Strongyloidiasis: The Worm That Can Live for Decades and Why Screening Matters Before Steroids
Almost every parasitic worm needs to leave the body and return through the environment to keep an infection going. Strongyloides stercoralis does not. Some of its larvae mature inside the intestine and burrow straight back through the bowel wall or the skin around the anus, restarting the cycle without ever leaving the host.
That single feature, called autoinfection, changes everything. It means an infection acquired by walking barefoot on contaminated soil at the age of twenty can still be present at seventy, with no further exposure. Cases have been documented more than 60 years after the person left an endemic area, most famously in veterans of tropical campaigns.
For most of those decades it causes little or nothing. The reason it matters is what happens when the immune system is deliberately suppressed.
🚨 The most important sentence on this page. Corticosteroids can convert a silent, decades-old infection into hyperinfection syndrome within weeks. The parasite multiplies unchecked, larvae disseminate through the lungs, bloodstream and brain, and they carry gut bacteria with them, producing Gram-negative sepsis and meningitis. Mortality in disseminated disease reaches 60 to 85 percent.
Which gives one rule: anyone who has ever lived in or travelled extensively through an endemic region should be screened for Strongyloides before starting corticosteroids or any immunosuppressive treatment. That includes prednisolone courses, methotrexate, leflunomide, biologics, JAK inhibitors, chemotherapy and transplant medication. Screening is a blood test. Not doing it is how people die of a treatable worm.
🌍 Who carries it
Between 30 and 100 million people worldwide are infected. Risk relates to soil contact in warm, humid places with poor sanitation.
| Region | Notes |
|---|---|
| Southeast Asia, South Asia | High prevalence, including Cambodia, Thailand, Laos |
| Sub-Saharan Africa | Widespread |
| Latin America and the Caribbean | Including Brazil, Peru, Caribbean islands |
| Rural southeastern United States and Appalachia | Endemic pockets persist, associated with historical poor sanitation and barefoot soil contact. Frequently forgotten |
| Southern and eastern Europe | Older residents of rural areas, particularly former rice-growing regions |
| Military veterans | Of tropical campaigns, decades after service |
Other risk groups: agricultural workers, people who have walked barefoot in rural tropics, migrants and long-term travellers, and anyone with HTLV-1 infection, which impairs the specific immune response to this parasite and both raises the risk of hyperinfection and causes treatment to fail more often.
🐛 How the cycle works
- Skin penetration. Larvae in soil penetrate intact skin, usually of the feet
- Migration. They travel through the bloodstream to the lungs, up the airways, and are swallowed
- Intestine. Female worms embed in the small bowel wall and produce eggs without needing a male, by parthenogenesis
- Autoinfection. Most larvae leave in the stool, but some mature inside the gut and re-penetrate the bowel wall or perianal skin, re-entering the circulation and starting again
Step four is why the infection is permanent without treatment, and why immune suppression is so dangerous: the loop that was running slowly accelerates.
🐚 The rash that identifies it
🐛 Larva currens, meaning running larva. This is close to diagnostic on sight. A raised, itchy, snaking red track appears on the buttocks, groin, abdomen or trunk, and it moves visibly, at 5 to 15 centimetres per hour — fast enough that a mark drawn at one end is left behind within the hour. It lasts hours to a couple of days, disappears, and recurs weeks or months later.
Nothing else behaves like this. Ordinary cutaneous larva migrans from dog or cat hookworm moves only a few millimetres to centimetres a day. Anyone with a recurring rapidly migrating track and any history of tropical exposure should be tested for Strongyloides.
🔍 What else it produces
Most chronic infections are silent or nearly so. When symptoms occur they are vague enough to be attributed to something else for years.
| System | Symptoms | Often mistaken for |
|---|---|---|
| Gut | Intermittent diarrhoea alternating with constipation, bloating, burning upper abdominal pain, nausea, weight loss | Irritable bowel syndrome, reflux, peptic ulcer |
| Skin | Larva currens, chronic urticaria, itching | Chronic idiopathic urticaria |
| Lungs | Dry cough, wheeze, breathlessness during larval migration | Asthma, bronchitis |
| Blood | Raised eosinophils, often the only abnormality | Allergy, unexplained eosinophilia |
The asthma connection deserves a flag: someone with unexplained wheeze, urticaria and eosinophilia who has lived in the tropics may be treated with corticosteroids for presumed asthma, which is precisely the trigger for hyperinfection.
