Encephalitis, meningitis, and granulomatous meningoencephalomyelitis (GME) all refer to inflammation of the nervous system.
Encephalitis means ‘inflammation of the brain,' whereas meningitis means ‘inflammation of the meninges.’ Certain breeds are more likely to be affected with encephalitis, including the Yorkshire Terrier, Maltese, Chihuahua, terrier breeds, and Pugs. Any age can be affected, but most commonly the pet is a young adult or adult. There are two broad categories of encephalitis in dogs: infectious causes and non-infectious causes. Non-infectious causes of encephalitis are much more common in dogs and will be the focus of this article.
Cause and Clinical Signs of GME
The true underlying cause for encephalitis in dogs remains unknown. There is evidence that encephalitis is due to autoimmune disease. In a normal dog, the immune system acts to protect the body from infection, etc. The body relies on the immune system to recognize itself and recognize infections as ‘non-self.’ In autoimmune disease, the immune system loses the ability to discriminate between self and non-self and mounts an inflammatory attack against itself. Examples of autoimmune disease in people include rheumatoid arthritis and lupus. In dogs with encephalitis, the immune system attacks the brain and coverings of the brain (meninges).
Clinical signs of encephalitis depend on the part of the brain that is affected but may include seizures, walking in circles, changes in behavior, seeming off balance, stumbling in the legs, blindness or pain. Sometimes multiple parts of the brain can be affected, and multiple clinical signs are present. A complete neurological examination is performed to determine what part of the brain is affected.
Diagnosis of GME
Encephalitis can be suspected based on age, breed, history, clinical signs, and neurological examination. However, other neurological diseases may have similar characteristics, and therefore tests are warranted. Blood tests are useful as a general health screen, but seldom are enough to diagnose encephalitis. Encephalitis in dogs is diagnosed by magnetic resonance imaging (MRI) of the brain and cerebrospinal fluid (CSF) analysis. Sometimes additional blood tests or CSF tests are warranted to rule out infectious causes of encephalitis. MRI may show characteristic areas of ‘inflammation.' CSF analysis may show an elevated nucleated cell count (pleocytosis), with increased protein levels.
High-field MRI of a dog’s brain with encephalitis. The arrows show ‘hyperintensity’ of the white matter on the left side of the screen. Also note that the ventricles (bright white) is asymmetrical.
Treatment of GME
Encephalitis in dogs is thought to be an autoimmune disease. Treatment involves medications to decrease the inflammation and suppress the overactive immune system. Prednisone is often used for this; however, dogs treated with prednisone alone often have shorter survivals. Recently, ‘newer’ drugs have been studied for use in dogs with encephalitis that may increase survival times. These medications include cytosine arabinoside, cyclosporine, azathioprine, leflunomide, and procarbazine.