A small intestinal roundworm, Strongyloides stercoralis, can live quietly in a person for decades and then, once corticosteroids or other immune-suppressing drugs are started, multiply out of control with frequently fatal results. This article covers why that happens and who is at risk. It compares the two main ways of looking for the infection, blood antibody tests (serology) and stool examination, and explains when doctors give the antiparasitic drug ivermectin presumptively, without waiting for a test result, to people who have lived in regions where the worm is common. The goal is to help patients and families ask the right question at the right time. Often that time is before the first steroid dose rather than after symptoms appear.
Why a dormant worm becomes dangerous when immunity is suppressed
Most intestinal worms cannot complete their life cycle inside a single human host. Strongyloides can. Some larvae mature inside the bowel and re-enter the body through the gut wall or the skin around the anus, a process called autoinfection. In a healthy person the immune system keeps this cycle in check. The result is a low-grade infection that may cause intermittent itching, vague abdominal discomfort, or no symptoms at all, sometimes for a lifetime.
The persistence is remarkable. Studies from the Liverpool School of Tropical Medicine of British former prisoners of war held in Southeast Asia during the Second World War found that a meaningful minority still carried the worm more than 30 years after they had left the tropics. These men had not been re-exposed. Autoinfection alone had sustained the parasite.
When immunity is suppressed, the balance tips. Corticosteroids such as prednisone, dexamethasone and methylprednisolone are the best-documented trigger. They suppress the eosinophils and type-2 immune responses that normally restrain the worm, and they may also speed the larvae's development directly. The result can be hyperinfection, an explosion in larval numbers within the gut and lungs. It can also be disseminated disease, in which larvae spread to the brain, liver and other organs. Migrating larvae carry gut bacteria with them, so these patients often develop bloodstream infections or meningitis with intestinal organisms. Published case series report mortality frequently above 50 percent, and higher in some series. Cases have followed steroid courses lasting only days.
Other risk factors include organ and stem-cell transplantation, some chemotherapy and biologic drugs, and co-infection with the virus HTLV-1, which strongly predisposes to severe disease. Because the danger arises from something done in the course of medical care, prevention is mostly a matter of foresight.
Who should be considered for screening
A 2020 analysis led by Dora Buonfrate and colleagues, published in the journal Pathogens, estimated that around 600 million people worldwide carry Strongyloides. The worm thrives in warm, moist soil with limited sanitation. It is acquired when larvae penetrate bare skin. People who should raise the question with their doctor include those who:
- were born in, or lived for a significant period in, Southeast Asia, sub-Saharan Africa, Latin America, the Caribbean, South Asia or the Pacific Islands;
- grew up in rural areas with endemic pockets in wealthier countries, such as parts of southern Europe, Appalachia and the southeastern United States, or remote communities in northern Australia;
- served in the military or worked for long periods in endemic tropical regions, even many years ago;
- have unexplained eosinophilia, recurrent itchy rashes around the buttocks or waist, or are infected with HTLV-1.
The time since leaving an endemic area does not rule out infection. A grandmother who emigrated 50 years ago remains a candidate for screening. Doctors cannot know this history unless they ask or the patient tells them. This is one of those places where a well-informed patient genuinely protects themselves.
Serology versus stool testing: what each can and cannot tell you
Diagnosing Strongyloides is difficult because larvae are shed in stool in small numbers and intermittently. The two main approaches have complementary strengths.
Stool examination. Standard microscopy of a single stool sample misses a large share of chronic infections. Published sensitivity estimates often fall well below 50 percent. Repeating the test on several samples helps. Specialised techniques such as agar plate culture and the Baermann concentration method perform better, but they are labour-intensive and not widely available. Stool PCR is highly specific, but its sensitivity has varied considerably between studies. A positive stool test is definitive and confirms active infection. A negative one offers limited reassurance.
Serology. Antibody tests, usually ELISA formats using crude larval antigen or a recombinant antigen such as NIE, are generally the most sensitive single screening test for chronic infection. Many evaluations report sensitivities in the range of roughly 80 to 95 percent. Their limitations matter:
- Cross-reactivity: antibodies against related worms, especially filarial parasites, can cause false positives.
- Persistence after cure: antibodies may remain detectable for months to years, so a positive result does not always mean current infection.
- False negatives in the immunosuppressed: people already on steroids, transplant recipients and those with HTLV-1 may not mount a detectable antibody response. These are exactly the people at highest risk.
- Turnaround time: results often take days to weeks, which may be too long if treatment cannot wait.
