Strongyloidiasis is an infection caused by the parasitic roundworm Strongyloides stercoralis, a soil-dwelling nematode that enters the body through the skin and can silently persist inside a person for decades. An estimated 614 million people worldwide carry the parasite, mostly in tropical and subtropical regions, though many never realize they are infected. What makes strongyloidiasis unusual among worm infections is its ability to reinfect the host from within, a trick that lets it maintain itself indefinitely and, under certain conditions, spiral into a life-threatening crisis.
How You Get Infected
The infection begins in contaminated soil. Microscopic larvae of S. stercoralis live in warm, moist earth, typically where human feces have been deposited. Walking barefoot on that soil is the classic route of entry. The larvae don’t need a cut or wound to get in. They actively bore through intact skin, secreting a powerful enzyme that dissolves connective tissue and lets them tunnel at speeds of up to 10 centimeters per hour.
1Experimental Parasitology. Strongyloides stercoralis: Identification of a protease that facilitates penetration of skin by the infective larvaeOnce inside, the larvae hitch a ride through the bloodstream to the lungs. They climb the airways, get swallowed, and settle in the small intestine, where they mature into adult worms. The adults burrow into the intestinal lining and start producing eggs. This skin-to-lung-to-gut journey is not unique to Strongyloides; hookworms follow a similar path. What happens next, though, is what sets this parasite apart from virtually every other human worm.
The Autoinfection Cycle
Most intestinal worms produce eggs or larvae that leave the body in stool. To reinfect, a new batch of larvae has to develop in the environment and penetrate the skin again. S. stercoralis skips that step. Some of the larvae hatching inside the gut transform into their infective form right there in the intestines, penetrate the gut wall or the skin around the anus, and re-enter the bloodstream to start the whole cycle over. This internal loop, called autoinfection, means the parasite can maintain itself inside the body without any new exposure to contaminated soil.
2PubMed Central. The Unique Life Cycle of Strongyloides stercoralis and Implications for Public Health ActionIn a person with a healthy immune system, autoinfection ticks along at a low level, kept in check by immune defenses. The worm burden stays small, and the person may feel nothing at all. But because the cycle never fully stops, the infection can persist for the rest of someone’s life if untreated. Case reports describe people who left tropical regions decades ago and were eventually diagnosed with strongyloidiasis long after any possible re-exposure.
3PubMed Central. Twenty-five Years of Chronic Strongyloidiasis in an ImmigrantSymptoms of Chronic Infection
Chronic strongyloidiasis is one of the great medical chameleons. When it produces symptoms at all, they tend to be vague, come and go unpredictably, and overlap with dozens of other conditions. As one clinical review put it, “if you don’t look for it, you won’t find it.”
4Australian Family Physician. Chronic strongyloidiasis – Don’t look and you won’t findSymptoms tend to cluster around the three body systems the parasite passes through:
- Skin: A recurring rash, sometimes described as raised red lines or hives, often around the buttocks, groin, or trunk. The most distinctive form is larva currens, a rapidly moving, snake-like rash caused by larvae migrating under the skin. It appears suddenly and fades within hours, making it hard to catch at a clinic visit.
- Lungs: Coughing, wheezing, and a scratchy feeling in the throat as larvae pass through the airways. This can be mistaken for asthma or bronchitis.
- Gut: Intermittent abdominal pain, bloating, watery diarrhea, and sometimes nausea. These come and go and are easily written off as irritable bowel syndrome or food intolerance.
Larva currens is considered pathognomonic for strongyloidiasis, meaning it doesn’t occur with other conditions. But it is rare and fleeting enough that most clinicians have never seen it, and patients may not mention a rash that disappears in a few hours.
5PubMed Central. Larva Currens: Report of Seven Cases and Literature ReviewMany people with chronic strongyloidiasis have no symptoms whatsoever. The infection is discovered incidentally, often when blood work done for another reason shows unexplained eosinophilia, an elevated count of a specific white blood cell that tends to rise in response to parasitic infections.
When the Infection Turns Dangerous
The autoinfection cycle normally stays low-grade because the immune system actively suppresses it. Eosinophils and neutrophils attack the larvae directly, and a specific type of immune response keeps the worm numbers in check.
6PubMed Central. Innate and adaptive immunity to the nematode Strongyloides stercoralis in a mouse modelIf that immune control is disrupted, the autoinfection cycle accelerates. Larvae reproduce unchecked, and the worm burden explodes. This is called hyperinfection syndrome if the larvae stay within the organs of their normal cycle (gut and lungs) or disseminated strongyloidiasis if they spread to organs they don’t normally reach, such as the brain, liver, or kidneys. Both are medical emergencies with high death rates.
