Intestinal & Tissue Helminth Infections (STH, Tapeworms, Schistosomiasis, Strongyloides, Filariasis)

Meet the patient

A 28-year-old who emigrated from rural Bangladesh six years ago presents with vague epigastric discomfort and intermittent diarrhoea. His full blood count shows an eosinophil count of 1.2 x10^9/L that nobody has yet explained, and the nephrologists want to start him on high-dose prednisolone this week for a new nephrotic syndrome.^[9]

Two questions decide whether this visit is routine or catastrophic: which worm is hiding behind that eosinophilia, and is it safe to give the steroids? Miss the second and a silent, decades-old Strongyloides carriage turns into fatal Gram-negative septicaemia within the week — which is why the eosinophilia is not a footnote on the form, it is a stop sign in front of the steroids.^[1][9]

Overview & Definition

Helminths (from the Greek helmins, worm) are multicellular, macroscopic parasitic worms that colonise the human host for years. Unlike protozoa, they do not multiply within the definitive human host — the worm burden reflects the inoculum (the number of eggs or larvae acquired), and intensity of infection drives disease. The three phyla of medical importance are:^[1]

• Nematodes (roundworms) — elongated, cylindrical, unsegmented worms with a complete digestive tract. The major human species are the intestinal nematodes (Ascaris lumbricoides, hookworm, Trichuris trichiura, Enterobius vermicularis, Strongyloides stercoralis) and the tissue nematodes (filarial worms: Wuchereria bancrofti, Brugia malayi, Onchocerca volvulus, Loa loa; also Trichinella spiralis, Dracunculus medinensis, and the zoonotic larva migrans).
• Cestodes (tapeworms) — segmented, ribbon-like, hermaphroditic flatworms without a digestive tract (nutrients absorbed through the tegument). The human-relevant species are Taenia solium (pork) and Taenia saginata (beef), Diphyllobothrium latum (fish), Hymenolepis nana (dwarf tapeworm), and Echinococcus granulosus/multilocularis (hydatid disease).
• Trematodes (flukes) — non-segmented, leaf-shaped flatworms. The human species are the blood flukes (Schistosoma species — the most clinically important trematodes), the liver flukes (Fasciola hepatica, Clonorchis sinensis, Opisthorchis species), the lung fluke (Paragonimus westermani), and the intestinal flukes (Fasciolopsis buski, Heterophyes). ^[1]

The clinical skill rests on four reflexes: (1) recognising the syndrome of unexplained eosinophilia and linking it to a travel, dietary, or environmental exposure; (2) choosing the right diagnostic sample (stool for most, urine for Schistosoma haematobium, sellotape for Enterobius, serology/antigen for tissue stages); (3) matching the right species-specific drug (benzimidazoles, praziquantel, ivermectin, or diethylcarbamazine — they are not interchangeable); and (4) screening for Strongyloides before any immunosuppression, because untreated strongyloidiasis causes fatal hyperinfection.^[9]

A note on terminology: soil-transmitted helminths (STH) refers specifically to the three major intestinal nematodes whose eggs require a soil-maturation phase — Ascaris lumbricoides, hookworm, and Trichuris trichiura. Enterobius and Strongyloides are intestinal nematodes but are not classed as STH (Enterobius eggs are immediately infective and do not need soil; Strongyloides larvae can complete the cycle entirely within the host). The WHO and the public-health literature use STH in this narrow sense. ^[1]

Classification

Helminths are classified along three axes: phylum (aetiology), route of acquisition, and primary site of pathology (intestinal vs tissue). The classification below is the practical framework examiners use. ^[1]

^{[1] [10] [11] [12] [4]}

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Epidemiology & Risk Factors

Helminths are among the commonest infections of humans. STH infect over 1.5 billion people globally and are associated with anaemia and stunting. More than a quarter of the world's population is at risk of STH (including Strongyloides). By conservative estimates, at least 230 million people are infected with Schistosoma spp. Strongyloidiasis is estimated to affect 300 to 600 million people.^{[27][1][3][10]}

^{[27] [3] [10] [11]}

The single most important determinant of helminth endemicity is poverty — specifically the combination of warm, moist climate; poor sanitation; use of untreated human faeces (night soil) as fertiliser; and lack of access to safe water and footwear. Worm eggs and larvae require warm (25 to 35 degrees C), moist, well-aerated soil to mature, which is why STH and schistosomiasis cluster in the tropics and disappear in temperate, arid regions with reticulated sewerage.^[1]

Host and environmental risk factors, and the worms they favour: ^[1]

^{[26] [28]}

Pathophysiology

The clinical and laboratory features of every helminth infection derive from where the worm lives, what it feeds on, and how the host immune system responds to it. The unifying immunological signature is the Th2 cytokine response — driven by IL-4, IL-5, IL-9, IL-13 and IL-31 — which drives eosinophilopoiesis (via IL-5), IgE class-switching, goblet-cell hyperplasia, mucin secretion, smooth-muscle hypercontractility, and macrophage alternative activation. This Th2 axis is the host's principal defence against worms (it expels them by mucous trapping, weep-and-sweep, and mechanical grip-loosening), which is why eosinophilia and high IgE are the laboratory hallmarks of tissue helminth infection, and why Th2-deficient states (corticosteroids, HTLV-1) tip the balance toward the parasite.^[9]

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The Ascaris lumbricoides life cycle — the paradigm of hepatopulmonary migration

