Encephalitis in Pets | Dog and Cat Brain Inflammation

What is encephalitis and what causes it?

Encephalitis means inflammation of the brain. Infectious causes of encephalitis include:

• Viruses
• Bacteria
• Fungal organisms
• Tick-borne disease

• Other parasites (Toxoplasma, Neospora, etc.)

Immune-mediated encephalitis is due to the body’s immune system malfunctioning and creating abnormal inflammation for unknown reasons. Immune-mediated encephalitis in dogs is more common than encephalitis caused by infection. There are a few varieties of immune-mediated encephalitis in dogs; “GME” (see below for more information on GME) and “necrotizing encephalitis” are examples of these. 

Who is at risk for encephalitis?

Younger to middle age dogs appear more predisposed to all forms of encephalitis than older dogs. Smaller breed, younger to middle aged female dogs, especially Pugs, Yorkies, and Chihuahuas, appear more predisposed to immune-mediated encephalitis than larger breeds. Encephalitis in cats appears to be less common than it is in dogs and is more commonly due to infection in these pets. Research into the causes and treatments for immune-mediated encephalitis is ongoing. Encephalitis of any kind can be rapidly life threatening and should be diagnosed and treated as early as possible.

What are the signs and symptoms?

The signs of encephalitis are dependent upon the part of the brain that is affected and may not be the same for every dog and cat. 

Signs can include:

• Seizures
• Disorientation
• Behavioral changes
• Weakness
• Circling
• Loss of balance
• Spinal pain (particularly neck pain)
• In severe cases, coma and death
• Fever may or may not be present

How is encephalitis diagnosed?

Your Veterinary Neurologist will likely start with routine blood tests (including white blood cell counts). These tests may be abnormal with bacterial encephalitis, but are typically normal for most varieties of encephalitis. Encephalitis is primarily diagnosed with an MRI (magnetic resonance imaging) and spinal tap (done to collect and then analyze spinal fluid). Spinal fluid analysis can tell us if the brain or the tissues covering the brain (meninges) are inflamed or not, but it may not tell us the underlying cause of the inflammation. Once encephalitis is diagnosed, additional testing on spinal fluid or blood may be necessary to try to determine if the cause is due to infection. 

The only way to definitively diagnose immune-mediated encephalitis is by biopsy of the inflamed part(s) of the brain. Brain biopsy is difficult in most practice settings, requiring special equipment and skills, and has risks. Because of these challenges, when infection has been ruled out (to the best of our ability), immune-mediated encephalitis is presumed.  

How do we treat encephalitis?

Treatment for encephalitis depends on the underlying cause. For example, antibiotics are used for bacterial encephalitis while anti-fungal medications may be used for fungal encephalitis. 

Medications that suppress the immune system are used for immune-mediated encephalitis. There are many options for treating immune-mediated encephalitis. No single medication or even specific combination of medications has had overwhelming success which is why there are many different medications and protocols used to treat immune-mediated encephalitis. 

Some medications your Veterinary Neurologist may use to treat immune-mediated encephalitis include:

• Steroids
• Cytosar (cytarabine)
• Cyclosporin
• Mycophenolate
• Azathioprine
• CCNU
• Procarbazine

Encephalitis can be rapidly life threatening and should be diagnosed and treated as soon as possible.

What is the prognosis for encephalitis in dogs and cats?

Even though any form of encephalitis can be rapidly life threatening, the prognosis is often good if identified early and treated aggressively. Overall, prognosis for encephalitis is variable, and depends on the underlying cause of the inflammation. Some dogs and cats can be cured of encephalitis but treatment typically is for many months or even years. Unfortunately, some dogs and cats will die of encephalitis despite treatment.

What is Granulomatous Meningoencephalitis (GME)?

“GME” stands for granulomatous meningoencephalomyelitis and is one of the more commonly diagnosed forms of encephalitis. This is a primary inflammatory condition of the brain, meninges or spinal cord and often involves all three. We do not know what causes this inflammation and since many scientific studies have found no evidence for an infectious cause, it is presumed to be an immune-mediated form of encephalitis.

GME can have the same clinical signs described above and diagnosis is most commonly made using a combination of MRI, spinal tap and negative blood or spinal fluid tests (ruling out infectious causes of encephalitis and meningitis). Long-term therapy with steroids and other medications is always necessary. Radiation therapy has also been used successfully for forms of GME limited to a small area. Two-thirds of dogs typically respond favorably to treatment for GME; however, it can be difficult to predict a prognosis in an individual dog before treatment is started. Some evidence and anecdotal experience suggest that the more aggressively a veterinarian treats GME, the more success they will have controlling the disease. Cats do not appear to get GME or any of the immune-mediated forms of encephalitis.

What is necrotizing encephalitis (NE)?

Necrotizing encephalitis (NE) in dogs is a rapidly progressive, fatal form of inflammation of the brain that is an immune-mediated disease. One severe form of necrotizing encephalitis (Pug dog encephalitis) has been linked to a genetic defect. This gene defect has been associated with an increased risk for encephalitis in Pug dogs. A blood test is available to identify this gene defect, but the test does not specifically identify when an individual has the disease. Therefore, this test can only identify dogs at increased risk for developing Pug dog encephalitis and may be best used to screen Pugs who may be used for breeding. A dog identified as having the gene defect associated with Pug dog encephalitis should not be used for breeding.