Giardia lamblia: Trophozoite and Cyst
Original bench-card visuals for recognizing Giardia trophozoite and cyst morphology in wet preparations and stained stool concentrates, including median body placement and nuclear arrangement.
Giardia lamblia: trophozoite and cyst
Trophozoite (en face): bilateral symmetry, paired nuclei ("owl eyes"), two comma-shaped median bodies, ventral sucking disc. Lateral view: "shepherd's crook" shape. Cyst: oval, up to 4 nuclei, retracted curved axonemes.
10-20 um. En face: bilateral symmetry, two nuclei ("owl eyes"), two comma-shaped median bodies (Giardia-specific). Lateral view: "shepherd's crook." Falling-leaf motility in saline wet mount.
8-14 um oval. Mature: 4 nuclei; immature: 1-2. Curved axoneme fibrils visible on iodine stain. Chlorine-resistant, environmentally stable.
10-20 um. En face: bilateral symmetry, two nuclei ("owl eyes"), two comma-shaped median bodies (Giardia-specific). Lateral view: "shepherd's crook." Falling-leaf mot...
8-14 um oval. Mature: 4 nuclei; immature: 1-2. Curved axoneme fibrils visible on iodine stain. Chlorine-resistant, environmentally stable.
What to look for by stage
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Pear shape, bilateral symmetry (en face) | Trophozoite | Mirror-symmetric from front to back when viewed face-on; most distinctive low-power clue |
| "Shepherd's crook" or spoon shape (lateral) | Trophozoite | Lateral view - sucking disc curves one face; aids ID when en-face view is unavailable |
| Paired anterior nuclei ("owl eyes") | Trophozoite | Two nuclei in anterior half, each with a small central karyosome; bilateral placement is key |
| Two comma-shaped median bodies | Trophozoite | Posterior to nuclei; claw- or comma-shaped; unique to Giardia among intestinal protozoa - best seen on trichrome |
| Oval cyst, <=4 nuclei, retracted fibrils | Cyst | Mature cyst has 4 nuclei; curved axoneme fibrils visible on iodine; nuclei may cluster at one end in immature forms |
| Falling-leaf motility | Live trophozoite | Characteristic tumbling/rotating movement in saline wet mount; lost in cold or old specimens |
Giardia morphology identification guide
| Pattern | What to call it | How to think about it |
|---|---|---|
| Bilateral symmetry, 2 nuclei, comma-shaped median bodies | Trophozoite | Trichrome stain shows median bodies and flagellar detail; saline wet mount captures motility |
| Oval, <=4 nuclei, curved axoneme fibrils, thick cyst wall | Cyst | Iodine highlights nuclei and retracted fibrils; concentration method (formalin-ethyl acetate) improves yield |
| Equivocal morphology or low-burden specimen | Confirm with EIA or NAAT | Antigen detection (EIA) and multiplex GI panels are more sensitive for light infections |
- Bilateral symmetry (en face) and "shepherd's crook" (lateral) are the two views to know.
- Median bodies - comma-shaped, posterior to the nuclei - are unique to Giardia and confirm the identification.
- Cysts are chlorine-resistant, environmentally stable, and the infective stage; trophozoites do not survive outside a host.
Two eyes (nuclei) + two commas (median bodies) = Giardia trophozoite. Oval + curved fibrils = cyst.