Many specialists therefore recommend serology as the main screening test, with stool studies added where possible, especially in already-immunosuppressed patients. A rising or high eosinophil count is a useful clue. Its absence means little, since many infected people have normal counts and steroids suppress eosinophils anyway.
When presumptive ivermectin is recommended
Because testing is imperfect and slow, and because ivermectin is well tolerated, several guidelines endorse treating at-risk people without waiting for confirmation. This approach is variously called presumptive, empiric or pre-emptive treatment.
The clearest example comes from the United States Centers for Disease Control and Prevention's guidance for refugees. It recommends presumptive ivermectin before departure for many refugees from endemic regions, with special precautions for those from areas where the filarial worm Loa loa occurs. Transplant infectious-disease guidelines advise screening candidates with epidemiological risk. They often recommend treating seropositive candidates, or treating empirically when testing is impractical before transplantation. In 2020, during the pandemic, Stauffer, Alpern and Walker wrote in JAMA that patients from endemic regions starting dexamethasone should be considered for empiric ivermectin to prevent hyperinfection. This recommendation concerned the parasite, not the virus. Large randomised trials have not shown ivermectin to be an effective treatment for COVID-19 itself, and the two questions should not be confused.
In practice, clinicians commonly reason as follows:
- Time allows testing: send serology, ideally with stool studies, and treat if positive.
- Steroids or immunosuppression cannot wait, or testing is unavailable: give presumptive ivermectin. Many clinicians send serology at the same time.
- High-dose or prolonged immunosuppression in a high-risk person: some experts favour treating even when serology is negative, given the risk of false negatives.
The usual regimen is 200 micrograms per kilogram of body weight taken by mouth, as a single dose or repeated once after one to two weeks or on consecutive days, depending on the guideline. A Cochrane systematic review led by Henriquez-Camacho in 2016 found ivermectin clearly more effective than albendazole for chronic strongyloidiasis, and better tolerated than thiabendazole. The STRONGTREAT randomised trial, coordinated from Negrar, Italy, and published in The Lancet Infectious Diseases in 2019, found that a single dose was not meaningfully less effective than multiple doses for uncomplicated infection. Oral ivermectin is approved in the United States for intestinal strongyloidiasis. Its use for disseminated or hyperinfection disease, often requiring prolonged daily dosing and sometimes non-oral routes, falls outside the approved labelling and belongs in specialist hands.
Ivermectin itself has a modest and fitting history. It is derived from avermectins produced by Streptomyces avermitilis, a bacterium Satoshi Ōmura isolated from a soil sample near a Japanese golf course. Its development, with William Campbell, earned the 2015 Nobel Prize. Few would have guessed that a handful of ordinary soil would yield a medicine that now protects hundreds of millions. It is a reminder that the created world holds provision for human need, waiting to be discovered by patient work.
Important cautions before taking ivermectin
Ivermectin is generally safe at standard doses, with mostly mild side effects such as dizziness, nausea or itching. Certain situations require care:
- Loa loa exposure: in people from parts of West and Central Africa where this eye worm is endemic, ivermectin given to someone with a very high blood count of Loa larvae can cause a rare but serious encephalopathy. Guidelines advise testing for Loa loa first, or using an alternative.
- Pregnancy: safety data in pregnancy are limited, particularly for the first trimester. Guidance generally favours testing and deferring or individualising treatment rather than giving presumptive ivermectin to pregnant women. When a pregnant mother truly needs steroids, her doctors must weigh two lives, hers and her unborn child's. Both deserve protection, and that is a decision best made carefully and together with her physician.
- Young children, liver disease and interacting medicines: very small children, people with significant liver impairment, and those on certain drugs need individual assessment.
- Self-medication: veterinary products are formulated for animals, vary in concentration, and should never be used by people.
Making the conversation happen
Strongyloidiasis is rarely in the forefront of a busy clinician's mind when prescribing a steroid burst for asthma, a flare of arthritis or a hospital admission. Responsible stewardship of one's own health, and that of an ageing parent or spouse, includes mentioning past residence in the tropics or subtropics whenever immunosuppression is proposed. A simple question opens the conversation: "I grew up in an area where Strongyloides is common. Should I be tested or treated before starting this?" Some families who care for relatives from endemic regions keep this history written down alongside medication lists. That small act of preparedness can save critical days in an emergency. The decision about testing, presumptive treatment or both belongs to the patient and their own doctor, made with full understanding of the options.
Key takeaway: Anyone who has lived in a region where Strongyloides is common should raise it before starting steroids or other immunosuppression, because serology, stool testing and, when appropriate, presumptive ivermectin can prevent a rare but often fatal complication.