Case #503 – November, 2019

A 17-year-old male sought medical attention after three days of intermittent diarrhea, abdominal pain and loss of appetite. Routine stool examination for ova and parasite (O&P) was ordered and a fecal specimen was collected in zinc PVA (Zn-PVA). Figures A–F show what was observed on a trichrome-stained smear made from the Zn-PVA preserved stool sample. The structures of interest were detected at 1000x oil magnification and measured 6-14 µm.

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Figure A

Figure B

Figure C

Figure D

Figure E

Figure F

This case presented a mix of two species of intestinal protozoa: Endolimax nana (Figures A, B, C, and F), which is generally considered non-pathogenic, and Dientamoeba fragilis (Figures D and E). Diagnostic morphologic features included:

trophozoites of E. nana with a single nucleus containing a large, central karyosome and no peripheral chromatin (Figures A, B, and F).
cysts of E. nana containing 2-4 nuclei with discrete karyosomes (Figures B and C).
trophozoite of Dientamoeba fragilis with two nuclei (which are fragmented karyosome, and no peripheral chromatin) within the size range for D. fragilis. (Figure D, E).

Although E. nana is among the amoebae generally considered nonpathogenic, this finding should be reported in all routine ova-and-parasite (O&P) examinations. The presence of E. nana in stool could indicate fecal contamination of a food or water source and does not rule out the presence of a true pathogen. The pathogenicity and clinical importance of D. fragilis is still under investigation. Both asymptomatic and symptomatic infection (e.g., with various nonspecific gastrointestinal symptoms) have been reported.

For more information on E. nana, please visit: https://www.cdc.gov/dpdx/intestinalamebae/index.html

For more information on D. fragilis, please visit: https://www.cdc.gov/dpdx/dientamoeba/index.html

Images presented in the dpdx case studies are from specimens submitted for diagnosis or archiving. On rare occasions, clinical histories given may be partly fictitious.