipolar mucoid plugs.
- Drug of choice: albendazole or mebendazole (mebendazole arguably more effective against Trichuris); a 3-day course is often needed for heavy infections as single-dose cure rates are lower.
High-yield: Rectal prolapse in a child + barrel-shaped bipolar-plugged eggs = Trichuris. Single-dose albendazole is less reliable for whipworm than for Ascaris/hookworm.
Strongyloides stercoralis
The most clinically dangerous intestinal nematode because of autoinfection, which lets it persist for decades and cause life-threatening disease in the immunosuppressed.
Three life cycles — the key concept
- Direct (free-living/heterogonic & homogonic) cycle: rhabditiform larvae in soil → develop into infective filariform larvae → penetrate skin → lungs → trachea → swallowed → adult females in small intestine reproduce by parthenogenesis (no parasitic males) → eggs hatch within mucosa → rhabditiform larvae passed in stool.
- Autoinfection: rhabditiform larvae in the gut transform into filariform larvae before leaving the body, penetrate the colonic mucosa (internal autoinfection) or perianal skin (external autoinfection) → re-enter circulation → perpetuate infection without re-exposure.
- Hyperinfection / disseminated strongyloidiasis: massive amplification of the autoinfective cycle, especially with corticosteroids, HTLV-1 infection, malignancy, transplant, malnutrition — larvae disseminate to lungs, CNS, etc., carrying gut bacteria → Gram-negative sepsis, meningitis, ARDS, high mortality.
Filariform larva penetrates skin / gut → lung → trachea → swallowed → parthenogenetic female → eggs hatch in mucosa → rhabditiform larva → (a) passed in stool OR (b) becomes filariform → autoinfection
High-yield: Never start steroids in an at-risk patient without screening/empirically deworming for Strongyloides — steroids precipitate fatal hyperinfection. HIV is a weaker risk factor than HTLV-1 for dissemination. Eosinophilia is often paradoxically absent/low in disseminated disease (poor prognostic sign).
Diagnosis & treatment
- Stool shows rhabditiform larvae (short buccal cavity, large genital primordium — distinguishes from hookworm larva, which has a long buccal cavity). Sensitivity of a single stool is low; use multiple stools, agar plate culture, Baermann concentration, or duodenal aspirate (string test). Serology (ELISA) is sensitive for screening.
- Drug of choice: ivermectin (200 µg/kg/day) — superior to albendazole. Hyperinfection requires prolonged ivermectin until larvae cleared.
Rhabditiform larva: Strongyloides vs hookworm
| Feature | Strongyloides rhabditiform | Hookworm rhabditiform |
|---|---|---|
| Buccal cavity (mouth) | Short | Long |
| Genital primordium | Prominent / large | Inconspicuous |
| Found in fresh stool? | Yes (usual diagnostic stage) | No — only if stool left standing |
Egg & larva morphology — the picture round
| Nematode | Egg / larva appearance |
|---|---|
| Ascaris (fertilised) | Round, bile-stained, thick mammillated albuminous coat |
| Ascaris (unfertilised) | Elongated, more disorganised internal contents |
| Hookworm | Oval, thin colourless shell, 4–8 cell segmented ovum, clear space within shell |
| Enterobius | D-shaped / planoconvex (flat on one side), colourless, contains larva |
| Trichuris | Barrel / lemon-shaped with bipolar mucoid plugs, bile-stained |
| Strongyloides | Rhabditiform larva with short buccal cavity + prominent genital primordium |
Complications & key associations
- Ascaris: intestinal/biliary obstruction, cholangitis, pancreatitis, appendicitis, Loeffler syndrome, vitamin A deficiency, intussusception.
- Hookworm: severe iron-deficiency anaemia, hypoproteinaemic oedema, high-output cardiac failure, developmental delay.
- Enterobius: secondary bacterial dermatitis from scratching, vulvovaginitis, rarely appendicitis/peritoneal granulomas.
