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?