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.
1 If the infection or resulting inflammation progresses past the membranes of the brain or the spinal cord, then the process is called encephalitis (inflammation of the brain).  The highest incidence of meningitis is between birth and 2 years, with the greatest risk immediately following birth and at 3-8 months of age.