Strongyloides stercoralis
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Direct answer
Strongyloides stercoralis is a nematode that can complete its life cycle inside a single human host, so infection may persist for many years through autoinfection. Its larvae are passed in stool, not eggs, which makes stool examination for larvae the key diagnostic step. The high-yield fact is that corticosteroid therapy or other immunosuppression can trigger hyperinfection with gram-negative bacteraemia, and ivermectin is the drug of choice.
What you must remember
- Larvae in stool: Rhabditiform larvae with a short buccal cavity are seen in stool, and eggs are rarely found.
- Filariform larva: The infective third-stage larva penetrates skin, usually of the feet.
- Larva currens: A rapidly migrating, serpiginous urticarial track on the trunk or buttocks is a classic skin sign.
- Autoinfection: Rhabditiform larvae can mature into filariform larvae inside the gut or perianal skin, sustaining infection without new exposure.
- Hyperinfection: Heavy larval load with dissemination follows corticosteroid use, HTLV-1 infection or other immunosuppression.
- Gram-negative sepsis: Enteric bacteria carried by migrating larvae cause bacteraemia in hyperinfection.
- Diagnosis: Agar plate culture is the most sensitive method, and the Baermann technique concentrates larvae.
- Treatment: Ivermectin 200 micrograms per kg daily for two days is the usual regimen, and albendazole is less effective.
A cycle that can restart inside the same patient
Strongyloides is unusual because the parasite can multiply inside the host without fresh exposure. The pathway below shows how a mild infection can progress to a life-threatening illness, and why immunosuppression changes the outcome.
- Skin entry: Filariform larvae in soil penetrate the skin, most often of the feet, and migrate through the blood to the lungs.
- Intestinal settlement: Larvae are coughed up, swallowed and mature into parthenogenetic females in the small intestine. This is the phase that establishes chronic infection.
- Larval output: Females lay eggs that hatch within the intestinal mucosa, releasing rhabditiform larvae that pass in stool.
- Autoinfection: Some rhabditiform larvae transform into filariform larvae inside the intestine or perianal skin and reinfect the same host, which allows infection to persist for decades.
- Hyperinfection: Immunosuppression increases the larval burden, and larvae can then disseminate to the lungs, liver and other organs.
- Diagnosis and treatment: Agar plate culture or the Baermann test finds larvae, serology supports the diagnosis when larvae are not seen, and ivermectin is given with repeat stool checks because relapse is possible.
Where the exam tries to catch you
Most mistakes come from looking for eggs, using albendazole first and ignoring the risk before steroids.
- Trap: Looking for eggs in stool to confirm strongyloidiasis Correct: larvae are seen in stool, since eggs hatch in the intestinal mucosa.
- Trap: Assuming a single negative stool excludes infection Correct: larval output is low and variable, so agar plate culture or serology is used.
- Trap: Using albendazole as the first choice Correct: ivermectin is the drug of choice, with albendazole reserved as a less effective alternative.
- Trap: Starting high-dose steroids without screening Correct: presumptive ivermectin is considered for patients from endemic regions before steroids, since hyperinfection can be fatal.
Frequently asked questions
What is the drug of choice for Strongyloides stercoralis?
Ivermectin is the drug of choice for Strongyloides stercoralis, usually given at 200 micrograms per kilogram daily for two days. Albendazole is less effective and is not preferred for this infection.
Why are larvae rather than eggs found in stool in strongyloidiasis?
Strongyloides females lay eggs that hatch within the intestinal mucosa, so rhabditiform larvae rather than eggs pass in stool. This makes larvae the key finding on stool examination for this infection.
What is hyperinfection in strongyloidiasis?
Hyperinfection is a heavy larval burden with dissemination to organs, typically triggered by corticosteroids or other immunosuppression. It can cause gram-negative bacteraemia and carries a high risk of death if untreated.
What is larva currens?
Larva currens is a rapidly migrating, serpiginous urticarial track on the skin caused by Strongyloides larvae moving through the epidermis. It is classically seen on the trunk and buttocks and is strongly suggestive of this infection.
Which test is most sensitive for Strongyloides stercoralis?
Agar plate culture is the most sensitive test for Strongyloides stercoralis, because larvae migrate across the plate and can be counted. Serology with ELISA is also useful when larvae are not seen in stool.