1. Egg ingestion. Fertilised eggs (rounded, thick-shelled, mammillated brown coat) are swallowed in soil-contaminated food or water. They require 2 to 6 weeks of soil maturation to become embryonated and infective — so fresh faecal-oral transit does not transmit.
2. Small-intestinal hatching and mucosal penetration. Eggs hatch in the jejunum, releasing rhabditiform larvae that penetrate the intestinal wall and enter the portal venous system.
3. Hepatic and pulmonary migration. Larvae reach the liver (4 to 7 days), then pass through the right heart to the lungs (days 7 to 14), where they lodge in alveolar capillaries, moult twice, and mature into third- and fourth-stage larvae (about 1 to 2 mm long). The host mounts a brisk eosinophilic inflammatory response (Loeffler syndrome) — cough, wheeze, transient pulmonary infiltrates, and marked peripheral eosinophilia.
4. Airway ascent and swallowing. Larvae ascend the bronchial tree to the glottis (day 14), are swallowed, and return to the small intestine, where they mature into adult roundworms (15 to 35 cm) in the jejunum. Adults do not attach — they live free in the lumen, consuming semi-digested food.
5. Egg laying and excretion. Adult females begin laying eggs at 8 to 12 weeks after infection. Eggs appear in stool. Adult worms live 1 to 2 years. ^[1]

The pathological consequences of Ascaris are mechanical: a heavy burden (hundreds of worms) can ball up and obstruct the small bowel (especially the ileum, in children), and an adult worm can migrate into the bile duct, pancreatic duct, or appendix, causing biliary colic, ascending cholangitis, acute pancreatitis, or appendicitis. The migrating larval phase causes the Loeffler syndrome.^[1]

The hookworm life cycle — the paradigm of blood loss

1. Skin penetration. Human hookworm is caused by Necator americanus or Ancylostoma duodenale and is transmitted primarily by larval invasion of exposed skin (usually the feet).
2. Localisation in the small intestine. The adults inhabit the host small intestine, where they consume host blood — the physiological basis of hookworm disease.
3. Blood loss and anaemia. Hookworm disease is generally asymptomatic in light infection, but heavy infection leads to iron-deficiency, hypochromic microcytic anaemia; chronic infection causes stunted growth and cognitive deficits in children, reduced work capacity in adults, and a variety of pregnancy complications.
4. Diagnosis and control. Diagnosis is made by finding the characteristic hookworm eggs on a direct faecal film; control relies on single-dose mass deworming of at-risk school-aged and preschool children.^[12][13]

The Strongyloides stercoralis cycle — the paradigm of auto-infection

Strongyloides is unique among human helminths in that it can complete its entire life cycle within a single human host (homogonic cycle), through auto-infection. This is the single most important fact about the parasite, because it explains both decades of asymptomatic carriage (in a person who left an endemic region years ago) and catastrophic hyperinfection when cell-mediated immunity is suppressed.^[9]

1. Skin penetration and migration. As for hookworm: filariform larvae penetrate skin, migrate to lungs, are swallowed, and adult parasitic females (2 to 3 mm) embed in the duodenal and jejunal mucosa, where they reproduce parthenogenetically (no male required).
2. Egg laying and rhabditiform larvae. Females lay eggs that hatch within the mucosa to release rhabditiform larvae which pass out in stool. In the external environment, rhabditiform larvae can either develop into free-living adult worms (the heterogonic cycle, unique among human nematodes) or moult into infective filariform larvae.
3. Auto-infection. Some rhabditiform larvae within the bowel lumen moult into filariform larvae before being excreted, then penetrate the perianal skin (external auto-infection) or the colonic mucosa (internal auto-infection), re-migrating through the lungs to re-establish infection. This loop, repeating every 2 weeks, sustains infection for the life of the host.
4. Hyperinfection. When cell-mediated (Th2) immunity is impaired — classically by corticosteroids (which also directly stimulate larval moult via ecdysteroid-like receptors), HTLV-1, haematological malignancy, solid-organ transplantation, or malnutrition — the auto-infection cycle accelerates enormously, producing thousands of migrating larvae. The larvae carry enteric bacteria (notably Escherichia coli, Klebsiella, Enterococcus, and Bacteroides) on their cuticle and disrupt the bowel wall, producing Gram-negative bacteraemia, meningitis, and pneumonia, often with ARDS, ileus, and shock. Hyperinfection/disseminated disease during immunosuppression has a more than 60 percent case fatality. Notably, HIV is NOT the dominant risk — HIV does not deplete the T-helper-2 lymphocytes that mediate helminth immunity — whereas HTLV-1 decreases the type-2 response and is clearly associated with hyperinfection.^[10][9]

The Schistosoma cycle — the paradigm of egg-driven granulomatous disease

1. Cercarial penetration. Cercariae released by infected freshwater snails (the intermediate host) penetrate the skin during swimming, bathing, or wading. They shed their forked tail, become schistosomula, and migrate via venous circulation through the lungs to the liver, where they mature and pair.
2. Adult worm pairing and venous migration. Mature male and female worms pair and migrate against portal venous flow to their species-specific final niche: S. mansoni and S. japonicum to the inferior mesenteric and superior mesenteric venules (bowel); S. haematobium to the vesical and pelvic venous plexus (bladder). Adult worms live 3 to 10 years (occasionally much longer), sheathed in a host-derived tegument that hides them from immunity.
3. Egg deposition — the pathogenic stage. The female lays eggs (each species has a characteristic spine: S. mansoni = lateral, S. haematobium = terminal, S. japonicum = small lateral). To exit the host, eggs must traverse the bowel or bladder wall, a process that only a minority achieve — the rest are swept by the portal/systemic circulation to the liver (S. mansoni/japonicum), lungs, brain, spinal cord, or genitourinary tract, where they elicit a granulomatous Th2 response around the egg.
4. Granulomatous disease. The egg-antigen-driven granuloma is the pathological hallmark of schistosomiasis. In the liver, periportal ('clay-pipestem') Symmers fibrosis develops over years, producing presinusoidal portal hypertension (splenomegaly, varices, hypersplenism) with preserved hepatocellular function (normal synthetic function — distinguishes schistosomiasis from cirrhosis). In the bladder, chronic S. haematobium egg deposition causes haematuria, bladder-wall calcification, obstruction, and, over decades, squamous cell carcinoma of the bladder.
5. Acute schistosomiasis (Katayama syndrome). Occurs most often in travellers or immigrants exposed to schistosome antigens for the first time. Typical presentation is sudden fever, malaise, myalgia, headache, eosinophilia, fatigue, and abdominal pain lasting 2 to 10 weeks. Adult worms live an average of 3 to 10 years (occasionally much longer).^[3]

