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.
Infections of the brain often also involve other parts of the central nervous system, including the spinal cord. The brain and spinal cord are usually protected from infection, but when they become infected, the consequences are often very serious. Infections can cause inflammation of the brain ( encephalitis). Viruses are the most common causes of encephalitis. Infections can also cause inflammation of the layers of tissue (meninges) that cover the brain and spinal cordâcalled meningitis. Often, bacterial meningitis spreads to the brain, causing encephalitis. Similarly, viral infections that cause encephalitis often also cause meningitis.