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 Disseminated Encephalomyelitis (ADEM) is a rare inflammatory demyelinating disease of the central nervous system. ADEM is thought to be an autoimmune disorder in which the bodyâs immune system mistakenly attacks its own brain tissue, triggered by an environmental stimulus in genetically susceptible individuals. More often it is believed to be triggered by a response to an infection or to a vaccination.