Answer to Case 454

Answer: Tapeworm, Diphyllobothrium species

This case shows the classic morphology of an adult tapeworm, consisting of a flat ribbon-like body composed of multiple repeating units called proglottids. This particular tapeworm can be identified by the morphology of its proglottids and eggs. The proglottids are broader/wider than they are long; hence the name the "broad fish tapeworm". Within the center of each proglottid is a central uterine structure which is often visible macroscopically. In this case, it can be seen as a central 'hump' in each proglottid:

The eggs are also characteristic for this genus and allow for differentiation between Diphyllobothrium and Taenia species. Note the thin wall, oval shape, intermediate size (55-75 micrometers long) of the egg and the operculum (lid-like opening) at one end. In many cases, a knob-like structure can be seen at the other end (abopercular knob), but it is not clearly visible in this case.

The most reliable means for identifying the infecting Diphyllobothrium species is through molecular testing. While D. latum is the most common cause of human infection worldwide, other species that can infect humans include D. pacificum, D. yonagoensis, D. dendriticum, D. dalliae, D. cordatum, D. ursi, and D. lanceolatum.