A peripheral blood smear from a patient presenting with fever, malaise, and lymphadenopathy is shown above. Which of the following organisms is most likely identified?

Explanation

## Image Findings * Numerous red blood cells are visible in the background. * Several elongated, slender, C- or S-shaped organisms are present extracellularly. * These organisms exhibit an undulating membrane and a free flagellum. ## Diagnosis **Key Point:** The presence of extracellular, elongated, C- or S-shaped organisms with an undulating membrane and a free flagellum in a peripheral blood smear is characteristic of **Trypanosoma trypomastigotes**. The image displays classic morphological features of trypomastigotes, the diagnostic stage of *Trypanosoma* species found in the peripheral blood. These parasites are typically 15-30 µm long, slender, and possess a kinetoplast, a nucleus, an undulating membrane, and a free flagellum. Their characteristic curved or S-shape is often seen in blood smears, distinguishing them from other blood parasites. ## Differential Diagnosis | Feature | *Trypanosoma brucei* trypomastigote (Correct) | *Leishmania donovani* amastigote | *Plasmodium falciparum* trophozoite | *Wuchereria bancrofti* microfilaria | | :------------------ | :-------------------------------------------- | :------------------------------- | :---------------------------------- | :--------------------------------- | | **Location** | Extracellular in blood | Intracellular (macrophages) | Intracellular (RBCs) | Extracellular in blood/lymph | | **Shape** | Elongated, slender, C/S-shaped | Oval, round | Ring-shaped, compact | Long, slender, worm-like | | **Flagellum** | Present (free) | Absent | Absent | Absent (motility by undulation) | | **Undulating Memb.**| Present | Absent | Absent | Absent | | **Size** | 15-30 µm | 2-4 µm | 1-2 µm (ring) | 240-300 µm | ## Clinical Relevance **Clinical Pearl:** *Trypanosoma brucei* causes African Trypanosomiasis (Sleeping Sickness), transmitted by the tsetse fly. Initial symptoms include fever, headache, joint pains, and lymphadenopathy (Winterbottom's sign). If untreated, it progresses to meningoencephalitis, leading to neurological symptoms and eventually death. *Trypanosoma cruzi* causes American Trypanosomiasis (Chagas disease), transmitted by triatomine bugs, and can lead to chronic cardiac and gastrointestinal complications. ## High-Yield for NEET PG **High-Yield:** The **trypomastigote** is the diagnostic stage found in human blood for both African and American Trypanosomiasis. **Key Point:** African Trypanosomiasis is transmitted by the **tsetse fly** (*Glossina* species), while American Trypanosomiasis is transmitted by **reduviid bugs** (kissing bugs). ## Mnemonic **Mnemonic:** **T**rypanosoma **T**rypomastigotes are **T**hin, **T**wisty, and have a **T**ail (flagellum). ## Common Traps **Warning:** Do not confuse trypomastigotes with microfilariae. Microfilariae are significantly larger, have a distinct sheath (or lack thereof), and different internal structures, lacking the characteristic undulating membrane and free flagellum of trypomastigotes. Also, distinguish from spirochetes like Borrelia, which are typically thinner and more tightly coiled without a prominent undulating membrane. ## Reference [cite:Jawetz, Melnick & Adelberg's Medical Microbiology, Ch 46]