A 35-year-old woman from rural Uttar Pradesh is brought to hospital in a comatose state. Her husband reports she was bitten by a bat 8 weeks ago while sleeping, but she did not notice the bite at the time and did not seek any medical intervention. Over the past 5 days, she has developed fever, confusion, hallucinations, and bizarre behaviour. On examination, she is unresponsive, with signs of brainstem involvement including irregular respiration and pupillary abnormalities. MRI brain shows non-specific signal changes in the limbic system and brainstem. What is the most appropriate immediate management?