Strongyloidiasis and soil-transmitted helminths

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Strongyloides stercoralis is unique among common soil-transmitted helminths because larvae can mature within the host and reinvade bowel or perianal skin. This autoinfective cycle sustains infection without repeat exposure and can accelerate catastrophically when corticosteroids or impaired cellular immunity allow uncontrolled larval multiplication.

Ascaris lumbricoides adults occupy small bowel and can form an obstructing mass or migrate into biliary and pancreatic ducts. Trichuris trichiura adults embed in colonic mucosa, causing dysentery and rectal prolapse when burden is high. Necator americanus and Ancylostoma duodenale hook onto small-bowel mucosa and cause chronic occult blood loss.

Exposure history should cover birth and residence as well as holidays, barefoot soil contact, sanitation, farming, military service and remote past travel. The major preventable error is focusing only on recent journeys before immunosuppression; autoinfective Strongyloides may remain clinically silent for many decades.

The wider investigation of eosinophilia requires assessment for schistosomiasis, filariae, toxocariasis, drugs, allergy and malignancy. The focus here is differentiating Strongyloides larvae from egg-producing intestinal nematodes and matching diagnostic sensitivity, urgency and drug choice to that biology.

Key points

Strongyloides larvae penetrate skin, migrate through lungs and mature in small bowel; internal autoinfection allows untreated infection to persist for life.
Chronic disease may be silent or cause abdominal pain, diarrhoea, urticaria, wheeze and rapidly moving serpiginous larva currens, often with intermittent eosinophilia.
Before substantial steroids, biologics, chemotherapy, transplantation or HTLV-1-associated immune change, screen anyone with plausible tropical or subtropical soil exposure.
First-line chronic Strongyloides assessment uses serology plus sensitive repeated stool testing such as concentration, Baermann, agar culture or PCR; ordinary single microscopy has poor sensitivity.
Hyperinfection diagnosis depends on abundant larvae in stool, sputum or other fluids, but treatment must not wait when epidemiology and critical illness are compelling.
Ivermectin 200 micrograms/kg orally once daily for one to two days is the treatment of choice for uncomplicated chronic strongyloidiasis under a specialist plan.
For hyperinfection, give ivermectin 200 micrograms/kg once daily until stool and sputum remain negative for two weeks; non-oral routes require specialist regulatory access.
Ascaris and Trichuris eggs are ingested from faecally contaminated soil or food; hookworm larvae penetrate skin and chronic blood feeding causes iron deficiency.
UK-licensed mebendazole treatment for ascariasis, trichuriasis and hookworm is 100 mg orally twice daily for three consecutive days in adults and children over two years.
Prevention combines footwear, sanitation, hand and food hygiene and population deworming in endemic programmes; individual UK travellers do not take routine prophylactic anthelmintics.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Strongyloides skin entry

Infective filariform larvae in contaminated soil penetrate bare skin, migrate through lungs and mature into parthenogenetic females within small bowel.

Ascaris and Trichuris ingestion

Embryonated eggs reach humans on hands, soil, food or water contaminated by faeces where sanitation is inadequate.

Hookworm skin entry

Necator and Ancylostoma larvae penetrate feet or other exposed skin before pulmonary migration and attachment to small-bowel mucosa.

Repeated endemic exposure

Poor sanitation, barefoot work and contaminated crops sustain high burdens, with children and agricultural communities experiencing cumulative nutritional effects.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Strongyloides autoinfection
Rhabditiform larvae transform into infective forms before leaving the host, penetrate bowel or perianal skin and restart migration indefinitely.
2
Accelerated larval migration
Corticosteroids and impaired cellular immunity amplify autoinfection, flooding gut and lungs with larvae and permitting dissemination to other organs.
3
Bacterial translocation
Migrating larvae disrupt bowel integrity and transport enteric organisms, causing bacteraemia, pneumonia, meningitis and septic shock.
4
Mechanical worm burden
Numerous adult Ascaris entangle within small bowel or enter hepatobiliary ducts, producing obstruction, perforation, cholangitis, cholecystitis or pancreatitis.
5
Blood and mucosal loss
Hookworms consume blood at attachment sites, while Trichuris damages colonic mucosa, producing iron deficiency, protein loss and dysentery.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Chronic strongyloidiasis

Intermittent abdominal pain, diarrhoea, nausea, wheeze or urticaria may occur, but many infected people remain asymptomatic for years.

Larva currens

A rapidly advancing, itchy, linear or serpiginous rash over buttocks, perineum or trunk reflects autoinfective larvae and is highly suggestive.

