Strongyloides stercoralis: Rhabditiform Larva
Original bench-card visuals for recognizing Strongyloides stercoralis rhabditiform larva in fresh stool: short buccal channel and visible genital primordium as the key morphologic clues against hookworm larval mimic.
Strongyloides stercoralis: rhabditiform vs. filariform larva
Rhabditiform (passed in stool): short buccal channel (key vs. hookworm: long buccal channel), genital primordium visible. Filariform (infective): longer, notched tail, no genital primordium. Size ~250 um (rhabditiform).
~250 um. Short buccal channel is the primary clue vs. hookworm larvae (long buccal channel). Genital primordium: a cluster of cells visible in the mid-body - not present in hookworm rhabditiform larvae. Found in fresh stool; Baermann or agar plate method improves sensitivity. The stage passed in stool in primary infections.
~550 um. Infective stage - develops from rhabditiform in soil or intestine (autoinfection). Notched tail (vs. hookworm filariform: pointed tail). No genital primordium. Finding filariform larvae in stool suggests hyperinfection - a clinical emergency in immunocompromised patients.
~250 um. Short buccal channel is the primary clue vs. hookworm larvae (long buccal channel). Genital primordium: a cluster of cells visible in the mid-body - not pre...
~550 um. Infective stage - develops from rhabditiform in soil or intestine (autoinfection). Notched tail (vs. hookworm filariform: pointed tail). No genital primordi...
What to look for by larval stage
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Short buccal channel | Strongyloides rhabditiform | Most critical clue vs. hookworm rhabditiform larvae which have a longer buccal channel |
| Genital primordium (cell cluster, mid-body) | Strongyloides rhabditiform | Not present in hookworm larvae; confirms Strongyloides when visible |
| Notched tail | Strongyloides filariform larva | Hookworm filariform has a pointed tail - use tail shape to differentiate infective stages |
| ~250 um length, actively motile | Rhabditiform in fresh stool | Must examine fresh warm stool (within 1-2 hours) or use Baermann/agar plate method |
| Filariform larvae in stool (not soil-exposed) | Possible hyperinfection or dissemination | In immunocompromised hosts, finding filariform in stool or other sites warrants urgent evaluation |
Strongyloides larval identification guide
| Pattern | What to call it | How to think about it |
|---|---|---|
| Rhabditiform larva, short buccal channel, genital primordium | Strongyloides stercoralis | Report and treat; evaluate for hyperinfection risk (immunosuppression, steroid use) |
| Rhabditiform larva, long buccal channel, no genital primordium | Hookworm rhabditiform (from hatched egg) | Confirm by finding hookworm eggs in the same specimen; examine fresh stool only |
| Filariform larva in stool, notched tail | Strongyloides filariform - possible hyperinfection | Urgent: assess immune status; filariform larvae may carry gut bacteria causing sepsis in hyperinfection syndrome |
- Short buccal channel + genital primordium = Strongyloides rhabditiform (vs. hookworm: long buccal channel, no genital primordium).
- Strongyloides passes larvae, not eggs - rhabditiform larvae in stool without eggs = Strongyloides until proven otherwise.
- Filariform larvae in stool = possible hyperinfection: assess immune status urgently.
Short buccal channel + genital primordium = Strongyloides. Long buccal channel = hookworm. Filariform in stool = hyperinfection risk.