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Health condition · Clinically reviewed

Ascariasis, roundworm infection in returning travellers - and how it’s treated.

Rare in the UK, common globally - a single course of albendazole clears most infections, and stool testing gives a clear answer.

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Why trust this guide

  • 01

    Clinically reviewed

    Written by our editorial team and reviewed by a UK-registered clinician with tropical medicine experience before publication.

  • 02

    Sourced from guidance

    Checked against WHO, UKHSA, CDC and peer-reviewed tropical medicine sources you can see at the end.

  • 03

    Current for 2026

    Reflects modern UK guidance on returning travellers, anthelmintic dosing and complication management.

Key facts

Ascariasis at a glance.

The essentials, in plain English - what the worm is, where it comes from, and how it’s treated in the UK today.

  • What it is

    Intestinal infection with Ascaris lumbricoides - the largest human roundworm, 15 to 35 cm long as an adult.

  • Where it lives

    Endemic across the tropics and subtropics - Asia, Africa and Latin America - anywhere sanitation is limited.

  • Global burden

    Around 800 million people infected worldwide according to WHO - the most common human helminth.

  • In the UK

    Rare and almost always imported - seen in returning travellers, migrants and children adopted from endemic regions.

  • How it spreads

    Faecal-oral - eggs on unwashed produce or in contaminated soil are swallowed, then hatch in the small intestine.

  • First-line treatment

    A single 400 mg dose of albendazole clears most infections - alternatives include mebendazole and pyrantel.

Why this guide matters

A tropical infection that walks into UK clinics.

Ascariasis is uncommon in the UK but easy to miss in a returning traveller with a cough, a raised eosinophil count or vague abdominal pain.

  • Think of it in returning travellers

    Cough plus eosinophilia after time in Asia, Africa or Latin America - Loeffler’s syndrome from Ascaris migration is a classic pattern.

  • Stool microscopy gives the answer

    Ascaris eggs are large and unmistakable - one or two stool samples usually settle the diagnosis.

  • Treatment is short and effective

    A single 400 mg dose of albendazole clears the great majority - complications need hospital-based care, not just tablets.

How the diagnosis is made

From travel history to a clear plan.

The steps a UK GP, infection specialist or tropical medicine clinic will typically follow, in order.

  1. 01

    Assessing

    Travel and exposure history

    Time spent in endemic regions, contact with contaminated soil or food, and duration since return - the single most useful step.

  2. 02

    Assessing

    Symptom review

    Cough and wheeze during larval migration, abdominal pain, malabsorption, growth failure in children, or an adult worm passed per rectum, mouth or nose.

  3. 03

    Assessing

    Full blood count

    Peripheral eosinophilia is common during the pulmonary migration phase and often persists in intestinal disease.

  4. 04

    Confirming

    Stool microscopy for ova

    The gold standard - Ascaris eggs are large and easily identified. Repeat samples if the index of suspicion is high.

  5. 05

    Confirming

    Chest X-ray if respiratory

    Transient migratory pulmonary infiltrates support a diagnosis of Loeffler’s syndrome during larval passage through the lungs.

  6. 06

    Confirming

    Abdominal imaging when needed

    Ultrasound or CT for suspected biliary migration, pancreatitis or intestinal obstruction - adult worms are often directly visible.

  7. 07

    Preparing

    Screen for co-infections

    Test for hookworm, Trichuris and Strongyloides - polyparasitism is common in returning travellers with eosinophilia.

Typical timeline: from clinic visit to cleared infection in a matter of weeks.

Symptoms

What ascariasis actually feels like.

Two phases matter - the pulmonary migration (Loeffler’s syndrome) and the intestinal phase - and both have hallmark features worth knowing.

  • Loeffler’s syndrome

    Cough, wheeze, low-grade fever and eosinophilia during the two-week larval migration through the lungs.

  • Abdominal pain

    Cramping mid-abdominal pain from adult worms in the small intestine - often intermittent and worse after meals.

  • Malabsorption

    Weight loss, steatorrhoea and micronutrient deficiency - a major cause of growth failure and stunting in endemic paediatric populations.

  • Intestinal obstruction

    Heavy worm burdens can form a bolus - especially in children - causing vomiting, distension and a surgical picture.

  • Biliary and pancreatic disease

    Adult worms migrating up the ampulla can cause cholangitis, obstructive jaundice, pancreatitis or a liver abscess.

  • Passed worm

    An adult worm passed per rectum, vomited up or seen in the nose is unmistakable - and often the moment of diagnosis.

  • Eosinophilia

    A raised eosinophil count on a routine full blood count in a returning traveller is a classic clue.

  • Red flag - acute abdomen

    Obstruction, intussusception, volvulus or biliary sepsis - a surgical and hepatobiliary emergency needing hospital review.

Treatment

How ascariasis is treated in the UK.

A single dose of albendazole for most patients - hospital-based care and ERCP for biliary migration or bowel obstruction.

  • Albendazole 400 mg single dose

    First-line UK treatment for uncomplicated infection - one dose clears the great majority of adult worms.

  • Mebendazole 100 mg twice daily

    A three-day course is an equally effective alternative - useful when albendazole is unsuitable or unavailable.

  • Pyrantel pamoate

    A single-dose alternative sometimes used in pregnancy after the first trimester or in young children under specialist advice.

  • Ivermectin

    Reserved for polyparasitism or where Strongyloides is suspected - given in specialist tropical medicine settings.