Hyperinfection lungsRed flag

Cough, wheeze, hypoxaemia, haemoptysis and diffuse pulmonary infiltrates in an immunosuppressed host can represent massive accelerated larval migration.

Translocated sepsisRed flag

Enteric bacteria carried across damaged bowel produce Gram-negative bacteraemia, polymicrobial infection or meningitis and contribute heavily to mortality.

Ascaris complicationsRed flag

Colicky pain, distension, vomiting or visible worms suggests heavy bowel burden; right-upper-quadrant pain, jaundice or pancreatitis indicates migration.

Hookworm anaemia

Fatigue, pallor, exertional breathlessness, pica and iron deficiency follow chronic intestinal blood loss, especially with repeated endemic exposure.

Trichuris colitis

Heavy whipworm infection causes chronic bloody diarrhoea, tenesmus, anaemia, growth impairment and sometimes rectal prolapse.

Red flags requiring action

Corticosteroids can precipitate fatal Strongyloides hyperinfection even decades after exposure; assess risk before the first dose whenever the clinical situation permits.
Eosinophilia may disappear during hyperinfection, so a normal count in an immunosuppressed deteriorating patient is not reassuring.
Unexplained Gram-negative bacteraemia, meningitis or polymicrobial sepsis with pulmonary or gastrointestinal symptoms should prompt an urgent search for disseminated larvae.
Ileus, gastrointestinal bleeding, hypoxaemia, diffuse infiltrates or respiratory failure indicates high-burden Strongyloides disease requiring inpatient daily treatment.
Severe abdominal pain, vomiting and obstruction can follow a heavy Ascaris burden; biliary migration may cause cholangitis or pancreatitis.
Profound iron-deficiency anaemia, rectal prolapse or growth and nutritional impairment suggests heavy hookworm or Trichuris infection and needs complication care as well as deworming.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
First-line FBC and exposure historyFirst stepFirst line
Why
Identify eosinophilia, anaemia and epidemiological risk while judging urgency.
Interpretation and limitations
Persistent eosinophilia supports tissue-invasive helminths but may fluctuate and is often absent in Strongyloides hyperinfection; microcytic anaemia supports heavy hookworm or Trichuris disease.
02
Strongyloides IgG serology
Why
Screen for chronic low-burden infection before immunosuppression or when stool sensitivity is limited.
Interpretation and limitations
A positive result supports infection but can cross-react with other helminths and may remain reactive after cure; immunosuppression can reduce sensitivity.
03
Repeated specialist stool testing
Why
Demonstrate Strongyloides larvae and diagnose egg-producing intestinal nematodes.
Interpretation and limitations
Send at least three separate specimens for concentration plus Baermann, agar-plate culture or PCR where available; explicitly request Strongyloides because routine microscopy can miss larvae.
04
Urgent sputum larval microscopy
Why
Confirm pulmonary hyperinfection in a deteriorating exposed patient.
Interpretation and limitations
Numerous larvae in sputum, bronchoalveolar fluid or stool confirm accelerated migration; absence in one specimen does not justify delaying treatment.
05
Blood and CSF cultures
Why
Detect bacterial translocation complicating disseminated Strongyloides infection.
Interpretation and limitations
Enteric Gram-negative or polymicrobial growth is a major clue; use sepsis-directed antimicrobial susceptibility results while treating the underlying larvae.
06
Stool ova identification
Why
Separate Ascaris, hookworm and Trichuris and estimate whether burden is substantial.
Interpretation and limitations
Egg morphology identifies genus, though hookworm eggs do not reliably distinguish species; concentration and repeated sampling improve yield after light infection.
07
Complication-directed imaging
Why
Assess obstruction, biliary migration, pancreatitis or pulmonary disease.
Interpretation and limitations
Abdominal radiography or CT assesses obstruction; ultrasound or MRCP investigates ducts; chest imaging maps infiltrates but cannot distinguish larvae from bacterial pneumonia alone.
08
Pre-treatment modifier assessment
Why
Prevent ivermectin neurotoxicity and choose safe anthelmintic therapy.
Interpretation and limitations
Check pregnancy, weight, liver function, interacting medicines and exposure to Loa loa-endemic Central or West Africa; high Loa microfilaraemia requires specialist quantification before ivermectin.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Schistosomiasis

Freshwater exposure, haematuria, bowel bleeding or portal disease with eosinophilia suggests schistosome eggs rather than intestinal nematode larvae.

Drug or allergic disease

Medication reactions, asthma and atopy cause eosinophilia and rash but do not explain characteristic larvae or ova on parasite testing.

Inflammatory bowel disease

Chronic diarrhoea, bleeding and anaemia overlap with Trichuris; endoscopy, histology and repeated stool tests separate causes.

