Answer to Parasite Case of the Week 552: Enteromonas hominis
This was a tough one! Not a surprise since it came from Blaine 😊
The organisms shown are small trophozoites, measuring only 5 to 7 micrometers in greatest dimension, and no cysts were seen, thus making identification a bit challenging. While I think that some of these trophozoites look a bit like Chilomastix mesnili (for example, some of the larger forms in the first image), I think that the smaller size, less elongated shape without a well-defined terminal point, and larger karyosome is more consistent with E. hominis.
The CDC doesn't have any good trophozoite photos on the DPDx webpage, but Henry Bishop kindly shared these following beautiful photos with me awhile back. They are a bit clearer than in the current case and nicely demonstrate the diagnostic features.
So what about that positive Giardia antigen? A bit of a red herring in this case. Giardia was actually present in this case, but not included in the images I presented.