Microscopy

Rhabditoid first-stage (L1) larvae

The first-stage rhabditoid larvae (L1) of Strongyloides stercoralis are 180-380 µm long, with a short buccal canal, a rhabditoid esophagus and a prominent genital primordium.  These L1 larvae are usually found in stool, as the eggs embryonate and hatch in the mucosa of the small intestine of the host.  They may also be found in soil and cultured feces.

A	B

A: Rhabditoid larva of S. stercoralis in an unstained wet mount of stool.  Notice the rhabditoid esophagus (blue arrow) and prominent genital primordium (red arrow).
B: Rhabditoid larva of S. stercoralis in an unstained wet mount of stool.  Notice the prominent genital primordium (blue arrow), rhabditoid esophagus (red arrow) and short buccal canal (green arrow).

C	D

C: Close-up of the anterior end of a rhabditoid larva of S. stercoralis, showing the short buccal canal (red arrow) and the rhabditoid esophagus (blue arrow).  Image taken at 1000x oil magnification.
D: Rhabditoid larva of S. stercoralis in unstained wet mounts of stool.  Notice the short buccal canal and the genital primordium (red arrows).

Filariform (L3) larvae

Infective, third-stage filariform larvae (L3) of Strongyloides stercoralis are up to 600 µm long.  The tail is notched and the esophagus to intestine ratio is 1:1.  Infective L3 larvae are found in soil and invade the human host by direct penetration of the skin.  They may be found in respiratory specimens during cases of autoinfection.

E	F

E, F:  Filariform (L3) larvae of S. stercoralis in a sputum specimen, stained with Giemsa.  Notice the characteristic notched tail in Figure F.  Image E taken at 200x magnification; Image F at 1000x oil magnification.

Adult parasitic females and larvae in tissue

Adults and larvae of Strongyloides stercoralis in tissue specimens, stained with hematoxylin and eosin (H&E).

G	H

G: Cross-sections of female S. stercoralis (blue arrows) in small intestine tissue, stained with H&E.  Image taken at 200x magnification.
H: Sections of S. stercoralis from a duodenal biopsy specimen, stained with H&E.  Although Strongyloidiasis could not be confirmed based on microscopy alone, this case was confirmed using molecular methods (PCR).  Image taken at 200x magnification.

I	J

I: Higher magnification (1000x oil) of a female of S. stercoralis from the same specimen as Figure G.  Notice the intestine (red arrow) and ovaries (blue arrows).
J: Higher magnification (1000x oil) of a gravid female of S. stercoralis from the same specimen as Figure G.  Notice the intestine (blue arrow), ovary (red arrow) and an egg within the uterus (green arrow).

 

K	L

K: Cross-sections of larvae of S. stercoralis in a intestinal biopsy specimen, stained with H&E.  Image taken at 1000x oil magnification.  The patient was infected with Strongyloides following transplant of an infected kidney.
L: Longitudinal-section of a larva of S. stercoralis from the same specimen as Figure K.  Image taken at 400x magnification.