Bacterial pneumonia or sepsis

Respiratory failure and bacteraemia may be primary infection or secondary to Strongyloides; both processes can coexist and need treatment.

Eosinophilic malignancy

Clonal haematological disease remains possible when eosinophilia is extreme, persistent or accompanied by abnormal film, splenomegaly or constitutional symptoms.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01BEFORE IMMUNITYScreen before immunosuppressionFirst stepSteroids, biologics, chemotherapy or transplantation are planned in someone with plausible endemic soil exposure.

1Take a lifetime country and rural-soil history rather than limiting questions to recent travel, and inspect for larva currens or unexplained gastrointestinal symptoms.
2Request Strongyloides serology and sensitive repeated stool testing where time allows; document eosinophils but do not use a normal count to end assessment.
3If urgent high-dose steroids cannot wait and exposure risk is material, discuss empirical ivermectin with infection specialists while samples are obtained.
4Assess Loa loa exposure, pregnancy, weight and medicine access before ivermectin; define how cure or suppression will be confirmed during immune therapy.

02HYPERINFECTIONStop accelerating autoinfectionAn exposed immunosuppressed patient develops severe respiratory, gastrointestinal or Gram-negative septic illness.

1Admit to monitored care, send repeated stool and respiratory samples for larvae, obtain blood cultures and assess organ failure without waiting for eosinophilia.
2Give ivermectin 200 micrograms/kg once daily through a reliably absorbed route; use specialist subcutaneous or rectal access only when oral absorption is impossible.
3Reduce corticosteroid or other immunosuppression as rapidly as the underlying disease safely permits and treat bacterial sepsis using local severe-infection guidance.
4Continue daily ivermectin until clinical improvement and stool and sputum examinations remain negative for at least two weeks.

03CHRONICTreat uncomplicated StrongyloidesSerology or larval tests support infection without hyperinfection, dissemination or inability to absorb tablets.

1Check for planned immunosuppression, HTLV-1 risk, pregnancy and Loa loa exposure because these alter urgency, safety and confirmation strategy.
2Give ivermectin 200 micrograms/kg orally once daily for one to two days through a tropical specialist or locally governed imported-medicine pathway.
3Repeat treatment or extend the course when immunosuppression, persistent larvae or inadequate response makes a short regimen insufficient.
4Reassess symptoms, eosinophils, stool and serology on a planned timetable; a falling antibody level supports response but is not an immediate binary test-of-cure.

04STHTreat egg-producing nematodesStool testing or a convincing complication identifies Ascaris, Trichuris or hookworm infection.

1Assess worm burden and correct iron deficiency, dehydration or malnutrition; obtain surgical, biliary or pancreatic review before giving tablets during an acute obstruction syndrome.
2For uncomplicated adult ascariasis, trichuriasis or hookworm, use licensed mebendazole 100 mg orally twice daily for three consecutive days when suitable.
3AlternativeChoose an alternative such as albendazole through specialist advice when species, pregnancy, age, resistance or availability makes mebendazole unsuitable.
4Repeat stool testing after heavy or persistent infection and investigate ongoing anaemia or bowel symptoms for treatment failure, reinfection or another diagnosis.

05PREVENTBreak soil transmissionA traveller or community has continuing exposure to faecally contaminated soil.

1Promote safe disposal of faeces, reliable latrines, handwashing and washing or cooking produce, because Ascaris and Trichuris eggs are ingested.
2Wear footwear on potentially contaminated ground to reduce hookworm and Strongyloides larval skin penetration and avoid bare-skin contact with sewage.
3Use WHO population deworming only within endemic public-health programmes based on prevalence and age criteria, not as an improvised traveller prescription.
4Before future immune suppression, retain the exposure and treatment record because reinfection and incomplete clearance may renew hyperinfection risk.

Key medicines and prescribing safety4 treatments · regimens, roles and cautions

Treatment of choice for chronic Strongyloides infection and prevention of future hyperinfection.

Ivermectin for uncomplicated strongyloidiasis

Give 200 micrograms/kg orally once daily for one to two consecutive days, with extension or repeat dosing defined by immune status, response and specialist follow-up.

Discuss possible Loa loa exposure before use, avoid casual prescribing in pregnancy or below 15 kg, check interactions and ensure oral absorption is reliable.

Emergency parasite clearance during accelerated autoinfection or disseminated strongyloidiasis.

Ivermectin for hyperinfection

Give 200 micrograms/kg once daily and continue until the patient improves and stool and sputum remain negative for larvae for at least 14 consecutive days.

