Case #395 May 2015
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Stool specimens were collected from an asymptomatic 12-year-old boy from Myanmar as part of a refugee screening program. The stool was collected in 10% formalin and zinc PVA (Zn-PVA) and sent to the county health department for routine ova-and-parasite (O&P) examination. Figures A-D show what was observed at 1000x magnification with oil on a trichrome-stained slide made from stool preserved in Zn-PVA. The objects shown in Figures A-C measured 12-15 µm in length, on average; the object in Figure D measured 7 µm. What is your diagnosis? Based on what criteria?
Figure A
Figure B
Figure C
Figure D
Case Answer
The organisms observed in this case represented the nonpathogenic flagellate, Chilomastix mesnili. Diagnostic morphologic features included:
- pyriform trophozoites (Figures A-C) possessing a large, anteriorly-positioned nucleus; the nucleus contains an eccentric karyosome and granular peripheral chromatin. Trophozoites also have three anterior flagella, although they are often not easily visualized on permanently stained specimens.
- a lemon-shaped cyst (Figure D), containing a single nucleus and fibrils; the nucleus contains a large karyosome and irregular peripheral chromatin.
Chilomastix mesnili is not considered pathogenic. It should, however, be reported in all O&P examinations, as its presence can be indicative of a fecal-contaminated food or water source.
More on: Chilomastix mesnili
Images presented in the monthly case studies are from specimens submitted for diagnosis or archiving. On rare occasions, clinical histories given may be partly fictitious.
DPDx is an education resource designed for health professionals and laboratory scientists. For an overview including prevention and control visit www.cdc.gov/parasites/.
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- Page last updated: August 24, 2016
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- Global Health – Division of Parasitic Diseases and Malaria
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