- Trichuris: rectal prolapse, chronic dysentery, clubbing (in massive chronic infection), anaemia.
- Strongyloides: hyperinfection syndrome, Gram-negative/polymicrobial sepsis and meningitis, ARDS, chronic urticaria with larva currens (rapidly migrating serpiginous skin track — pathognomonic).
Key differentials & "spot the worm" logic
- Eosinophilia + migratory pulmonary infiltrates → Loeffler syndrome — think Ascaris, hookworm, Strongyloides (lung-migrating nematodes); differentiate from tropical pulmonary eosinophilia (filaria) and ABPA.
- Perianal itch at night → Enterobius (children) vs. simple haemorrhoids/eczema; the tape test settles it.
- Iron-deficiency anaemia in a barefoot agricultural worker → hookworm before assuming dietary cause; look for occult GI loss.
- Rectal prolapse in a malnourished child → Trichuris (worms on prolapsed mucosa) vs. cystic fibrosis, chronic constipation.
- Unexplained Gram-negative sepsis/meningitis in a steroid-treated or HTLV-1 patient with GI/pulmonary symptoms → Strongyloides hyperinfection.
Recently asked / exam angle
- Diagnostic stage of Strongyloides in stool = rhabditiform larva (repeatedly tested; contrast with all others where it is the egg).
- Scotch-tape (cellophane) test and the D-shaped egg identify Enterobius; stool exam is typically negative.
- Drug of choice for Strongyloides = ivermectin, not albendazole — a favourite distractor.
- Steroids + Strongyloides → hyperinfection: classic single-best-answer "which is contraindicated/most dangerous" stem.
- Barrel-shaped egg with bipolar plugs → Trichuris and its link to rectal prolapse.
- Differentiating Ancylostoma vs Necator by mouthparts (teeth vs. cutting plates), blood loss, and route of entry (oral possible in Ancylostoma only).
- Bile-stained mammillated egg = Ascaris; largest intestinal nematode; piperazine paralyses worms and is preferred in obstruction.
- Most common helminth worldwide = Ascaris; most common in temperate regions/children = Enterobius.
- Larva currens (migrating urticarial track) → Strongyloides; cutaneous larva migrans (serpiginous, slower) → Ancylostoma braziliense (a zoonotic hookworm).
Rapid revision
- Ascaris = largest intestinal nematode; bile-stained mammillated egg; Loeffler syndrome; biliary obstruction; albendazole (piperazine in obstruction).
- Strongyloides is the only one diagnosed by the rhabditiform larva in stool; DOC = ivermectin.
- Hookworm = chief cause of helminthic iron-deficiency anaemia; filariform larva penetrates skin; thin-shelled segmented egg.
- Ancylostoma bleeds more, has teeth, and can be acquired orally/transmammary; Necator has cutting plates, skin-only entry.
- Enterobius = scotch-tape test, D-shaped egg, nocturnal perianal itch; repeat dose at 2 weeks + treat family.
- Trichuris = barrel-shaped egg with bipolar plugs; rectal prolapse in children; needs a longer albendazole/mebendazole course.
- Lung-migrating nematodes: Ascaris, hookworm, Strongyloides (→ eosinophilia + Loeffler); Enterobius and Trichuris do not migrate.
- Steroids in undiagnosed Strongyloides → fatal hyperinfection; HTLV-1 is the strongest dissemination risk.
- Larva currens (fast migrating urticaria) = Strongyloides; cutaneous larva migrans = dog/cat hookworm (A. braziliense).
- Albendazole 400 mg single dose treats Ascaris, hookworm and Enterobius; ivermectin for Strongyloides.
- Most common helminth worldwide = Ascaris; most common in developed countries/children = Enterobius.
- Distinguish Strongyloides larva (short buccal cavity, big genital primordium) from hookworm larva (long buccal cavity) in standing stool.