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 literally means an inflammation of the brain. In most cases, this inflammation is caused by a virus. Encephalitis is a rare disease that occurs in approximately 0.5 per 100,000 individuals â most commonly in children, the elderly, and people with weakened immune systems (e.g., those with HIV/AIDS or cancer).