Case #520 – July, 2020
A 67-year-old male from Houston, TX sought medical evaluation following 3 days of abdominal pain, diarrhea, and fatigue after returning from a two-week summer vacation in Nairobi, Kenya. Examination of a trichrome stained Zn-PVA stool smear submitted for routine ova and parasite (O&P) revealed objects shown in Figures A–D, ranging in size from 12-25μm. What is your diagnosis? Based on what criteria?
Figure A
Figure B
Figure C
Figure D
This was a case of amebiasis caused by Entamoeba polecki (Figures A and B) and blastocystosis caused by Blastocystis hominis (Figures C and D). Diagnostic morphologic features included:
- Trophozoites within the size range of E. polecki 10-25µm (Figures A and B).
- Large nucleus with evenly distributed peripheral chromatin and large central karyosome (Figures A and B).
- Vacuoles in cytoplasm (Figures A and B, blue arrow).
- Structures within the size range of B. hominis 5–40 µm (Figures C, D).
- Large central vacuole surrounded by several small nuclei (Figures C, D). Blastocystis hominis is a pleomorphic organism and the predominant form found in human stool specimens is referred to as the vacuolar (or central body) form.
Both E. polecki and B. hominis are considered non-pathogenic parasites. However, the pathogenicity of Blastocystis must be considered when found in large numbers in patients with abdominal symptoms who have no other plausible etiology.
 Figure A |
 Figure B |
More on amebiasis
More on blastocystosis
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.
DPDx is an educational resource designed for health professionals and laboratory scientists. For an overview including prevention, control, and treatment visit www.cdc.gov/parasites/.