Strongiloidiasis

01Pathophysiology

Strongyloides stercoralis has a life cycle unlike any other soil-transmitted helminth, and this unique biology is the reason it appears so frequently on licensing exams.

Infection begins when filariform (L3) larvae in contaminated soil penetrate intact skin, typically the feet of individuals walking barefoot. Once in the bloodstream, the larvae travel to the lungs, penetrate the alveoli, ascend the tracheobronchial tree, are swallowed, and eventually reach the small intestine. This lung migration phase is the reason patients can present with cough, wheezing, and transient pulmonary infiltrates, a pattern sometimes called Loeffler-like syndrome. The pulmonary symptoms are caused by the inflammatory reaction to larvae migrating through lung parenchyma.

In the small intestine, larvae mature into adult female worms that burrow into the duodenal and jejunal mucosa. The adults produce eggs by parthenogenesis (no males are required in the human host). These eggs hatch within the intestinal wall, releasing rhabditiform (L1) larvae into the intestinal lumen. This is a critical point: unlike hookworm or Ascaris, you will not see eggs on stool microscopy. You will see larvae.

The feature that makes Strongyloides dangerous is internal autoinfection. Some rhabditiform larvae transform into infective filariform larvae while still inside the gut. These filariform larvae can then re-penetrate the intestinal mucosa or perianal skin and re-enter the bloodstream, restarting the entire cycle without any external re-exposure. This is why a patient can harbor the infection for decades after leaving an endemic area, maintaining a low-grade, chronic infection indefinitely. A vignette describing a veteran who served in Southeast Asia 30 years ago and now presents with eosinophilia is a classic setup.

In an immunocompetent host, the autoinfection cycle is kept in check by the immune system, particularly cell-mediated (Th2) immunity and the eosinophilic response. However, when cell-mediated immunity is suppressed, particularly by corticosteroids or by HTLV-1 co-infection, the autoinfection cycle accelerates out of control. This is called hyperinfection syndrome. Larvae multiply exponentially, penetrate the bowel wall in enormous numbers, and disseminate to the lungs, liver, CNS, and virtually every organ. Each larval penetration of the gut drags enteric bacteria into the bloodstream, which is why hyperinfection is frequently complicated by gram-negative sepsis and meningitis (often E. coli or Klebsiella). This connection between corticosteroid administration and overwhelming Strongyloides dissemination is one of the most commonly tested concepts in parasitology.

Larva currens (the signature skin finding) occurs when filariform larvae re-penetrate perianal or abdominal skin during external autoinfection. The host mounts a local urticarial reaction to the migrating larva, producing a serpiginous, erythematous track that moves at roughly 5 to 10 cm per hour. This speed distinguishes it from cutaneous larva migrans caused by hookworm larvae, which advances only a few centimeters per day.

02Classification and Clinical Manifestation

03Diagnostic Workup

When you encounter a vignette with a patient from an endemic region (or with remote exposure history) who has eosinophilia and GI or pulmonary symptoms, stool examination for larvae is the best initial diagnostic step. The critical distinction here is that Strongyloides passes larvae in stool, not eggs. If an answer choice says "stool O&P for ova," that is a distractor designed to catch you. A single stool exam has poor sensitivity (around 30%) because larval output is intermittent and low in chronic infection. Repeating the exam on at least three separate specimens, or using the agar plate culture method, significantly improves detection.

Serology (ELISA) is the most sensitive single test and is the preferred screening method, especially in patients with low worm burdens or when stool exams are negative. However, it has two important limitations you should remember: it cannot reliably differentiate current from prior infection, and it cross-reacts with antibodies to other helminthic infections. In clinical practice, a positive serology in a patient with compatible symptoms and exposure history is usually sufficient to initiate treatment.

In hyperinfection or disseminated disease, the diagnosis becomes easier in one sense: the larval burden is so high that larvae can be found in sputum, BAL fluid, peritoneal fluid, CSF, or biopsies from almost any organ. Finding filariform larvae outside the GI tract is essentially diagnostic of hyperinfection/dissemination.

