Answer to Case 816

Answer to the Parasite Case of the Week 816: Trichuris trichiura adult.

Our readers presented an excellent differential diagnosis based on the appearance of the nematode and its location in the intestinal tract. Even if you don't have a lot of experience with histopathologic sections, you can narrow the differential based on a few key features:

First, the gross appearance is consistent with a small nematode (10mm long according to the measurements provided, but we can also approximate the size based on the diameter of the intestinal lumen). The small size helps us narrow the differential to a few potential nematodes: Enterobius vermicularis, hookworm, Trichuris trichiura (assuming that part of the worm broke off during removal), an anisakid larva, and an immature Ascaris lumbricoides. It's great that we have 2 sequential photos in this case, showing how the tail curled up on itself, leading me to think that it might be a male nematode:

Second, we can see from the microscopy images that the worm is embedded in the colonic mucosa.

A. lumbricoides and E. vermicularis do not invade the intestinal wall, so we can rule them out, leaving us with just T. trichiura, hookworm, and anisakids in the differential.  

Lastly, the location in the cecum helps us rule out the hookworms as they are typically found in the small intestine rather than the colon. As Blaine Mathison mentioned, "Trichuris’ favorite childhood playground game is hide-in-cecum 😎" E. vermicularis also likes this location. Our job is now much easier!

But how do we make a final identification? This is where the histopathologic features play a key role:

• There are no lateral alae, so we can rule out E. vermicularis.

• There is a stichosome and bacillary band. A stichosome is a multicellular organ comprised of stichocytes (see white arrows below) that is very prominent in some nematodes, include members of the Trichinellidae and Trichuridae.

• Another very helpful feature would have been the presence of eggs. However, there are none seen in this case, possibly because the worm might be a male. However, the other features mentioned above still allow us to make a definitive diagnosis. 
• The last helpful histopathologic feature is when the embedded portion is much smaller in diameter than the portion left hanging in the intestinal lumen. As many of you know, the narrow end is the head, and this is the end that embeds itself in the intestine. The thicker end hangs free in the lumen, allowing eggs to be easily shed into the stool from the female worms.

Thanks again to Dr. Daniel Montelong for donating this great case!