A 32-year-old male from rural West Bengal presents with progressive swelling of the left lower limb for 6 months. On examination, he has pitting edema of the foot and ankle, with thickened skin and prominent lymphatic vessels. He reports recurrent episodes of fever with chills over the past year, particularly at night. Blood smear examination at 11 PM shows microfilariae with a sheath. What is the most likely causative organism?

## Diagnosis: Wuchereria bancrofti Lymphatic Filariasis ### Clinical Presentation Analysis **Key Point:** The combination of lower limb lymphedema, nocturnal fever, and microfilariae with a sheath in the blood smear at 11 PM is pathognomonic for Wuchereria bancrofti infection. ### Distinguishing Features of W. bancrofti | Feature | W. bancrofti | Brugia malayi | Loa loa | O. volvulus | |---------|--------------|---------------|---------|-------------| | **Microfilariae in blood** | Yes (sheathed) | Yes (sheathed) | Yes (sheathed) | No (in skin/subcutaneous) | | **Periodicity** | Nocturnal (urban), subperiodic (rural) | Nocturnal, subperiodic | Diurnal | None (subcutaneous) | | **Lymphatic involvement** | Marked (lymphedema, elephantiasis) | Mild to moderate | Minimal | None | | **Fever pattern** | Nocturnal, recurrent | Nocturnal | Calabar swellings | Absent | | **Geographic distribution** | Tropical Africa, Asia, Pacific | Southeast Asia (Malaysia, Indonesia) | West/Central Africa | Africa, Yemen, Latin America | **High-Yield:** The **nocturnal periodicity** (microfilariae appear in peripheral blood at night) is a critical diagnostic clue for W. bancrofti. The patient's 11 PM blood smear showing microfilariae confirms this temporal pattern. ### Pathophysiology of Lymphatic Filariasis 1. **Adult worm location:** Lymph nodes and lymphatic vessels (especially inguinal and axillary) 2. **Microfilaremia:** Nocturnally periodic in urban strains; subperiodic in rural strains 3. **Immune response:** Chronic inflammation → lymphatic obstruction → lymphedema and elephantiasis 4. **Fever episodes:** Associated with microfilaremia surges and host inflammatory response **Clinical Pearl:** The recurrent nocturnal fever with chills in this patient corresponds to the timing of microfilariae release into the bloodstream, which triggers acute inflammatory episodes. ### Why W. bancrofti Over Brugia malayi? Although both cause lymphatic filariasis with sheathed microfilariae: - **W. bancrofti** causes more severe and extensive lymphedema and elephantiasis (especially of lower limbs and genitals) - **Brugia malayi** typically affects the lymphatics of the lower limbs but with less severe manifestations; also endemic to Southeast Asia (Malaysia, Indonesia), not West Bengal - West Bengal is a hyperendemic zone for W. bancrofti **Mnemonic:** **WUCHERERIA = Worst (most severe) lymphedema** — remember W. bancrofti for the most dramatic lymphatic disease. ### Microfilariae Identification **Key Point:** Sheathed microfilariae in blood at night → W. bancrofti or Brugia. The nocturnal periodicity and geographic location (West Bengal) make W. bancrofti the answer. [cite:Park 26e Ch 3]