Case of the Week 663

 Answer to the Parasite Case of the Week 663: Not a human parasite; "Beaver bodies".

Beaver bodies are sporocysts of the unicellular, eukaryotic protozoon, Psorospermium cf. haeckeli (Hilgendorf 1883), a parasite of crayfish. They may be seen in the stool of individuals after eating crayfish, and thus obtaining a dietary history may be helpful. Sporocysts are the host-dependent stage, and are released into the environment after the crayfish host dies. In nature, free-living amoeboids emerge from the sporocyst and are capable of infecting new crayfish.

The genus Psorospermium is located phylogenetically near the animal-fungal divergenc and was formerly thought to be an alga. Only one species (haeckeli) has been described, but molecular analysis suggests that there may be multiple species. 

Psorospermium sporocysts have a distinct appearance, but may be mistaken for various helminth eggs such as those of Trichuris trichiura and Enterobius vermicularis, as well as diatoms. Fortunately, Psorospermium sporocysts can be differentiated by their large size and characteristic shape.

Here is a comparison between beaver bodies and diatoms:

Thanks again to Monica and Lilian for sharing this great case!