Tuesday, September 29, 2020

Answer to Case 608

 Answer to the Parasite Case of the Week 608: Taenia sp. egg. The eggs are those of T. solium, T. saginata, or T. asiatica, but it is not possible to tell the species from the egg alone. You can find more informations about these interesting cestodes on the CDC's DPDx website.

Note the classic small size (30-35 micrometers in diameter) of the eggs. Each have a thick radially-striated outer wall and internal 6-hooked oncosphere. 

The hooks aren't easily visible with H&E, but they are refractile when viewed with a narrowed condenser. It is likely that the eggs were released from a mature gravid proglottid in the large intestine and made their way into an ulcerated region of the mucosa where they were an incidental finding on biopsy. Taenia solium and T. saginata do not invade the mucosa (other than at the point where the hooklets anchor the adult worm's scolex to the small intestine), so there must have been some other condition present that caused the intestinal disruption in this patient.

Some readers commented that this could also be Echinococcus sp. if this was a canid rather than a human host. Echinococcus spp. adults are found within the canid definitive host and the eggs are shed into the environment where they are ingested by herbivore intermediate hosts. Humans are accidental intermediate hosts. After ingesting the eggs from the environment, the eggs hatch in the small intestine to release the oncosphere. Thus we wouldn't expect to see intact Echinococcus sp. eggs in humans. The oncospheres penetrate the wall of the human host and enter the circulatory system to travel to the organs (usually the liver) and form the cystic form of the parasite.

Thanks again to Dr. Gilligan for donating this fascinating case!

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