A 32-year-old farmer from rural Maharashtra presents with a 5-day history of fever, headache, myalgia, and conjunctival suffusion. He waded through flooded rice fields 10 days ago. Blood culture is pending. On examination, he is febrile (39.2°C) with no rash, normal meningeal signs, and mild hepatosplenomegaly. What is the most appropriate immediate next step in management?