The Taenia solium cycle — the paradigm of cysticercosis

Humans are the definitive host (adult tapeworm) for both T. solium (pork) and T. saginata (beef), and the intermediate host (larval cysticercus) for T. solium alone. This dual role is the source of the most important exam distinction: eating undercooked pork causes taeniasis (the adult worm in the gut); ingesting T. solium eggs causes cysticercosis (larval cysts in brain, muscle, and eye).^[4]

1. Taeniasis (adult worm). Humans eat undercooked pork containing cysticerci. The cysticercus evaginates in the small intestine, attaches to the jejunal mucosa by its scolex (with four suckers and a double ring of hooks — T. solium; hooks absent in T. saginata), and matures into an adult tapeworm of 2 to 8 metres in 2 to 3 months. The adult is usually single and asymptomatic; gravid proglottids detach and pass in stool or migrate actively per rectum/per anus.
2. Cysticercosis (larval cysts). T. solium eggs (indistinguishable from T. saginata eggs morphologically) ingested in faecally contaminated food or water, or by auto-infection (a taeniasis carrier swallowing their own eggs via hand-to-mouth), release oncospheres that penetrate the intestinal wall, enter the bloodstream, and lodge in skeletal muscle, subcutaneous tissue, brain, and eye, where they develop into cysticerci — fluid-filled bladders of about 5 to 20 mm containing an invaginated scolex.
3. Neurocysticercosis (NCC). Cysticerci in the brain parenchyma, subarachnoid space, ventricles, spinal cord, or eye provoke disease when they degenerate and the host immune system recognises them — releasing antigen and eliciting inflammation, oedema, and seizures. Live (viable) cysticerci evade immunity and are often silent for years; calcified cysts are inactive but remain epileptogenic foci.^[4]

^[1]

Clinical Presentation

Most helminth infections are asymptomatic or pauci-symptomatic at low intensity; clinical disease is a function of worm burden, larval migration, tissue deposition of eggs, and host immune response. The syndromes below are organised by worm. ^[1]

Soil-transmitted helminths (Ascaris, hookworm, Trichuris)

Ascaris lumbricoides. Light infection: asymptomatic. Heavy infection produces three syndromes: (1) the migrating-larval phase (Loeffler syndrome) — at days 7 to 14, dry cough, wheeze, dyspnoea, low-grade fever, and transient pulmonary infiltrates with marked eosinophilia (resolves in 1 to 2 weeks); (2) established intestinal infection — non-specific abdominal pain, anorexia, nausea, occasionally malabsorption and failure to thrive in children; and (3) mechanical complications — acute small-bowel obstruction (a heavy burden balls up in the ileum, classically in a child with a palpable abdominal mass of worms), biliary migration (biliary colic, obstructive jaundice, ascending cholangitis), pancreatic duct migration (acute pancreatitis), and appendicitis. A worm may be vomited or passed per rectum, which is often the presenting complaint.^[1]

Hookworm. The classical presentation is insidious iron-deficiency anaemia in an endemic or returned-travel patient — pallor, fatigue, exertional dyspnoea, koilonychia, angular cheilitis, pica (geophagia), and in children, growth and cognitive impairment. Hypoalbuminaemia produces dependent oedema. At skin penetration, 'ground itch' (pruritic erythematous papule at the entry site) may be recalled. The migrating-larval phase can produce a mild Loeffler-like pneumonitis.^[1]

Trichuris trichiura. Light infection is asymptomatic. Heavy infection produces the Trichuris dysentery syndrome (TDS) — chronic dysentery, rectal prolapse, anaemia, poor growth, and clubbing of the fingers in children with heavy colonic worm burdens; TDS should be considered in endemic areas among children presenting with chronic bloody diarrhoea and anaemia, and failure to thrive with persistent anaemia can continue despite treatment. An estimated 1049 million people harbour T. trichiura, including 114 million preschool-age and 233 million school-age children.^[11][14]

Strongyloides stercoralis

Chronic strongyloidiasis (decades of auto-infection) is usually mild and non-specific — epigastric pain, intermittent diarrhoea alternating with constipation, nausea, urticaria, and 'larva currens' (a rapidly migrating, serpiginous, pruritic urticarial weal, typically perianal, buttock or trunk, that moves several centimetres per hour — pathognomonic and distinct from cutaneous larva migrans which moves only millimetres per day). Irritable-bowel-like symptoms are common.^[9]

Hyperinfection / disseminated disease (in the immunosuppressed) is a medical emergency: Gram-negative sepsis (often with E. coli, Klebsiella, Enterococcus, Bacteroides — the larvae carry them from the gut), pneumonia and ARDS, meningitis, ileus and abdominal distension, shock, and Gram-negative bacteraemia of gut origin without an obvious source. The clue is astonishing peripheral eosinophilia may be ABSENT in the immunosuppressed (paradoxically), but larvae are found in sputum, bronchoalveolar lavage, and stool in large numbers. Suspect it in any patient with HTLV-1, recent corticosteroids, transplant, or haematological malignancy who develops unexplained Gram-negative sepsis.^[9]

Enterobius vermicularis (threadworm / pinworm)

The classical presentation is nocturnal perianal pruritus in a child (the gravid female migrates out at night to lay eggs on the perianal skin), with restless sleep, irritability, and secondary excoriation or vulvovaginitis. Whole-household infection is the rule. Eosinophilia is absent (no tissue migration). ^[1]

Tapeworms (Taenia, Diphyllobothrium)

Intestinal taeniasis (adult worm) is usually asymptomatic or mild — vague abdominal discomfort, nausea, weight loss, and the distressing passage of motile proglottids per anus (the proglottid may crawl out onto underclothes — a pathognomonic complaint). The key clinical importance of T. solium taeniasis is that the carrier is at risk of auto-infection with cysticercosis and is the source of cysticercosis for household contacts. ^[1]