A notable exam pearl: in a patient on corticosteroids who develops gram-negative meningitis or sepsis and has eosinophilia (or a history of eosinophilia that has now resolved), always consider disseminated strongyloidiasis. The bacteremia is secondary to larval translocation of gut flora.

04Management and Treatment

For uncomplicated chronic strongyloidiasis, ivermectin 200 mcg/kg/day orally for 2 days is the first-line treatment and the answer the exam is looking for. Ivermectin is superior to albendazole in both efficacy and tolerability. Albendazole at 400 mg twice daily for 7 days is the second-line agent, reserved for situations where ivermectin is unavailable or contraindicated. Thiabendazole, which older resources may mention, is no longer recommended due to its side-effect profile.

The "Next Best Step" logic that exams test most often revolves around hyperinfection in an immunosuppressed host. If a patient on chronic corticosteroids develops worsening pulmonary symptoms, GI distress, or signs of gram-negative sepsis, and you suspect hyperinfection, the immediate priorities are:

1. Start ivermectin immediately and continue daily until all specimens are negative for at least 2 weeks.

2. Initiate broad-spectrum antibiotics to cover enteric gram-negative organisms (the polymicrobial bacteremia and meningitis are the immediate cause of death).

3. Reduce or discontinue immunosuppression if clinically feasible.

4. Provide ICU-level supportive care as needed.

Ivermectin is generally well tolerated. It is contraindicated in pregnancy (Category C, with teratogenic effects in animal studies) and should be used with caution in patients co-infected with Loa loa (risk of fatal encephalopathy, relevant mainly in West/Central African patients). In patients who cannot take oral medications (e.g., intubated, severe ileus), rectal or subcutaneous formulations of ivermectin have been used, though these are off-label.

A preventive concept that appears on exams: any patient from an endemic area who is about to receive corticosteroids, chemotherapy, or organ transplant immunosuppression should be screened for Strongyloides (with serology) and treated if positive before immunosuppression begins. Failing to do so can trigger fatal hyperinfection. Vignettes may describe a transplant patient or a new leukemia patient who develops disseminated strongyloidiasis after starting immunosuppressive therapy to test this principle.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common way students lose points on strongyloidiasis questions is by failing to connect corticosteroid use to the risk of hyperinfection. The classic vignette describes a patient who has been started on prednisone (for asthma, COPD exacerbation, autoimmune disease, or transplant rejection) and then deteriorates with overwhelming sepsis, respiratory failure, or gram-negative meningitis. The student fixates on the bacterial infection and misses the underlying parasitic cause. The clue is always in the history: the patient is from an endemic area, or has remote military service in a tropical region, or has unexplained eosinophilia on a prior lab. When you see corticosteroids plus gram-negative sepsis plus any hint of tropical exposure, think Strongyloides hyperinfection immediately.

A second trap involves the diagnostic technique. Students trained to look for "ova and parasites" on stool exams instinctively choose that answer. But Strongyloides does not shed eggs in stool. It sheds rhabditiform larvae. If the answer choice says "stool for ova," it is wrong. The correct phrasing is stool examination for larvae, or a stool agar plate culture.

Third, watch for the disappearing eosinophilia trap. In chronic uncomplicated infection, eosinophilia is expected and is often the initial clue. But in hyperinfection, the very immunosuppression that triggered the crisis (corticosteroids) also suppresses eosinophil production. A normal eosinophil count in a critically ill, immunosuppressed patient does not rule out Strongyloides. HTLV-1 co-infection similarly blunts the eosinophilic response.

Finally, remember the decades-long latency. No other soil-transmitted helminth can maintain itself in the host for years or decades without re-exposure. If a vignette describes a patient who left an endemic region 20 or 30 years ago and has never returned, the autoinfection cycle of Strongyloides is the explanation. This feature alone can clinch the diagnosis and eliminate hookworm, Ascaris, and other helminths from the differential.

The core competency being tested is the ability to recognize that Strongyloides is not just another intestinal worm. Its autoinfection cycle creates a uniquely dangerous relationship with immunosuppression, and the exam expects you to identify the at-risk patient, order the correct diagnostic test (larvae, not eggs), and intervene with ivermectin before or during immunosuppressive therapy.