Answer to Case 790

Answer to the Parasite Case of the Week 790: Rodentolepis (formerly Hymenolepis) nana egg.

This is a classic example of an R. nana egg showing the inner and outer membranes, oncosphere with large splayed hooklets, and polar thickenings from which filaments extend into the space between the oncosphere and outer membrane. It's hard to make out the polar filaments in this particular example, but the smaller size allows us to differentiate it from the larger Hymenolepis diminuta egg.

Interestingly, most readers on social media are still referring to this parasite as Hymenolepis nana. However, I can confirm that Rodentolepis is now the preferred genus. For further reading, I recommend the systematic study of hymenolepidid cestodes by Haukisalmi and colleagues which supports the split into separate genera for a more practical and stable classification. 

Thanks again to Dr. Tudo Rares Olariu for donating this classic case.