Neurocysticercosis (NCC). The presentation depends on cyst location, number, and viability: (1) parenchymal NCC — new-onset seizures (the commonest cause of adult-onset epilepsy in endemic regions), headache, focal deficits; (2) subarachnoid NCC — chronic meningitis, basal arachnoiditis, hydrocephalus, vasculitis and stroke; (3) intraventricular NCC — obstructive hydrocephalus with episodic intracranial-pressure spikes (Brun syndrome — positional vertigo and drop attacks); (4) spinal NCC — radicular pain, myelopathy; (5) ocular NCC — visual loss from intraocular cysts. Subcutaneous cysticerci present as firm, painless nodules over muscle.^[4]

Diphyllobothrium latum (fish tapeworm) is usually asymptomatic but can cause vitamin B12 deficiency (the worm competes for dietary B12 in the terminal ileum) — megaloblastic anaemia, glossitis, neuropathy, and rarely subacute combined degeneration of the cord. ^[1]

Schistosomiasis

• Acute schistosomiasis (Katayama syndrome). Most often in travellers after first exposure: sudden fever, malaise, myalgia, headache, eosinophilia, fatigue, and abdominal pain lasting 2 to 10 weeks. A treated traveller cohort presented at weeks 4 to 5 after infection.
• Chronic intestinal/hepatic schistosomiasis (S. mansoni/japonicum). Years after infection: abdominal pain, diarrhoea (± blood/mucus), hepatosplenomegaly, progressing to periportal fibrosis (Symmers), presinusoidal portal hypertension — splenomegaly, varices, ascites, hypersplenism (pancytopenia) — with preserved hepatic synthetic function (no encephalopathy, normal albumin/coagulation — the key distinction from decompensated cirrhosis). Death is from variceal haemorrhage.
• Urogenital schistosomiasis (S. haematobium). Terminal (end-of-stream) haematuria is the classical presentation, with dysuria, frequency, suprapubic pain, and, on examination, bladder-wall nodularity, sandy patches, and hydro-ureter/hydronephrosis from chronic granulomatous inflammation and fibrosis. Chronic disease leads to squamous cell carcinoma of the bladder (presents years later), infertility, and in women, genital lesions (worms in the cervix/vagina — 'female genital schistosomiasis').
• Neuroschistosomiasis. Ectopic egg deposition in the spinal cord (acute transverse myelitis, especially S. mansoni) or brain (seizures, focal deficit).^[3]

Lymphatic filariasis (Wuchereria, Brugia)

• Acute adenolymphangitis (ADLA). Recurrent episodes of fever, tender lymphadenitis, and lymphangitis (classically inguinal), often preceded by a 'filarial fever', lasting days. Repeated episodes scar and obstruct lymphatics.
• Chronic lymphatic disease. Lymphoedema of the limbs (usually lower, asymmetric) progressing to elephantiasis — massive, warty, weeping limb swelling with recurrent bacterial cellulitis (the skin fissures and is superinfected with streptococci). Hydrocoele (the commonest chronic manifestation in men), chylocele, chyluria (milky white urine from lymphatico-urinary fistula), and genital elephantiasis.
• Tropical pulmonary eosinophilia (TPE). A hypersensitivity reaction to microfilariae trapped in the lungs — nocturnal cough, wheeze, dyspnoea, marked eosinophilia, and miliary pulmonary infiltrates; microfilariae are absent from blood (they are trapped/dying in the lungs).^[7]

Onchocerciasis (river blindness) and loiasis

• Onchocerciasis. Subcutaneous nodules (onchocercomata) over bony prominences (iliac crest, greater trochanter, skull), pruritic papular dermatitis ('lizard skin', 'leopard skin' from depigmentation and atrophy), and ocular disease (punctate and sclerosing keratitis, anterior uveitis, chorioretinitis, optic atrophy) progressing to blindness. Microfilariae are in the skin and eye, not blood.
• Loiasis (African eye worm). Calabar swellings (transient 5 to 10 cm subcutaneous angio-oedematous swellings, often on the limbs) and the dramatic migration of an adult worm across the conjunctiva ('eye worm') — alarming but usually benign. ^[1]

Hydatid disease (Echinococcus granulosus)

Often asymptomatic for years until the cyst grows large enough to cause mass effect. Hepatic hydatid presents as a painless right-upper-quadrant mass, occasionally with biliary obstruction (jaundice, cholangitis). Pulmonary hydatid may cause cough, haemoptysis, or chest pain. Intraperitoneal rupture of a hepatic hydatid cyst has an incidence of up to 16 percent in some series and can result in anaphylaxis in up to 12.5 percent of ruptures — rupture can be fatal without surgery.^[15][6]

Atypical presentations (deliberately tested)

• Immunosuppressed patient with Gram-negative sepsis, ARDS, or meningitis — Strongyloides hyperinfection; larvae in sputum/stool; ivermectin urgently.
• Returned traveller from Africa with terminal haematuria — S. haematobium; urine for ova.
• Adult-onset seizures in a pork-endemic region — neurocysticercosis; CT/MRI brain.
• Chronic bloody diarrhoea and rectal prolapse in a child — Trichuris dysentery syndrome.
• Painless RUQ mass with a folded membrane on ultrasound — hydatid cyst.
• Iron-deficiency anaemia with eosinophilia in a returned traveller / barefoot child — hookworm. ^[1]

Differential Diagnosis

Because helminths cause diverse syndromes (eosinophilia, anaemia, diarrhoea, seizures, portal hypertension), the differential is syndrome-driven. ^[1]

The exam trap: cutaneous larva migrans (CLM) and larva currens are easily confused. CLM is zoonotic (dog/cat hookworm larvae in soil), produces an erythematous, serpiginous, pruritic track, and a typical regimen is albendazole 400 mg twice daily for three days. Larva currens is Strongyloides and requires systemic ivermectin. ^[29][9]

