Hookworm: Oval Thin-Shelled Egg with Morula
Original bench-card visuals for recognizing hookworm eggs (Ancylostoma duodenale and Necator americanus) in stool: oval, thin-shelled, with segmented morula and a clear space between shell and embryo.
Hookworm egg: thin shell, morula, clear space
Oval, 55-75 x 35-45 um, thin colorless shell with a visible clear space between shell and embryo. Contains 2-8 cell morula in freshly passed stool. A. duodenale and N. americanus eggs are morphologically identical.
55-75 x 35-45 um. Oval. Thin, colorless shell. Clear (unstained) space between shell wall and embryo. Contains 2-8 cell morula in freshly passed stool. In old specimens, larva may develop and hatch - do not examine stool older than 24 h without refrigeration.
If stool is not examined fresh, hookworm eggs embryonate and may hatch, releasing rhabditiform larvae that mimic Strongyloides. Key: fresh hookworm stool shows morula (not larvae). Finding larvae in fresh stool (not old specimen) = Strongyloides. Refrigerate stool if processing is delayed.
55-75 x 35-45 um. Oval. Thin, colorless shell. Clear (unstained) space between shell wall and embryo. Contains 2-8 cell morula in freshly passed stool. In old specim...
If stool is not examined fresh, hookworm eggs embryonate and may hatch, releasing rhabditiform larvae that mimic Strongyloides. Key: fresh hookworm stool shows morul...
What to look for in fresh stool
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Oval, thin colorless shell, 55-75 um | Hookworm egg | Thin shell + clear space between shell and morula is the bench clue; size distinguishes from Strongyloides (no eggs in stool) |
| 2-8 cell morula inside | Fresh hookworm egg | Morula = recently passed egg; well-developed larva inside = old specimen held at room temperature |
| Clear (unstained) space between shell and embryo | Hookworm - present in fresh specimens | Space shrinks as embryo develops; its presence confirms freshness |
| Thin shell without knobbed outer coat | Hookworm - not Ascaris | Ascaris lumbricoides eggs have a thick outer coat (fertile) or no outer coat (unfertile); hookworm has a thin, smooth shell |
| Larvae in fresh stool (no eggs) | Strongyloides, not hookworm | Hookworm larvae appear only in old stool; Strongyloides passes larvae in fresh stool |
Hookworm egg identification guide
| Pattern | What to call it | How to think about it |
|---|---|---|
| Oval, thin shell, morula, clear space - fresh stool | Hookworm egg - positive | Report and treat; species ID (A. duodenale vs. N. americanus) not required clinically |
| Oval, thin shell, larvae inside - old stool | Hookworm egg (embryonated) - positive, note specimen age | Confirm specimen was not fresh; larvae in old stool from hookworm eggs do not indicate Strongyloides |
| Thin-shelled egg, no outer coat, similar size | Hookworm vs. other nematodes | Compare size and shell structure; Trichuris is barrel-shaped with bipolar plugs; Ascaris has thick coat |
- Thin colorless oval shell + morula + clear space = hookworm egg in fresh stool.
- Never examine stool older than 24 h at room temperature for helminth larvae - hatched larvae may be mistaken for Strongyloides.
- A. duodenale and N. americanus eggs cannot be morphologically distinguished; species ID is not clinically required.
Oval + thin shell + morula + clear space = hookworm. Fresh stool only for larvae differentiation.