🧪 Testing, and why the stool test lets you down
This is where the diagnosis is usually missed.
- ❌ A single stool microscopy misses most infections — sensitivity is roughly 30 percent, because larval output is low and intermittent. Three or more samples, or specialised agar plate culture, are needed to approach reliability
- ✅ Serology is the test of choice for screening, with sensitivity above 90 percent. It is the right test before immunosuppression and the right test in someone with unexplained eosinophilia and tropical exposure
- ℹ️ Serology has limits: it can cross-react with other helminths, and it can be falsely negative in the immunosuppressed, which is exactly the group in most danger. Where suspicion is high and the test is negative, treating empirically is often the safer course
- 📉 Antibody levels fall after successful treatment, so repeat serology at 6 to 12 months is used to confirm cure
⚠️ The eosinophil trap. Raised eosinophils are the classic clue in chronic infection, present in most cases. But in hyperinfection they are often normal or low, partly because corticosteroids suppress them, and a falling eosinophil count in a deteriorating patient is a poor prognostic sign. A normal eosinophil count never excludes the diagnosis in someone who is immunosuppressed and unwell.
💊 Treatment: ivermectin, dosed by weight
Ivermectol (Ivermectin 12 mg) is the drug of choice. It binds glutamate-gated chloride channels present in invertebrate nerve and muscle cells but not in humans, paralysing and killing the parasite. Cure rates are high, and it clearly outperforms albendazole.
📏 The dose
| Situation | Regimen |
|---|---|
| Uncomplicated chronic infection | 200 micrograms per kg once daily for 2 days, with many protocols repeating the two-day course at day 14 to cover the autoinfection cycle |
| Immunosuppressed, or before starting immunosuppression | The same, with confirmation of clearance before proceeding |
| Hyperinfection or disseminated disease | Daily ivermectin continued until stool and sputum have been clear for at least two weeks, in hospital, with reduction of immunosuppression where possible and antibiotics for the accompanying bacterial sepsis |
🧮 Working out the dose from 12 mg tablets. At 200 micrograms per kilogram, a 60 kg adult needs about 12 mg, which is one tablet. A 75 kg adult needs about 15 mg, and a 90 kg adult about 18 mg, so prescriptions are rounded to the nearest practical combination of tablets. The dose is per day, taken on two consecutive days, and it is worth confirming the exact number of tablets with the prescriber rather than assuming one tablet is a course.
- 💧 Taken with water on an empty stomach, as the labelling directs
- 👶 Not used under 15 kg body weight
- 🤰 Avoided in pregnancy unless the infection is severe and the benefit clearly outweighs the uncertainty; treatment is usually deferred until after delivery in chronic infection
- 🧠 One geographic caution: in parts of Central and West Africa where Loa loa is endemic, ivermectin given to someone with a high Loa burden can cause a severe brain reaction, so that co-infection is excluded first
- 🙂 Side effects are usually mild — headache, dizziness, nausea, muscle aches, and itching that reflects the dying parasites rather than an allergy
Where ivermectin cannot be used, Albenza (Albendazole) at 400 mg twice daily for 7 days is the alternative, with lower cure rates. The fixed combination ZBD Plus (Albendazole and Ivermectin) is used where mixed helminth infection is likely, which is common in returning travellers. The full range is in the anti-parasitic category.
📋 Follow-up matters more here than with most infections
Because a single surviving worm can restart the whole cycle, cure is confirmed rather than assumed.
- 📉 Eosinophil count should normalise within a few months
- 🧪 Repeat serology at 6 to 12 months, looking for a fall in antibody titre
- 💩 Stool examination after treatment where it was positive initially
- 🔁 Retreatment where symptoms or eosinophilia persist, which is more common in the immunosuppressed and in HTLV-1 co-infection
🛡️ Prevention and who to screen
Prevention for travellers is simple: wear shoes. Skin penetration is the entry route, so footwear in rural areas with poor sanitation removes most of the risk, along with avoiding sitting or lying directly on bare soil.