Ileus can prevent oral absorption; specialist teams may obtain unlicensed non-oral formulations. Reduce immunosuppression, treat bacterial sepsis and monitor neurological status closely.

UK-licensed broad intestinal anthelmintic for uncomplicated roundworm, whipworm and hookworm infection.

Mebendazole

Give 100 mg orally twice daily for three consecutive days for adult ascariasis, trichuriasis or hookworm infection, using the licensed tablet or suspension instructions.

Avoid routine first-trimester use without individual assessment, review significant liver disease and interactions, and do not rely on mebendazole for Strongyloides hyperinfection.

Replaces chronic blood loss from hookworm or severe Trichuris infection after effective deworming.

Oral iron replacement

Use a formulary oral elemental-iron regimen and continue for approximately three months after haemoglobin normalises, adjusting preparation and frequency for tolerance and response.

Confirm iron deficiency, investigate disproportionate or persistent blood loss, separate from interacting medicines and use intravenous iron only for a defined indication.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Disseminated strongyloidiasis

Larvae invade lungs, central nervous system and other organs, causing respiratory failure, haemorrhage, encephalopathy and very high mortality.

Enteric septic shock

Barrier disruption and larval carriage produce severe Gram-negative or polymicrobial bloodstream infection, meningitis and multiorgan failure.

Intestinal or duct obstruction

Heavy Ascaris burdens obstruct small bowel or migrate into biliary and pancreatic ducts, causing perforation, cholangitis or pancreatitis.

Severe iron deficiency

Chronic hookworm blood loss causes symptomatic anaemia, impaired pregnancy outcomes, reduced exercise tolerance and developmental effects in children.

Trichuris dysentery syndrome

Heavy colonic infection causes persistent bloody diarrhoea, tenesmus, protein loss, growth impairment, anaemia and rectal prolapse.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

In hyperinfection, examine stool and sputum repeatedly while tracking respiratory support, ileus, cultures, renal and liver function and response to antibacterial treatment.
After chronic Strongyloides therapy, reassess clinical features and eosinophils, then repeat serology at a specialist-defined interval because antibody decline takes months.
Use repeat sensitive stool testing when larvae were previously demonstrated, immune suppression continues or serological response is inadequate.
After hookworm treatment, repeat FBC and ferritin to confirm marrow recovery and replenish stores; investigate ongoing anaemia rather than prescribing indefinite iron.
After heavy Trichuris or Ascaris infection, verify bowel recovery and repeat stool testing when symptoms, exposure or high burden creates treatment-failure risk.
Record the species, regimen, response and future steroid warning prominently so remote past infection is not missed during later emergency care.
Offer prevention counselling matched to route: footwear for skin-penetrating larvae and sanitation plus food hygiene for faecally transmitted eggs.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Autoinfection changes everything

Strongyloides does not need repeated environmental exposure to persist, making a decades-old residence history relevant before steroids today.

Eosinophils can vanish

The sickest hyperinfection patients may lack eosinophilia because corticosteroids and overwhelming disease suppress the usual signal.

One stool sample underperforms

Larval output is intermittent and routine concentration is insensitive; repeated Baermann, culture or PCR materially improves chronic Strongyloides detection.

Bacteria reveal the parasite

Unusual enteric bacteraemia or meningitis can result when migrating larvae carry bowel organisms across injured mucosa into blood and tissues.

Worm biology predicts symptoms

Ascaris obstructs or migrates, hookworm bleeds, Trichuris inflames colon and Strongyloides autoinfects; management follows that distinction.

Scope boundary

These intestinal nematodes are developed here; schistosomiasis, filariasis and the full non-infectious eosinophilia differential require their own targeted evaluation.

11Common pitfallsFrequent interpretation and management errors.

01
Do not give high-dose corticosteroids without considering lifelong Strongyloides risk in anyone who has lived in an endemic setting.
02
Do not exclude hyperinfection because eosinophils are normal; look for larvae and the combined respiratory, gastrointestinal and Gram-negative septic phenotype.
03
Do not send one routine stool microscopy request and assume a negative result excludes Strongyloides; request sensitive repeated methods explicitly.
04
Do not give ivermectin before assessing substantial Loa loa exposure, because high microfilaraemia can lead to severe neurological reactions.
05
Do not treat acute Ascaris bowel or biliary obstruction as an uncomplicated outpatient worm infection; obtain surgical or endoscopic assessment first.
06
Do not assume WHO single-dose population deworming and individual UK treatment are interchangeable; use the licensed or specialist species-specific course.
07
Do not stop follow-up when eosinophils fall if immune suppression continues or larvae were previously demonstrated.