  • ERCP for biliary worms

    Endoscopic retrieval of adult worms from the common bile duct - the definitive intervention for biliary ascariasis and cholangitis.

  • Conservative bowel management

    Nasogastric decompression, intravenous fluids and anthelmintic therapy resolve most partial obstructions without surgery.

  • Surgery for complicated obstruction

    Laparotomy with milking or resection is reserved for perforation, ischaemia, volvulus or failed conservative care.

  • Nutritional and public-health support

    Iron, vitamin A, catch-up growth support and household deworming - plus water, sanitation and hygiene (WASH) measures.

What this guide is based on

The sources behind every claim on this page.

International guidance from WHO and CDC, UK infection-service standards and specialist tropical medicine literature.

Key references

Guidelines and standards we relied on.

A quiet reminder

This guide is for information, not medical advice.

Your GP or infection specialist knows your history, exposures and travel and can tell you which parts apply to you. If in doubt, get seen.

  • World Health Organization. Soil-transmitted helminthiases - fact sheet and preventive chemotherapy guidance.

  • UK Health Security Agency (UKHSA). Guidance on parasitic infections in returning travellers.

  • CDC. Parasites - Ascariasis: diagnosis and treatment.

  • British Infection Association. Investigation of eosinophilia in returning travellers.

Red flags

When ascariasis needs urgent attention.

Most infections are cleared with a single dose. These are the situations that need the emergency department, surgery or specialist infection input.

  • Intestinal obstruction

    Bilious vomiting, distension and absolute constipation - especially in a child from an endemic area - is a surgical emergency.

  • Biliary obstruction and cholangitis

    Fever, jaundice and right upper quadrant pain in someone with confirmed or suspected ascariasis needs urgent hepatobiliary input.

  • Acute pancreatitis

    Ascaris migrating into the pancreatic duct is a recognised cause of pancreatitis - severe epigastric pain warrants immediate assessment.

  • Liver abscess

    Persistent fever, tender hepatomegaly and raised inflammatory markers after ascariasis suggests hepatic seeding.

  • Loeffler’s in a pregnant traveller

    Anthelmintic choice in pregnancy is nuanced - pyrantel and delayed treatment may be preferred and specialist input is essential.

  • Severe eosinophilia

    Very high eosinophil counts or systemic features can point to hyperinfection or a different helminth - escalate to tropical medicine.

  • Failure of first-line therapy

    Persistent eggs on stool testing after treatment needs repeat dosing and a review of adherence and re-exposure risk.

  • Malnutrition in a child

    Stunting, iron deficiency and vitamin A deficiency need active nutritional rehabilitation alongside deworming.

  • Household transmission

    Positive stool in one family member warrants screening and empirical treatment of close contacts to prevent reinfection.

Living with it

A treatable infection, with a clear plan.

Four habits that make the biggest difference - safe food and water, careful hand hygiene, follow-up stool testing and nutritional recovery.

A quiet reminder

The tablet is only half the job.

Preventing reinfection - and rebuilding what a heavy worm burden has taken - matters as much as the anthelmintic itself.

  1. 01 Prevention

    Wash food and hands

    Peel or thoroughly wash raw fruit and vegetables in endemic areas - and wash hands after any contact with soil.

  2. 02 Water

    Drink safe water

    Bottled, boiled or filtered water when travelling - and avoid ice made from tap water in high-risk regions.

  3. 03 Follow up

    Repeat stool testing

    A follow-up sample two to four weeks after treatment confirms clearance and catches early reinfection.

  4. 04 Nutrition

    Rebuild what was lost

    Iron and vitamin A repletion, protein-rich meals and paediatric growth monitoring after treatment matter as much as the anthelmintic itself.

Frequently asked

Everything we get asked about ascariasis.

Quick answers on how it spreads, what it does inside the body and how it’s treated.

  • What is ascariasis?

    It is an intestinal infection caused by Ascaris lumbricoides, the largest human roundworm. Adult worms measure 15 to 35 cm and live in the small intestine, where the female sheds thousands of eggs a day into the stool. The eggs mature in soil and are swallowed by the next person, restarting the cycle.

  • How would I catch it?

    By swallowing eggs from food, water or hands contaminated with human faeces - typically from unwashed produce or soil in an area with limited sanitation. It is not spread person-to-person like a cold. In the UK, almost every case is imported from a tropical or subtropical region.

  • What happens inside the body?

    Eggs hatch in the small intestine. The larvae burrow through the intestinal wall, enter the bloodstream and travel to the lungs - causing Loeffler’s syndrome with cough, wheeze and eosinophilia. They are then coughed up, swallowed, and mature into adult worms in the small intestine, where eggs are shed in faeces.

  • Do most people have symptoms?

    No - light infections are usually asymptomatic and are picked up incidentally on stool testing or when an adult worm is passed. Symptoms tend to emerge with heavier worm burdens, during pulmonary migration, or when complications like obstruction or biliary migration develop.

  • How is it treated?

    A single 400 mg oral dose of albendazole clears most uncomplicated infections. Mebendazole 100 mg twice daily for three days is an equally effective alternative. Complications - biliary migration, cholangitis, pancreatitis or intestinal obstruction - need hospital-based care alongside anthelmintic therapy.

  • Is it dangerous?

    For most travellers, no - a single course of treatment is curative. But heavy infections in children can cause growth failure, and complications such as intestinal obstruction, biliary migration and pancreatitis are genuine surgical and hepatobiliary emergencies. Any acute abdominal or biliary picture in a person from an endemic area deserves urgent review.

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