Clinical & Bedside Assessment

The bedside assessment is syndrome-driven and depends on the suspected worm, but every assessment should include a structured exposure history, growth assessment in children, and a search for the anaemia, eosinophilia, and organ-specific signs that betray tissue disease. ^[1]

History. Establish (1) residence or travel in a tropical endemic region (sub-Saharan Africa, India, Southeast Asia, Latin America, China); (2) the specific exposure — barefoot walking, open defecation, untreated water, consumption of undercooked pork, beef, freshwater fish, crab, or raw watercress, and freshwater swimming/wading (ask specifically about Lake Malawi, the Nile, Lake Victoria, the Mekong, and Philippines freshwater); (3) occupational exposure — sheep-farming and dogs (hydatid), abattoir work (Taenia), rice farming (filariasis, schistosomiasis); (4) the symptom tempo and pattern — perianal pruritus at night (Enterobius), terminal haematuria (S. haematobium), new seizures (NCC), chronic bloody diarrhoea and rectal prolapse (Trichuris), milky urine (chyluria in filariasis), a worm passed per mouth/rectum (Ascaris), Calabar swellings and an eye worm (Loa loa); and (5) immunosuppression — corticosteroids, HTLV-1, transplant, haematological malignancy (the trigger to screen for Strongyloides). ^[1]

Examination. Look for pallor (anaemia), koilonychia, angular cheilitis (hookworm iron deficiency); growth parameters — weight, height, mid-upper-arm circumference — in children (STH growth impairment); abdomen — hepatosplenomegaly (schistosomiasis, katayama, capillaria), mass (Ascaris bolus, hydatid cyst), tenderness (acute pancreatitis/cholangitis from Ascaris migration); skin — urticaria (acute helminthiasis), larva currens (Strongyloides), creeping eruption (CLM), subcutaneous nodules (onchocercomata, cysticerci, hydatid, Loa), 'leopard/lizard skin' (onchocerciasis); lymphatics — lymphoedema/elephantiasis of limbs and genitalia, hydrocoele (filaria); eyes — subconjunctival worm (Loa), chorioretinitis (onchocerciasis), intraocular cyst (cysticercosis); neurology — focal deficit, seizures, signs of raised intracranial pressure (NCC), acute transverse myelitis (neuroschistosomiasis); respiratory — Loeffler pneumonitis (wheeze, crackles, transient infiltrates), tropical pulmonary eosinophilia (TPE). Rectal examination for fresh blood/mucus (Trichuris dysentery, schistosomiasis) and rectal prolapse (Trichuris in children). ^[1]

Monitoring. In severe disease — Ascaris obstruction, Strongyloides hyperinfection, Katayama fever — serial observations (temperature, heart rate, blood pressure, respiratory rate, oxygen saturation, GCS), hourly urine output, and serial full blood count (haemoglobin, eosinophil trend) and organ function. ^[1]

Investigations

The diagnostic strategy is (a) parasitological (find the worm, eggs, or larvae), (b) immunological (antibody, antigen), (c) haematological/biochemical (eosinophilia, anaemia), and (d) imaging (for tissue disease). The choice of sample is species-specific and is the single most important practical decision. ^[1]

Parasitological diagnosis — stool, urine, and sellotape

Stool microscopy for ova, cysts, and parasites (OCP) is the cornerstone. Eggs of Ascaris, hookworm, Trichuris, Taenia, Schistosoma mansoni/japonicum, Fasciola, Clonorchis, and Fasciolopsis are shed in stool. Three samples on alternate days plus a concentration technique (formol-ether or zinc-sulphate flotation) improve yield over a single smear. The Kato-Katz thick smear quantifies eggs per gram and is used in mass-drug-administration programmes.^[1]

Immunological (serology/antigen)

Strongyloides serology is effective for screening but less sensitive in immunocompromise, and low larval output limits stool detection — pair serology with stool (agar-plate culture or Baermann). Schistosoma serology supports exposure in returned travellers but cannot distinguish species — eggs in stool or urine confirm active disease. Cysticercosis serology supports neurocysticercosis. Echinococcus serology supports hydatid diagnosis. Filarial antigen tests detect W. bancrofti without regard to nocturnal periodicity.^[9][10]

Haematology and biochemistry

• Eosinophilia is the screening red flag for tissue helminthiasis — marked in Katayama syndrome, fascioliasis, tropical pulmonary eosinophilia, and the migrating-larval phase; absent in established intraluminal gut infection and often paradoxically absent in Strongyloides hyperinfection in the immunosuppressed.
• Iron studies (low ferritin, low iron, high TIBC) confirm hookworm-related iron-deficiency anaemia.
• B12 and folate — megaloblastic picture in Diphyllobothrium.
• Liver function — preserved in schistosomal portal hypertension (normal synthetic function); cholestatic/obstructive in Ascaris biliary migration and Fasciola.
• Urinalysis — haematuria (macroscopic terminal, or dipstick micro) in S. haematobium.
• IgE — markedly elevated in tissue helminthiasis and TPE. ^[1]

Imaging

• Chest X-ray — transient pulmonary infiltrates (Loeffler), miliary pattern (TPE), 'water-lily' sign (collapsed hydatid cyst membrane floating in fluid), pleural effusion.
• Abdominal ultrasound — hydatid cyst (WHO-IWGE classification: CE1 active simple cyst with hydatid sand; CE2 active multivesicular; CE3a/CE3b transitional with detached membrane; CE4 inactive heterogeneous; CE5 inactive with calcified wall), periportal fibrosis (Symmers) and hepatosplenomegaly in schistosomiasis, Ascaris bolus (target sign, intra-biliary worms).
• CT / MRI brain — neurocysticercosis: vesicular (live cyst, CSF-density fluid, scolex — 'hole-with-dot' or 'starry sky'), colloidal vesicular (degenerating, ring-enhancing, oedema), granular nodular (shrinking, calcifying), calcified nodular (dense, inactive but epileptogenic). Hydrocephalus and subarachnoid cysts in extraparenchymal NCC.
• Endoscopic retrograde cholangiopancreatography (ERCP) — diagnostic and therapeutic for biliary Ascaris and biliary Fasciola. ^[1]

