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.
Acute encephalitis is a rare and a severe condition that affects children. It is characterized by an inflammation of the brain tissue and the meninges (the latter concerning meningoencephalitis). In most cases, this condition is secondary to a viral infection. The importance of neurological sequelae is the nature of the viral infection and the age of onset.