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Microbiology · Nematodes

Strongyloides stercoralis !

  • Parasite
  • Exam priority: high

A worm that reinfects its host and can live in the body for decades. Under steroids it multiplies out of control and can cause fatal infection.

Updated 2026-11-01

Also called: threadworm (Strongyloides)

At a glance

Shape
Threadlike parasitic female embedded in the small-bowel mucosa
Family
Strongyloididae
Type
Intestinal nematode with a free-living soil cycle
Size
Parasitic female about 2–3 mm

Morphology

  • Stool shows rhabditiform larvae, not eggs.
  • Rhabditiform larva with a short buccal canal and a prominent genital primordium (hookworm larvae have a long buccal canal).
  • Filariform larva with a notched tail.

Transmission

  • Skin penetration by filariform larvae from contaminated soil.
  • Internal or perianal autoinfection, which keeps the infection going for decades.
  • Rarely, transplant of an organ from an infected donor.

Reservoir: Humans; Warm moist soil (free-living cycle)

Isolation precautions

What the care team adds to standard precautions.

  • Standard

Life cycle

  1. The filariform larva in soil penetrates skin and migrates to the small bowel.
  2. Females embed in the mucosa and reproduce without males (parthenogenesis).
  3. Eggs hatch in the mucosa: rhabditiform larvae, not eggs, leave in stool.
  4. In soil the larvae become filariform or form free-living adults that reproduce.
  5. Autoinfection: larvae turning filariform in the gut reinvade the bowel wall or perianal skin.
  6. With steroids or HTLV-1, autoinfection speeds up: hyperinfection and dissemination.

Virulence factors

  • Autoinfection allows decades-long infection, even far from endemic areas.
  • In hyperinfection, millions of larvae cross the bowel wall carrying enteric bacteria.
  • This explains gram-negative bacteraemia and meningitis in disseminated disease.
  • Larva currens: a fast-moving serpiginous rash on the buttocks, perineum and thighs.

Diseases it causes

Clinical picture

  • Many chronic carriers are asymptomatic or have mild GI symptoms.
  • Recurrent larva currens and mild eosinophilia in chronic infection.
  • Hyperinfection: lung infiltrates, ileus, GI bleeding, gram-negative sepsis and meningitis.
  • Eosinophilia is often absent in hyperinfection; its absence does not rule it out.

Diagnosis

  • Larvae in serial stool samples: direct microscopy is insensitive and several samples are needed.
  • Agar plate culture (Koga method), the most sensitive stool method; Baermann also used.
  • Duodenal aspirate or string test; larvae in bronchoalveolar lavage in hyperinfection.
  • Serology (EIA) for screening and follow-up; PCR in reference centres.

Treatment

Ivermectin is the drug of choice; albendazole is an alternative. People from endemic areas are screened (and treated if positive) before steroids, chemotherapy or transplant, as are those with HTLV-1.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Shoes and basic sanitation.
  • Screening before steroids or other immunosuppressants in exposed people.
  • Consider Strongyloides in anyone with HTLV-1.
  • Screening of organ donors from endemic areas.

Test yourself

Think of the answer, then tap the question to see it.

What is looked for in stool to diagnose Strongyloides?

Rhabditiform larvae, because the eggs hatch in the mucosa before leaving the body.

Which drug class most often triggers Strongyloides hyperinfection?

Corticosteroids, which accelerate autoinfection.

Why does hyperinfection come with gram-negative sepsis?

Larvae cross the bowel wall en masse and carry enteric bacteria into the blood.

What is the most sensitive stool method for Strongyloides?

Agar plate culture (Koga method).

Which virus is linked to severe strongyloidiasis?

HTLV-1.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: CDC - DPDx - Strongyloidiasis (CDC, 2019); Clinical Overview of Strongyloides | Strongyloides | CDC (CDC, 2026); Clinical Care of Strongyloides | Strongyloides | CDC (CDC, 2024); Strongyloidiasis: MedlinePlus Medical Encyclopedia (MedlinePlus (NLM), 2024).

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