Answer: Echinococcus multilocularis
This is a difficult case, especially for those that are not used to looking at tissue sections. The history of a rapidly growing liver mass helps in formulating a differential diagnosis, and as one of the readers pointed out, Entamoeba histolytica must also be considered. The lack of travel history outside of the U.S. helps to exclude E. histolytica amebic liver abscess. The fact the patient is an outdoorsman is consistent with potential exposure to the natural hosts of this parasite: foxes and rodents. Infection with E. multilocularis, otherwise known as alveolar hydatid disease, is spreading throughout the American Midwest, and is also endemic in parts of Europe.
The diagnostic features of this case include the cystic nature of the lesion, the cellular septae (walls separating the cysts), calcaneous bodies/corpuscles, and hooked protoscoleces within the cysts. These features are pointed out on the images below:
The calcaneous corpuscles are very helpful in identifying cestodes in tissue sections, since both adults and intermediate forms have them. They simply represent non-specific calcified material, and can be seen in sections of Taenia solium cysticercosis and sparganosis, in addition to Echinococcus spp.
An important differential in this case is Echinococcus granulosus, since this species typcially causes more benign disease. While the two closely related species may cause similar appearing tissue masses, the cysts of E. granulosus are surrounded by layers of acellular material (the laminated layer), instead of being separated by cellular sepatae.
Here is a classic example of the laminated layer of E. granulosus:
Here, a small cyst of E. granulosus containing a single protoscolex is surrounded by a laminated layer.
My thanks to everyone who wrote in on this challenging case!