Case #270 – February, 2010

A six-year-old boy was taken to a local clinic for abdominal discomfort. A stool specimen was collected in formalin for routine ova-and-parasite (O&P) examination. Figures A–B show what was observed on an unstained wet mount preparation after the specimen was concentrated. All images were taken at 400x magnification. The objects of interest measured approximately 40-43 micrometers in diameter. What is your diagnosis? Based on what criteria?

Figure A

Figure B

This was a case of hymenolepiasis caused by Hymenolepis nana (the dwarf tapeworm). Diagnostic morphologic features shown in the figures included:

eggs with a thin, hyaline shell and an oncosphere in which hooks can be seen. Six hooks should be present, but they are not always visible in the same focal plane.
eggs within a size range consistent with those of H. nana (30 to 50 micrometers in diameter). Size is the primary feature used to distinguish eggs of H. nana from those of H. diminuta (70-85 micrometers by 60-80 micrometers).
the presence of polar filaments extending from opposite ends of the oncosphere (lacking in H. diminuta).