Corticosteroids are the most common trigger. These drugs suppress the very immune pathways that keep autoinfection in check. The risk is not limited to long courses of steroids; case reports describe hyperinfection developing after a single dose of dexamethasone given for an unrelated reason.
7PubMed Central. Strongyloides Hyperinfection Syndrome Triggered by a Single Dose of Dexamethasone Administered for Fetal Lung Development Other drugs that suppress immunity, including those used after organ transplants and some cancer chemotherapies, also carry risk. The danger is that someone with a silent chronic infection receives one of these treatments and the parasite erupts with no warning.
During hyperinfection and dissemination, massive numbers of larvae breach the intestinal wall, dragging gut bacteria into the bloodstream. This can cause severe bacterial infections on top of the parasitic disease, including bloodstream infections and bacterial meningitis involving organisms that are normally confined to the gut.
8PubMed Central. Enterococcal meningitis associated with Strongyloides infection: a case report and literature reviewHTLV-1 and Strongyloidiasis
One particular co-infection deserves special attention. Human T-cell lymphotropic virus type 1 (HTLV-1) is a chronic retrovirus that alters the immune system in a way that specifically benefits Strongyloides. In people with both infections, the virus expands a population of regulatory immune cells that dampen the body’s parasite-fighting response, dramatically reducing the eosinophil activity and chemical signals that normally keep worm numbers low.
9PLOS Neglected Tropical Diseases. Regulatory T Cell Expansion in HTLV-1 and Strongyloidiasis Co-infection Is Associated with Reduced IL-5 Responses to Strongyloides stercoralis AntigenThe result is a massively increased risk of severe disease. A systematic review and meta-analysis found that people co-infected with HTLV-1 had roughly 60 times the odds of developing severe strongyloidiasis compared to those infected with the worm alone.
10PubMed Central. Human T-Cell Lymphotropic Virus Type 1 and Strongyloides stercoralis Co-infection: A Systematic Review and Meta-Analysis HTLV-1 also makes treatment harder; the worm often fails to clear or recurs quickly after therapy in co-infected patients. Because both infections are common in overlapping regions of the world, including parts of the Caribbean, South America, Japan, and sub-Saharan Africa, screening for one should prompt screening for the other.
Why Diagnosis Is Difficult
Strongyloidiasis is chronically underdiagnosed, and a big reason is the limitations of the most widely available test: looking for larvae in a stool sample. Unlike many other intestinal worms, S. stercoralis sheds larvae intermittently and in small numbers during chronic infection. A single standard stool exam catches only a fraction of true cases. In one Ethiopian study comparing five diagnostic methods, a routine stool concentration technique detected only about 5% of infections that were identified when all methods were combined.
11PubMed Central. Evaluation of five diagnostic methods for Strongyloides stercoralis infection in Amhara National Regional State, northwest EthiopiaSpecialized stool techniques improve detection. The agar plate culture method, where a stool sample is placed on a nutrient gel and observed for tracks left by crawling larvae, picks up many more cases than simple microscopy, though it takes a couple of days and is not available in most labs. A study in Ontario over 14 months found only 14 stool samples positive for Strongyloides larvae out of nearly 18,000 examined, while blood serology identified 200 reactive results out of about 3,300 specimens tested, illustrating the enormous gap between stool-based and antibody-based detection.
12PubMed. Strongyloidiasis in Ontario: Performance of diagnostic tests over a 14-month periodBlood-based antibody tests (serology) are the most sensitive screening tools for chronic infection. The best-performing assays reach sensitivities above 90%, making them far more reliable for ruling out infection than any stool-based method.
13PubMed Central. Accuracy of Serologic Tests for S. stercoralis The trade-off is that antibody levels can remain elevated for months after successful treatment, making it harder to confirm cure. Serology also cross-reacts with other parasitic infections in some settings.
Molecular testing using PCR on stool samples offers a middle ground: higher sensitivity than microscopy (one study found about 77% compared to a combination reference standard) with the ability to distinguish Strongyloides from look-alike hookworm larvae under the microscope.
14PubMed. Real-time PCR for detection of Strongyloides stercoralis in human stool samples from Côte d’Ivoire PCR is increasingly used in referral labs but remains unavailable in many of the tropical settings where the parasite is most common.
Treatment
Ivermectin is the drug of choice. Head-to-head comparisons consistently show it outperforms the main alternative, albendazole. A Cochrane systematic review of randomized trials found that ivermectin achieved parasitological cure at roughly 1.8 times the rate of albendazole.
15PubMed Central. Ivermectin versus albendazole or thiabendazole for Strongyloides stercoralis infection Individual trials back this up: one reported cure in about 83% of patients given ivermectin versus 38% with albendazole, and another found ivermectin cured close to 99% of cases compared to about 79% with albendazole.