Special: the Strongyloides screen before immunosuppression

Every patient about to start corticosteroids (especially high-dose or prolonged), transplantation immunosuppression, biologicals, or haematological-malignancy chemotherapy, and every patient with HTLV-1, who has lived in or travelled to an endemic region, should be screened with Strongyloides serology plus stool (with agar plate culture where available) and treated with single-dose ivermectin 200 micrograms/kg before immunosuppression (multiple doses if already immunocompromised). This is among the most important and most overlooked reflexes in clinical medicine.^[9][10]

Management — Resuscitation

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Most helminth infections are outpatient, oral-therapy conditions, but several scenarios are emergencies. ^[1]

Ascaris intestinal obstruction. Obstruction has been estimated to occur in 2 per 1000 Ascaris-infected children per year, and most uncomplicated cases respond to conservative treatment: nil by mouth, nasogastric decompression, intravenous fluids, and correction of electrolytes (some centres add gastrografin through the nasogastric tube, which shortened resolution in a randomised paediatric trial). Surgery is reserved for refractory obstruction, peritonitis, perforation, or ischaemia — never for uncomplicated obstruction.^[31][30]

Strongyloides hyperinfection. This is a critical-care emergency: ICU admission, broad-spectrum empiric antibiotics covering gut-origin Gram-negatives and anaerobes, fluid resuscitation and vasopressors for shock, mechanical ventilation for ARDS, and urgent ivermectin 200 micrograms/kg orally, in multiple doses whose duration is based on disease severity and larval burden, until stool and sputum are repeatedly negative. Consider adding albendazole if infection is severe. Reduce or hold immunosuppression where possible. Case fatality remains more than 60 percent.^[9][10]

Ruptured hydatid cyst / anaphylaxis. Intraperitoneal rupture of a hepatic hydatid cyst is a serious complication (an incidence of up to 16 percent in some series) and can result in anaphylaxis in up to 12.5 percent of ruptures — rupture can be fatal without surgery. Manage anaphylaxis with prompt intramuscular adrenaline as first-line treatment (plus oxygen and IV fluids), obtain urgent surgical consultation for source control, and treat active cysts with albendazole.^[15][17][16]

Severe hookworm anaemia in pregnancy. Chronic hookworm infection causes iron-deficiency anaemia and a variety of pregnancy complications — treat the anaemia (iron supplementation, transfusion if clinically indicated). The WHO preventive-chemotherapy strategy targets women of reproductive age alongside children in endemic areas, delivering single-dose albendazole (400 mg) or mebendazole through mass drug administration.^[12][26][27]

Status epilepticus due to neurocysticercosis. Terminate seizures with an IV benzodiazepine first-line; if seizures progress to established (benzodiazepine-refractory) status epilepticus, the three comparator agents in the ESETT randomised trial — levetiracetam, fosphenytoin, or valproate — are the evidence-based second-line options. Add a corticosteroid to blunt the inflammatory response to dying cysts (the landmark neurocysticercosis trial used 6 mg dexamethasone daily), with definitive cysticidal therapy once seizures are controlled.^[18][5]

Katayama fever. Therapy currently relies on poorly consistent combinations of corticosteroids and praziquantel. In a treated cohort of travellers, early symptoms were suppressed with oral methylprednisolone 0.5 mg/kg once daily in cycles of three consecutive days until symptoms abated, with praziquantel deferred to weeks 7 to 8 after infection, once the acute reaction had settled.^[19]

Management — Definitive & Stepwise

The choice of antihelminthic is species-specific and not interchangeable. The benzimidazoles (albendazole, mebendazole) are first-line for STH; praziquantel for cestodes and trematodes; ivermectin for Strongyloides and onchocerciasis; diethylcarbamazine (DEC) for lymphatic filariasis and loiasis (with care). ^[1]

Anthelmintic formulary — drug, dose, route, rationale

^{[27] [20] [5] [24] [23] [25] [28] [8]}

Neurocysticercosis — the management algorithm

Management of NCC is sub-type specific and depends on cyst location, viability, and number:^[4][5]

1. Antiepileptics. Seizures are the dominant manifestation — in the landmark randomised trial, patients with viable cysts and seizures were managed on antiepileptic drugs while receiving antiparasitic therapy.^[5]
2. Corticosteroids. Given alongside cysticidal therapy to blunt the inflammatory response to dying parasites — 6 mg dexamethasone daily in the trial regimen.^[5]
3. Cysticidal therapy for viable parenchymal cysts. Albendazole 800 mg daily with dexamethasone 6 mg daily for 10 days produced a significant 67 percent reduction in seizures with generalisation and resolved more intracranial cysts than placebo; published albendazole schedules span 8 to 30 days (8 days sufficed for most in a dose-finding series), an Indian randomised trial of 15 mg/kg/day for 7 days did not change the natural course of parenchymal NCC. Combined albendazole + praziquantel may improve parasite clearance by raising albendazole-sulfoxide concentrations.^{[5][20][21][22]}
4. Individualised treatment. Management accounts for parenchymal versus extraparenchymal involvement, the number and form of parasites, and the extent of degeneration and associated inflammation — the basis for choosing antiparasitic therapy, surgery, or both.^[4]

Public-health (mass drug administration) therapy

The WHO preventive chemotherapy strategy targets at-risk populations rather than individuals: ^[1]

