Answer to Case 594

Answer to Parasite Case of the Week 594: Entamoeba coli cysts and trophozoite.

This case shows all of the classic features of Entamoeba coli, with large organisms and, most importantly, cysts that contain more than 4 nuclei. According to the CDC DPDx, "karyosomes may be compact or diffuse and are usually eccentrically located. Peripheral chromatin is present and is often coarse, granular, and irregularly arranged along the nuclear membrane but may be more uniform. The cytoplasm of mature cysts may contain diffuse glycogen. Chromatoid bodies are seen less frequently than in E. histolytica. When present, they are usually splinter like with pointed ends, whereas the chromatoid bodies of E. histolytica have rounded ends. The cytoplasm [of trophozoites] is usually coarsely granular and vacuolated (often described as “dirty” cytoplasm). Pseudopodia may be seen and are often short and blunt."

While I would hesitate to make the diagnosis on any one of these features alone, together they strongly support the diagnosis of E. coli. Again, the presence of >4 nuclei in the cyst is the strongest diagnostic feature.

Thanks again to Florida Fan for donating this classic case!

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.