## Discriminating Features: *Trypanosoma brucei* vs *Leishmania donovani* **Key Point:** The **undulating membrane with a free flagellum** is the hallmark morphological feature of *Trypanosoma brucei* and is **completely absent in *Leishmania donovani***. This is the single most reliable discriminator on peripheral blood smear. ### Morphological Comparison Table | Feature | *Trypanosoma brucei* | *Leishmania donovani* | | --- | --- | --- | | **Flagellum** | Present, free, extends beyond body | Absent | | **Undulating membrane** | Present (characteristic wave-like structure) | Absent | | **Kinetoplast** | Present (large, prominent) | Present (rod-shaped, eccentric) | | **Location in blood** | Extracellular, free in plasma | Intracellular (in macrophages/monocytes) | | **Shape** | Elongated, spindle-shaped | Oval/round | | **Size** | 15–30 μm long | 2–3 μm | | **Nucleus position** | Central | Eccentric | ### Clinical Pearl **High-Yield:** *Trypanosoma brucei* is **extracellular** and visible on peripheral blood smear because it circulates freely in plasma. *Leishmania donovani* is **intracellular** and found within macrophages/monocytes — it is NOT typically seen on peripheral smear but on bone marrow or splenic aspirate. The undulating membrane is the **pathognomonic feature** of *T. brucei*. ### Mechanism of the Undulating Membrane The undulating membrane is a wave-like structure formed by the fusion of the flagellum with the body wall of *T. brucei*. This creates a characteristic rippling appearance under light microscopy and is absent in all Leishmania species. **Mnemonic:** **T**rypanosoma = **T**ravels (extracellular, free in blood); **L**eishmania = **L**ives inside (intracellular, in macrophages) ### Why This Matters for NEET PG - *T. brucei* (African sleeping sickness) presents with fever, anaemia, and CNS involvement; parasites are visible on blood smear - *L. donovani* (kala-azar) presents with hepatosplenomegaly and anaemia; parasites are found in bone marrow/spleen, NOT blood - The undulating membrane is tested as the **gold standard morphological discriminator** [cite:Baveja 6e Ch 42, 43]
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