Answer to Case 77

The correct answer for this case is: Entamoeba histolytica/E. dispar
Note that I've listed 2 separate protozoan parasites, separated by a slash. It is not possible to distinguish these organisms by morphologic grounds unless you see ingested RBCs in the trophozoites which are strongly suggestive of E. histolytica. Since there are none present in these images, this specimen should be reported out as E. histolytic/E. dispar, with a recommmendation for correlation with clinical presentation and additional lab testing such as serology, stool antigen testing, and/or stool PCR. Clinically, E. histolytica is a known pathogen, and can cause bloody diarrhea and disseminated disease. E. dispar, on the other hand, is NOT thought to cause disease. Since E. dispar is much more prevalent than E. histolytic, it is important to perform additional testing that will allow definitive speciation.

Congratulations to "Rabies Testing Lab" who got all of the caveats correct in this case!

On a stool preparation, the diagnosis of E. histolytica/E. dispar is made on the classic appearance of the cyst and trophozoite forms seen here, paying particular attention to the "ring and dot" nuclear chromatin pattern of Entamoeba spp. Size is also very important for making an accurate diagnosis, since other Entamoeba can look very similar, but differ in size.