Give 1mg of Benadryl per pound such that if your dog weighs 30 pounds, you should give it 30 mg of Benadryl. Talk your veterinarian - while antihistamines are generally safe, there are the potential for side effects particularly if your dog has chronic organ failure (renal failure, liver failure, congestive heart failure, etc.).
1 Encephalitis is inflammation of the brain.  Meningitis is inflammation of the membranes (called meninges) that surround the brain and spinal cord.
Encephalitis is inflammation of the brain. It is usually caused by a viral infection. In the UK, the most common virus to cause encephalitis is herpes simplex virus. Symptoms usually start with the common symptoms of a viral infection such as high temperature (fever), headache, muscle aches, feeling tired and feeing sick (nausea). Confusion, drowsiness and eventually coma can develop.