16Transactions of the Royal Society of Tropical Medicine and Hygiene. Treatment of Strongyloides stercoralis infection with ivermectin compared with albendazole: results of an open study of 60 cases17PubMed Central. Prevalence of strongyloides in Northern Thailand and treatment with ivermectin vs albendazole
For uncomplicated chronic infection, a standard course is typically one or two doses of oral ivermectin. The drug paralyzes and kills the larvae, and side effects are usually mild. Albendazole, given over several days, remains an option where ivermectin is unavailable, but its lower cure rate makes it a second choice.
Hyperinfection and disseminated disease are far harder to treat. Patients are often critically ill with gut dysfunction that impairs absorption of oral medications. In these cases, subcutaneous ivermectin, though not formally licensed for human use, has been administered with increasing frequency. Pharmacokinetic data from such cases suggest that higher loading doses may be needed to achieve adequate drug levels throughout the body.
18PubMed Central. Subcutaneous Ivermectin Pharmacokinetics in Disseminated Strongyloides Infection: Plasma and Postmortem Analysis Treatment is prolonged, often continuing daily until larvae are no longer detectable. Even with aggressive therapy, mortality in disseminated cases remains high, which is why the focus in medicine has shifted toward prevention through screening.
Who Should Be Screened
Because hyperinfection is preventable if the infection is found and treated before immune suppression occurs, screening the right populations matters enormously. Current expert guidance recommends testing anyone who has lived in or traveled extensively to tropical and subtropical regions before they are started on corticosteroids, chemotherapy, or other immunosuppressive drugs, particularly before organ transplantation.
19PubMed Central. Screening, prevention, and treatment for hyperinfection syndrome and disseminated infections caused by Strongyloides stercoralisSerology is the preferred screening test in these situations because of its high sensitivity. A negative antibody test makes active infection very unlikely; a positive one warrants treatment, even if stool tests come back negative, because the stakes of missing the diagnosis before immune suppression are so high.
Global Burden and the Poverty Connection
Strongyloidiasis is classified as a neglected tropical disease. One global modeling study estimated the worldwide prevalence at about 8%, translating to roughly 614 million people infected as of 2017. The heaviest burden falls on Southeast Asia, Africa, and the Western Pacific, which together account for more than three-quarters of global cases.
20PubMed Central. The Global Prevalence of Strongyloides stercoralis Infection In many endemic countries, survey data suggest infection rates between 10% and 40% of the population, with rates climbing as high as 60% in communities where sanitation is poor and conditions favor soil contact.
21PLoS Neglected Tropical Diseases. Strongyloides stercoralis: Global Distribution and Risk FactorsThe disease tracks poverty more closely than geography. Research has found that strongyloidiasis prevalence is driven more by socioeconomic conditions, infrastructure, and sanitation access than by climate alone.
22PubMed Central. Strongyloidiasis: A Disease of Socioeconomic Disadvantage Concrete floors, functional toilets, and wearing shoes are all associated with lower infection risk. A study in Cambodia found that the risk of acquiring a new infection after treatment dropped as sanitation coverage in a village increased, and that rice farmers who regularly defecated in fields or water and who went barefoot were at highest risk.
23PLoS Neglected Tropical Diseases. Ivermectin Treatment and Sanitation Effectively Reduce Strongyloides stercoralis Infection Risk in Rural Communities in CambodiaControl programs that combine mass drug administration with environmental health improvements have shown results. In Indigenous communities in northern Queensland, Australia, where strongyloidiasis was once highly prevalent, seroprevalence declined substantially over an 18-year period during which expanded environmental health programs and screening-and-treatment campaigns were implemented together.
24PubMed Central. Rapid Progress toward Elimination of Strongyloidiasis in North Queensland, Tropical Australia, 2000-2018Can Animals Spread It to Humans
The primary species causing human disease is Strongyloides stercoralis, which cycles mainly between humans and soil. But dogs can also harbor S. stercoralis, and studies have detected infected dogs in shelter settings, raising questions about whether household pets could be a reservoir.
25PubMed Central. Occurrence of strongyloidiasis in privately owned and sheltered dogs: clinical presentation and treatment outcome A related species, Strongyloides fuelleborni, primarily infects nonhuman primates but can occasionally cause human infection. Zoonotic transmission has implications for control efforts, since eliminating human-to-soil contamination alone may not break the cycle if animals continue to seed the environment.
26International Journal for Parasitology: Parasites and Wildlife. Genetic characterization of Strongyloides fuelleborni infecting free-roaming African vervets (Chlorocebus aethiops sabaeus) on the Caribbean island of St. KittsIn practice, the overwhelming majority of human cases come from contact with soil contaminated by human feces, so improving sanitation remains the most effective prevention strategy. Still, the zoonotic angle is an active area of research, and people who work closely with dogs or primates in endemic areas may face additional exposure that sanitation alone would not address.