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.
encephalomyelitis. acute disseminated encephalomyelitis inflammation of the brain and spinal cord after infection (especially measles) or, formerly, rabies vaccination.quine encephalomyelitis a type of encephalomyelitis in horses and mules, caused by an alphavirus and spread to humans by mosquitoes; it occurs in summer epizootics in the Western Hemisphere. Three forms are recognized: eastern, western, and Venezuelan. Called also equine encephalitis.