Answer to Case 821

Answer to the Parasite Case of the Week 821: Cystic echinococcosis (hydatid cyst) due to Echinococcus granulosus sensu lato.

Great job to everyone who got this one! As Florida Fan, Patrik, Jaime and many others noted, the objects in the cyst fluid are protoscolices, and the additional structure in the final image is a free hooklet. Taken together with the source of liver cyst, these findings are diagnostic of cystic echinococcosis (CE).  

The free protoscolices and detached hooklets are collectively referred to as hydatid sand. Each protoscolex has the potential to develop into an adult tapeworm if ingested by a suitable definitive canid host. 

Sarah Sapp noted "Wow, I’ve never seen such a tight cluster on an aspirate wet mount! Cool 😎", and many other readers agreed. The photos in this case are beautiful! 

Here is a closer view of a protoscolex, demonstrating the invaginated scolex, complete with suckers and a rostellum armed with hooklets. 

Depending on their orientation and degree of evagination, protoscolices can have somewhat different appearances, as demonstrated nicely in this case. 

The small oval-shaped structures within the protoscolices are calcareous corpuscles - small, round-to-oval mineralized inclusions found in the parenchyma of cestodes, including adult and larval stages. They are a characteristic histologic feature of cestodes and can be quite useful diagnostically. Despite the name, they aren't made just of calcium, but also contain magnesium salts, phosphorus, and an organic matrix. 

I've always found calcareous corpuscles to be fascinating, so I'd like to share a little bit more about them. Calcareous corpuscles are produced through a regulated process of biomineralization, usually within cells of the cestode parenchyma, although the precise mechanism varies among species. In at least some cestodes, they begin with an organic matrix that serves as a nidus, onto which calcium, magnesium, phosphate, carbonate, and other ions are progressively deposited. Continued mineral deposition produces the characteristic rounded or ovoid corpuscle, often with concentric lamination.

Their function is not fully understood, but several roles have been proposed, including mineral storage and homeostasis, buffering and acid-base regulation, sequestration of waste products and potentially toxic substances, and osmoregulation. It's quite possible that they perform more than one of these functions.

Finally, a brief word about the name. Historically, cystic echinococcosis was attributed simply to Echinococcus granulosus. We now know that what was called E. granulosus encompasses a genetically diverse group of closely related taxa, several of which infect humans. Unless molecular identification has been performed, Echinococcus granulosus sensu lato is therefore the most accurate designation for this case. Florida Fan gets bonus points for including the sensu lato!

Many thanks to Loranne Lambregts and our colleagues at the Institute of Tropical Medicine, Antwerp, for sharing this beautiful case.