Toxoplasma gondii: Tachyzoite and Tissue Cyst
Original bench-card visuals for recognizing Toxoplasma gondii tachyzoites and bradyzoite-filled tissue cysts: crescent-shaped tachyzoites, nucleus toward the broad end, and bradyzoites in PAS-positive tissue cysts.
Toxoplasma gondii: tachyzoite and tissue cyst with bradyzoites
Tachyzoite: crescent/banana shape, 2-3 um wide x 4-8 um long, Giemsa: pale blue cytoplasm, red nucleus at broad end. Bradyzoites: within tissue cysts (up to 100 um), cyst wall PAS-positive. Brain, eye, heart, and skeletal muscle a...
2-3 um wide x 4-8 um long. Crescent or banana shape. Giemsa: cytoplasm stains pale blue, nucleus stains red toward the broader end. One end is more pointed than the other. Found in acute infections, immunocompromised hosts, and congenital toxoplasmosis. Multiply rapidly (acute phase) by endodyogeny. Seen in BAL, CSF, peritoneal washings, and tissue impression smears.
Tissue cyst: up to 100 um, thin but distinct wall, PAS-positive. Contains hundreds of bradyzoites (slow-multiplying form, chronic stage). Bradyzoites are morphologically similar to tachyzoites but smaller and within the cyst. Preferentially located in brain, retina, cardiac and skeletal muscle. Cysts persist lifelong; reactivate under immunosuppression (CD4 <100 cells/umL).
2-3 um wide x 4-8 um long. Crescent or banana shape. Giemsa: cytoplasm stains pale blue, nucleus stains red toward the broader end. One end is more pointed than the...
Tissue cyst: up to 100 um, thin but distinct wall, PAS-positive. Contains hundreds of bradyzoites (slow-multiplying form, chronic stage). Bradyzoites are morphologic...
What to look for in stained preparations
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Crescent/banana shape, 4-8 um, nucleus at broad end | Toxoplasma tachyzoite | Giemsa or Wright stain shows pale blue cytoplasm and red nucleus; BAL or CSF are the most common diagnostic specimens |
| One pointed end, one rounded end | Tachyzoite morphology | Asymmetry is the key shape clue; distinguishes from Leishmania amastigotes (round, smaller, intracellular) |
| Round cyst up to 100 um, PAS+ wall, many organisms inside | Tissue cyst with bradyzoites | Hematoxylin-eosin shows cyst outline; PAS highlights cyst wall; bradyzoites visible at high magnification |
| Intracellular clusters in macrophages (acute) | Tachyzoites or early tissue cysts | In acute phase, tachyzoites replicate in any nucleated cell; in immunocompromised, brain cysts reactivate |
| No diagnostic stage in feces from human | Oocysts only from cats | Humans shed no diagnostic stages in stool; serology (IgM/IgG) and PCR are primary diagnostic tools |
Toxoplasma gondii identification guide
| Pattern | What to call it | How to think about it |
|---|---|---|
| Crescent tachyzoites, pale blue cytoplasm, red nucleus at broad end | T. gondii tachyzoites - active infection | Report with specimen source; confirm with PCR and serology; urgent in CNS or congenital cases |
| Round cyst, PAS+ wall, bradyzoites inside, in brain/muscle tissue | T. gondii tissue cyst - latent infection | Indicates seropositive/past infection; reactivation risk under immunosuppression; no treatment unless reactivation occurs |
| Equivocal morphology | Confirm with PCR or serology | Tachyzoites may be confused with other intracellular organisms; PCR on specimen is definitive |
- Crescent shape + nucleus at broad end = T. gondii tachyzoite on Giemsa/Wright stain.
- Tissue cyst (up to 100 um, PAS+ wall, bradyzoites inside) in brain/muscle = latent toxoplasmosis.
- Serology and PCR are the primary diagnostic tools; morphology is confirmatory when organisms are visualized.
Crescent tachyzoite = acute. PAS+ tissue cyst with bradyzoites = latent/chronic. Serology and PCR drive diagnosis, not smear.