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 irritation and swelling (inflammation) of the brain, most often due to infections. Encephalitis is a rare condition. It occurs more often in the first year of life and decreases with age. The very young and older adults are more likely to have a severe case. Encephalitis is most often caused by a virus.