Case #360 – November 2013

A county health department in Kansas routinely collects stool specimens as part of their refugee screening program. Stool is collected in 10% formalin and zinc-PVA (Zn-PVA) and forwarded to the state health laboratory for ova-and-parasite (O&P) examination. Images of any suspicious objects and organisms are captured and sent to the DPDx Team whenever there is a need for confirmatory diagnosis. Figures A–D show three such objects observed on a trichrome-stained slide of stool preserved in Zn-PVA. A scale bar was included in Figure B to show the average size of the objects. The objects in Figures C and D represent the same structure in two different focal planes. What is your diagnosis? Based on what criteria?

Figure A

Figure B

Figure C

Figure D

This was a case of Dientamoeba fragilis infection. All three organisms demonstrated the binucleate form of the trophozoite and were within the size range (5-15 micrometers) of the species. The nuclei lack peripheral chromatin and possess a fragmented karyosome. The trophozoite in Figure C shows the large central vacuole sometimes observed in D. fragilis.