Enterobius vermicularis: Perianal Tape Prep Egg
Original bench-card visuals for recognizing Enterobius vermicularis (pinworm) eggs on perianal tape prep: asymmetric D-shape, thin colorless shell, and visible internal larva.
Enterobius vermicularis egg: tape prep morphology
D-shaped (one flat side), thin colorless shell, 50-60 x 20-30 um. Contains fully developed larva when freshly deposited. Collected by perianal tape prep (cellophane tape or Swube paddle), not routine O&P stool exam.
50-60 x 20-30 um. Asymmetric: one side flat, one side curved - the definitive shape clue. Thin, colorless shell. Fully developed larva visible inside when freshly collected. NOT found by routine stool O&P - requires perianal tape prep.
Older or post-hatch eggs lose internal larval detail and the shell may appear wrinkled or collapsed. Still D-shaped in outline. Distinction from fresh egg is primarily internal content - shell outline persists. Correct collection timing (early morning, before bathing) maximizes fresh egg yield.
50-60 x 20-30 um. Asymmetric: one side flat, one side curved - the definitive shape clue. Thin, colorless shell. Fully developed larva visible inside when freshly co...
Older or post-hatch eggs lose internal larval detail and the shell may appear wrinkled or collapsed. Still D-shaped in outline. Distinction from fresh egg is primari...
What to look for on tape prep
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| D-shaped egg, one flat side | Enterobius vermicularis | Asymmetric shape is pathognomonic - no other intestinal helminth egg has this morphology |
| Visible larva inside | Fresh egg - confirms identity | Larval detail fades in older preparations; shape alone is sufficient for ID |
| Thin colorless shell, 50-60 um | E. vermicularis | Shell is much thinner than Ascaris or Trichuris; no thick outer coat |
| Round or oval symmetric eggs on tape | Not E. vermicularis | Symmetric oval eggs on tape prep may be environmental contaminants or artifact; pinworm eggs are D-shaped |
| No eggs on tape | Possible false negative | Collect 3 consecutive early-morning preps before calling negative; eggs deposited nocturnally |
Enterobius tape prep interpretation guide
| Pattern | What to call it | How to think about it |
|---|---|---|
| D-shaped egg, flat side, thin shell, larva inside | Enterobius vermicularis - positive | Report and treat; household contacts should also be evaluated given fecal-oral and environmental transmission |
| D-shaped outline, collapsed shell, no larval detail | Enterobius vermicularis - positive (older egg) | Shape alone is sufficient for ID; report as positive |
| No eggs on 1 tape prep | Indeterminate - false negative likely | Collect 3 consecutive early-morning preps; sensitivity increases to ~90% with 3 exams |
- D-shaped (asymmetric, one flat side) is the diagnostic clue - no other worm egg has this shape.
- Tape prep only: routine stool O&P will miss Enterobius in virtually all cases.
- Collect early morning on 3 consecutive days for best sensitivity.
D-shape + thin shell + larva inside = Enterobius egg. Tape prep, not O&P.