Answer to Case 765

 Answer to the Parasite Case of the Week 765: Strongyloides stercoralis L4 larvae

As noted by Idzi Potters, "The presence of a relatively large genital primordium confirms Strongyloides. The apparently tapered tail (not notched) together with the size of the larva point towards the fourth-stage autoinfective larva (L4)."

For those of you who were thinking these were autoinfective filariform (L3) larvae, you were very close! L4 larvae are the next stage in development after autoinfective L3 larvae and may be potentially detected in human respiratory specimens. Richard Bradbury and his colleagues just published a very interesting article in the Journal of Clinical Microbiology on the presence of L4 S. stercoralis larvae in two separate cases, and we received permission from JCM and the Editor-in-Chief to reproduce some of the images here. 

The major differences between S. stercoralis L3 and L4 larvae are as follows:

• Length: autoinfective L3 are <600 µm and L4 are ~800 µm
• Genital rudiment: small in L3 and elongated in L4 larvae
• Vulva: may be visible in L4 (not L3)
• Tail: L3 is notched whereas L4 is more tapered

Here are some images to illustrate these features; I refer you to the excellent article by Dr. Bradbury and his colleagues for more information, including an image of a L4 larvae/pre-adult female with a vaginal opening. 

Figure legend: Strongyloides stercoralis autoinfective L4 larvae. Abbreviations: An, anus; Bc, buccal cavity; Gr, genital rudiment; Nr, nerve ring; Oes/In, esophageal-intestinal junction.

Thanks again to Drs. Cameron Truarn and Damien Bradford from PathWest Laboratory Medicine, and Drs. Huan Zhao and Richard Bradbury from James Cook University, Australia for donating this interesting case!