• STH — the current strategy is annual or twice-annual preventive chemotherapy, typically school-based deworming targeting children and women of reproductive age in endemic regions, using single-dose albendazole (400 mg) or mebendazole.^[26][27]
• Schistosomiasis — praziquantel 40 mg/kg in a single dose is the WHO-recommended treatment for all forms of schistosomiasis.^[23]
• Lymphatic filariasis — annual mass drug administration with diethylcarbamazine plus albendazole (DA), or the WHO-recommended triple-drug regimen ivermectin-DEC-albendazole (IDA) introduced in India from 2018.^[28]
• Strongyloides — the 2024 WHO guideline conditionally recommends mass drug administration with single-dose ivermectin (200 micrograms/kg, oral) in endemic settings where Strongyloides stercoralis prevalence is 5 percent or higher, for all age groups from 5 years and older.^[10]

Specific Subtypes & Scenarios

• Neurocysticercosis — covered above; management is individualised to cyst location, number, viability, and associated inflammation; seizures are the dominant presentation of parenchymal disease.^[4]
• Strongyloides hyperinfection — covered above; the cardinal reflex is to screen before immunosuppression.
• Acute schistosomiasis (Katayama fever) — corticosteroid cycles to suppress early symptoms, with praziquantel deferred until the acute reaction settles.^[19]
• Cutaneous larva migrans — zoonotic hookworm larvae from soil contaminated by dogs and cats, producing an erythematous, serpiginous, pruritic eruption; a typical regimen is albendazole 400 mg twice daily for three days.^[29]
• Hydatid disease — covered above; management options are surgery, percutaneous treatment, albendazole for active cysts, and watch-and-wait for uncomplicated inactive cysts, staged by the WHO Informal Working Group on Echinococcosis (WHO-IWGE) ultrasound classification; never spill a cyst (anaphylaxis risk).^[16]
• Lymphatic filariasis — covered above; annual mass drug administration with the DA or IDA regimens is the elimination strategy.^[28]
• Diphyllobothrium latum — fish tapeworm; treat with single-dose praziquantel (efficacious for adult cestodes in clinical series).^[24]
• Enterobius vermicularis — threadworm; treat the whole household, and combine drug therapy with hygiene measures (morning shower, washing bedding and nightclothes, nail care) to prevent re-infection.
• Dracunculus medinensis (Guinea worm) — on the verge of eradication; emerges through the skin (usually the foot); extract by slow winding on a stick over days to weeks; no drug is effective; metronidazole may ease extraction.
• Trichinella spiralis — undercooked pork/bear; larvae encyst in muscle; fever, periorbital oedema, myalgia, eosinophilia; severe systemic disease is treated with corticosteroids plus antiparasitic therapy.
• Fasciola hepatica — sheep liver fluke from contaminated water or aquatic vegetables; acute then chronic biliary phases; treat with triclabendazole 10 mg/kg (WHO single dose; CDC two doses 12 hours apart) — not praziquantel.^[25]

Complications & Pitfalls

The cardinal pitfalls in helminth diagnosis and management: ^[1]

1. Failure to screen for Strongyloides before immunosuppression — the most important and most preventable fatal error in helminthology. Screen any patient from (or who has lived in) an endemic region with serology before corticosteroids, transplant, biologicals, or chemotherapy.^[9]
2. Aspirating a hydatid cyst without albendazole cover — risks anaphylaxis and spillage. Use peri-interventional albendazole chemoprophylaxis with PAIR or surgery; uncomplicated inactive CE4/CE5 cysts are followed, not operated.
3. Over-reliance on a single stool sample — conventional microscopy misses light infection. Repeat with concentration; three samples on alternate days.
4. Assuming eosinophilia must be present — it is absent in established intraluminal gut infection (Ascaris, Enterobius, intraluminal taeniasis) and paradoxically absent in Strongyloides hyperinfection in the immunosuppressed.
5. Confusing taeniasis (gut adult worm) with cysticercosis (tissue larval cysts) — both are caused by T. solium; eating undercooked pork causes taeniasis, ingesting eggs (or auto-infection) causes cysticercosis. A taeniasis carrier must be treated to prevent auto-infection and household spread.
6. Using praziquantel for Fasciola — Fasciola hepatica is resistant; use triclabendazole.

Prognosis & Disposition

Most treated helminth infections have an excellent prognosis with species-specific anthelmintics, and most tissue infections respond to single-dose or short-course therapy. The major exceptions and high-mortality scenarios are: ^[1]

• Strongyloides hyperinfection — case fatality more than 60 percent; survivors need prolonged treatment until stool/sputum are repeatedly negative.
• Neurocysticercosis with hydrocephalus or subarachnoid disease — significant morbidity; calcified parenchymal lesions remain epileptogenic and may require lifelong antiepileptics.
• Advanced schistosomal periportal fibrosis with portal hypertension — irreversible; risk of variceal haemorrhage is the dominant determinant of survival. Early disease is reversible with praziquantel.
• Ruptured hydatid cyst with anaphylaxis — surgical and ICU emergency; rupture can be fatal without surgery.
• Bladder squamous cell carcinoma complicating chronic S. haematobium — presents years to decades after infection; prognosis is that of bladder cancer. ^[1]

Disposition is usually outpatient with oral therapy and follow-up stool/serology at 2 to 4 weeks. Admit for Ascaris obstruction/migration, Strongyloides hyperinfection, ruptured hydatid, Katayama fever, neurocysticercosis with raised ICP or status epilepticus, and severe anaemia (especially in pregnancy). All patients with tissue disease need repeat stool/serology to confirm cure and re-treatment if eggs persist at 2 to 4 weeks. ^[1]

Special Populations

• Children in endemic areas — the highest-burden group: STH infect over 1.5 billion people and are associated with anaemia and stunting. School-based deworming via mass drug administration with albendazole or mebendazole, recommended by WHO, delivers single-dose therapy to at-risk school-aged and preschool children to reduce morbidity.^[27][26]
• Pregnancy — chronic hookworm infection causes iron-deficiency anaemia and a variety of pregnancy complications, so treat the anaemia (iron supplementation, transfusion if clinically indicated); WHO preventive chemotherapy explicitly targets women of reproductive age in endemic areas.^[12][26]
• Returned traveller / unexplained eosinophilia — clinicians should consider STH infection in individuals living in or returning from endemic regions; take a structured travel, food, water, and freshwater-exposure history and request stool ova and parasites plus species-specific serology as guided by exposure.^[1]
• Before immunosuppression (corticosteroids, transplantation, biologicals, chemotherapy) — host immunosuppression can trigger catastrophic, fatal hyperinfection/dissemination (case fatality more than 60 percent): screen for Strongyloides and treat with ivermectin BEFORE immunosuppression — single dose for stable disease, multiple doses in the immunocompromised.^[10][9]

