A biology graduate student who recently visited a tropical region of Africa presents with new visual impairment and the sensation that something is moving in her eye. She tells you that she is concerned because she had been warned about eye disease transmitted by black flies. When in Africa, she was in a river area, and despite her best efforts she received a lot of black fly bites. She also has some subcutaneous nodules. If her infection was acquired by black fly bite, what is the most likely causative agent?