Screening is the part that saves lives, and the list of who should have it is short and specific:
- 💉 Before starting corticosteroids or any immunosuppressive drug, in anyone with a history of living in or travelling through an endemic region
- 🩸 Before organ transplantation, in both recipient and, where relevant, donor assessment
- 📈 Unexplained eosinophilia with any tropical exposure history
- 🐛 Recurrent unexplained urticaria or a migrating rash
- 🌍 Migrants from endemic areas, as part of initial health assessment
- 🧬 Anyone with HTLV-1 infection
🚨 When to seek help immediately
In someone with known or possible Strongyloides infection who is immunosuppressed:
- Fever with rapidly worsening breathlessness or coughing blood
- Severe abdominal pain, vomiting or a swollen tender abdomen
- Confusion, severe headache or neck stiffness
- Widespread pinpoint bruising over the abdomen, which can mark larvae migrating through the skin
- Any picture of sepsis, particularly with Gram-negative organisms and no obvious source
- Deterioration within weeks of starting a steroid in someone with tropical exposure
That last point is worth repeating to any clinician involved, because the connection between the new steroid and the collapsing patient is the thing most often missed.
❓ Frequently asked questions
How long can Strongyloides live in the body?
Indefinitely. Because some larvae mature inside the intestine and re-enter the body without leaving it, the infection sustains itself. Cases have been confirmed more than 60 years after the person last visited an endemic area.
What is the rash that moves?
Larva currens: a raised itchy snaking track that migrates 5 to 15 centimetres per hour, usually on the buttocks, groin or trunk, lasting hours to days and recurring later. Nothing else moves that fast, and it is close to diagnostic.
Why must I be tested before taking steroids?
Because corticosteroids can turn a silent decades-old infection into hyperinfection within weeks, with larvae spreading through the lungs, bloodstream and brain and carrying gut bacteria with them. Mortality in disseminated disease reaches 60 to 85 percent, and screening is one blood test.
Which test detects it?
Serology, with sensitivity above 90 percent. A single stool microscopy misses roughly 70 percent of infections because larval output is low and intermittent, so stool testing needs three or more samples or agar plate culture.
What is the ivermectin dose for strongyloidiasis?
Two hundred micrograms per kilogram once daily for two consecutive days, with many protocols repeating the two-day course after two weeks to cover the autoinfection cycle. From 12 mg tablets, a 60 kg adult needs about one tablet per dose and a 90 kg adult about 18 mg.
Is one dose of ivermectin enough?
Usually not for this parasite. Because larvae are continuously re-entering the body, a two-day course is standard and repeating it at two weeks is common practice. Cure is then confirmed by falling antibody levels and a normalised eosinophil count.
Can I have it with normal eosinophils?
Yes. Raised eosinophils are the usual clue in chronic infection, but in hyperinfection they are often normal or low, partly because steroids suppress them. A normal count never excludes the diagnosis in someone immunosuppressed and unwell.
Where do people catch it?
Through skin contact with contaminated soil in warm humid regions with poor sanitation: Southeast Asia, sub-Saharan Africa, Latin America and the Caribbean, parts of southern Europe, and endemic pockets in the rural southeastern United States and Appalachia.
How do I avoid it when travelling?
Wear shoes. The parasite enters through intact skin, usually the feet, so footwear in rural areas with poor sanitation removes most of the risk, along with not sitting or lying directly on bare soil.
Could it explain long-standing gut symptoms?
Sometimes. Intermittent diarrhoea alternating with constipation, bloating and burning upper abdominal pain are often attributed to irritable bowel syndrome or reflux for years. With a history of tropical exposure, particularly alongside raised eosinophils or urticaria, it is worth testing.
📑 Sources and editorial
- Infectious disease guidance on the diagnosis and management of strongyloidiasis, including screening before immunosuppression
- Comparative studies of stool microscopy, agar plate culture and serology sensitivity
- Trials comparing ivermectin with albendazole, and dosing protocols for chronic infection and hyperinfection
- Case series of corticosteroid-associated hyperinfection syndrome and reported mortality
- Prescribing information for ivermectin and albendazole, including weight-based dosing and the Loa loa caution
- Epidemiological surveys of Strongyloides prevalence, including persistent foci in the rural southeastern United States
- Related reading: acute diarrhea, early rheumatoid arthritis and methotrexate
- Related products: Ivermectol (Ivermectin), Albenza (Albendazole), ZBD Plus (Albendazole and Ivermectin), anti-parasitic category
- RXshop Editorial Team — reviewed by Emily Chen, MD, MPH — Internal Medicine Specialist
Medical Disclaimer: The information in this article is for educational and informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek guidance from a qualified healthcare provider with any questions you may have regarding a medical condition, and before starting, stopping or changing any medication.