A 32-year-old male farmer from Tamil Nadu presents with a 5-day history of fever, myalgia, and headache. On examination, a painless eschar with surrounding erythema is noted on the left thigh. Laboratory investigations show thrombocytopenia (platelets 85,000/μL) and elevated transaminases (AST 156 U/L, ALT 142 U/L). Widal test is negative. What is the most appropriate immediate management?