^{[9] [16] [28] [26]}

Evidence, Guidelines & Regional Differences

^{[27] [11] [10] [5] [23]}

Key guideline changes and landmark evidence: ^[1]

• WHO 2021–30 neglected tropical disease roadmap — now incorporates strongyloidiasis among public-health control targets, alongside existing STH programmes.^[10]
• WHO 2024 strongyloidiasis guideline — WHO's first guideline on public health control of strongyloidiasis: a single conditional recommendation for mass drug administration with single-dose ivermectin (200 micrograms/kg, oral) in endemic settings with prevalence of 5 percent or higher, ages 5 years and older.^[10]
• Garcia 2004 NEJM trial — randomised placebo-controlled trial showing albendazole reduces seizures and accelerates cyst resolution in neurocysticercosis; established cysticidal therapy as standard of care for viable parenchymal NCC.^[5]
• Taylor 2005 Lancet / Hoerauf — doxycycline kills Wolbachia, the obligate intracellular endosymbiont of filarial worms, sterilising adult worms (macrofilaricidal effect) and reducing lymphatic inflammation — a paradigm-shifting addition to filariasis therapy.^[8]
• WHO-IWGE hydatid classification (CE1 to CE5) — guides whether to use PAIR, surgery, or 'watch and wait' for cystic echinococcosis, replacing the older Gharbi classification.
• The Wolbachia story is one of the great translational triumphs of tropical-medicine research — targeting a bacterial endosymbiont to treat a worm infection.

Regional empirical differences: ^[1]

• India — lymphatic filariasis elimination uses annual DEC plus albendazole (DA), with the WHO-recommended triple regimen (ivermectin + DEC + albendazole, IDA) introduced from 2018 and implemented in 63 endemic districts by December 2023; DeWorm3 evaluated community-wide albendazole 400 mg MDA alongside school-based deworming.^[28][26]
• WHO global — school-based deworming with albendazole or mebendazole for STH; praziquantel 40 mg/kg for schistosomiasis; and, from 2024, conditional ivermectin 200 micrograms/kg MDA for strongyloidiasis where stool prevalence is 5 percent or higher.^[27][23][10]

Exam Pearls

^{[27] [10] [20] [16] [23] [24] [1] [1]}

Ward-round test — three stems, thirty seconds each

Stem 1 — the returned traveller with eosinophilia (answer)ShowHide

The man from the top of the topic: six years out of Bangladesh, eosinophils 1.2 x10^9/L, and a steroid prescription waiting. What is the first investigation, and what is the one infection you must exclude before he takes a single prednisolone tablet? Model: The eosinophilia is a tissue-helminth marker, so start a stool ova, cysts and parasites examination (at least three samples, with concentration) plus species-specific serology for the tissue stages. But the infection you must exclude before any immunosuppression is Strongyloides stercoralis — it is the only common worm that auto-infects, so carriage persists for decades after leaving an endemic region, and steroids ignite fatal hyperinfection (Gram-negative sepsis, ARDS, meningitis, case fatality more than 60 per cent). Stool OCP alone is insensitive — add an agar-plate culture and Strongyloides serology, and treat with single-dose ivermectin 200 micrograms/kg (not albendazole first-line) before the steroids begin. HIV is not the major risk here; corticosteroids and HTLV-1 are.^[1][9][10]

Stem 2 — the immunosuppressed patient with Gram-negative sepsis (answer)ShowHide

A 60-year-old on high-dose prednisolone for three weeks, originally from Southeast Asia, is admitted with E. coli bacteraemia, ARDS and meningitis, and is found to have larvae in his sputum. What happened, what was not done, and what is the treatment? Model: This is Strongyloides hyperinfection — the auto-infection cycle accelerated out of control when cell-mediated immunity was suppressed, and the migrating larvae carried gut bacteria into the bloodstream and meninges. The eosinophil count is often paradoxically absent in the immunosuppressed, so do not be reassured by a normal eosinophil. The preventable error upstream was not screening for Strongyloides before the steroids — serology and stool agar-plate culture should precede any prolonged corticosteroid, transplant or HTLV-1-related immunosuppression. Treat now with oral ivermectin 200 micrograms/kg in multiple doses (duration based on severity and larval burden) until stool and sputum are repeatedly negative, add broad-spectrum antibiotics for the Gram-negative sepsis, support in ICU, and consider adding albendazole if infection is severe.^[9][10]

Stem 3 — new-onset seizures in a pork-endemic region (answer)ShowHide

A 30-year-old from a region where pork is eaten undercooked has his first-ever generalised seizure. CT brain shows several small ring-enhancing lesions, one with a bright dot inside it. What is the diagnosis, and why does the drug choice depend on whether the lesion is viable? Model: This is neurocysticercosis — the main cause of adult-onset seizures in the developing world. Swallowing T. solium eggs (from a carrier or by auto-infection — not from eating cysticerci in pork) releases oncospheres that lodge in the brain and form cysticerci; seizures arise when cysts degenerate and provoke inflammation. A viable cyst is the target of cysticidal therapy: the landmark randomised trial gave albendazole 800 mg daily with dexamethasone 6 mg daily for 10 days, producing a significant two-thirds reduction in seizures with generalisation and resolving more intracranial cysts than placebo — so treat with albendazole plus a corticosteroid and antiepileptics, individualised to cyst number, form, and location. Eating undercooked pork causes the adult tapeworm (taeniasis); swallowing eggs causes cysticercosis.^[4][5]

References31ShowHide

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