Neurology: Encephalitis in Dogs & Cats
Inflammatory disease of the central nervous system (brain and spinal cord) is one of the most common causes of neurological disease in veterinary medicine.
The clinical diagnosis is named after the area of the nervous system that is most affected. For example, “encephalitis” refers to inflammation of the brain ( encephalo- = brain and – itis = inflammation) and “myelitis” refers to inflammation of the spinal cord. If the meninges (the fibrous membrane that surrounds the brain and spinal cord) are primarily affected, the disease is called “meningitis”. If the meninges, brain and spinal cord are all involved then the disease is called “meningoencephalomyelitis”. Most inflammatory disease predominantly affects the brain and so “encephalitis” is often used as a general term for inflammatory disease of the nervous system.
What Causes Encephalitis
Infectious and idiopathic encephalitis are the two basic types of encephalitis. Infectious causes include bacteria, viruses, parasites, fungi and tick transmitted (rickettsial) disease. Idiopathic encephalitis, a medical term meaning encephalitis of unknown cause, is diagnosed if an infectious cause cannot be found. Idiopathic encephalitis is the most common cause of inflammatory brain disease at NC State Veterinary Hospital (VH).
Clinical signs of encephalitis
Clinical signs (the animal equivalent of symptoms) of encephalitis usually reflect the area of the brain that is affected. Seizures, blindness, behavior changes, mental depression and circling are seen with forebrain (cerebral) disease. Imbalance or incoordination, head tilt, tremors or facial paralysis are associated with brainstem disease. Animals with encephalitis may have neurologic abnormalities that come from a single area (focal) or multiple areas (multifocal) of the brain. Many diseases other than encephalitis can cause focal neurologic signs whereas multifocal brain disease is most often associated with encephalitis. If the neurologic abnormalities are focal, other diseases such as stroke or brain tumor must be ruled out. Most animals with encephalitis will develop neurologic abnormalities over a short period of time (days to weeks). Although less common, we have seen animals with clinical signs of encephalitis for months before referral.
How is encephalitis diagnosed?
Routine bloodwork and urinalysis may be suggestive of infectious encephalitis but are rarely diagnostic. Chest x-rays, abdominal ultrasound or lymph node aspirates may find signs of an infection; however, many animals with encephalitis will not have systemic abnormalities. Therefore, encephalitis is usually diagnosed by a spinal tap.
The brain and spinal cord are surrounded by cerebrospinal fluid (CSF or just spinal fluid) and this fluid directly reflects what is going on inside the nervous system. A significant increase in white blood cells in the spinal fluid usually indicates encephalitis. A spinal tap is a routine procedure at most veterinary schools and referral practices but does carry a slight risk. Animals with elevated pressure inside the skull (intracranial pressure) because of brain swelling or a tumor are at higher risk for complications with a spinal tap. If intracranial pressure is elevated, removal of spinal fluid may cause a suction effect. This in turn may pull part of the hindbrain out of the back of the skull (brain herniation), a complication which is often fatal.
Magnetic Resonance Imaging (MRI) helps to differentiate between encephalitis and other causes of neurologic disease.
The brain may be imaged with an MRI or CT (CAT) scan before a spinal tap to look for signs of elevated intracranial pressure and to rule out other causes of neurologic disease. Brain and spinal cord imaging can be costly and so in cases where there is a high suspicion for encephalitis, a spinal tap alone may be sufficient as long as there are no clinical signs of elevated intracranial pressure. Imaging of the brain is usually done first in cases where encephalitis is not the chief concern. This is because certain brain tumors can cause significant inflammation in spinal fluid and thereby “mimic” encephalitis.
What causes encephalitis
The most common causes of infectious encephalitis vary in different parts of the country. In North Carolina, tick borne infections are the most common infectious causes in dogs. In areas such as Arizona and New Mexico, San Joaquin Valley Fever or coccidioidomycosis is very common, whereas other fungal infections are more common in the Ohio River valley. A few examples of each kind of infectious encephalitis are listed below:
- Bacteria – Bacterial meningitis is rare in companion animals mostly because the main bacteria that cause this disease in people do not seem to infect dogs and cats. When present, bacterial infections usually result from extension of a deep ear or sinus infection or spread through the blood stream.
- Virus – Canine distemper virus, feline infectious peritonitis virus, rabies virus.
- Parasites – Toxoplasma gondii, Neospora caninum
- Fungi – Cryptococcus neoformans, Blastomyces dermatididis, Coccidioidomyces immitis (“Valley Fever“)
- Tick transmitted disease – Rocky Mountain Spotted Fever, Ehrlichia canis, Lyme disease
What is idiopathic encephalitis?
Idiopathic encephalitis is a general term for cases in which an infectious cause cannot be found. In many cases of idiopathic encephalitis, an underlying immune-mediated cause is suspected because these animals improve after suppression of the immune system. Immune-mediated diseases occur when the body’s immune system (white blood cells) mistakenly attacks normal tissue, in this case brain or spinal cord. There have been a number of different types of idiopathic encephalitis described in dogs. A few examples of idiopathic encephalitis are listed below along with characteristics of each disease:
Granulomatous meningoencephalomyelitis (GME)
- Synonyms: inflammatory reticulosis
- Breed/Age: any breed but usually small breed dogs (Terriers and Miniature Poodles over represented), predominantly middle aged (3 to 6 years-old)
- Clinical features: may affect any area of the brain or spinal cord, may be focal or multifocal
Necrotizing menginogencephalitis (NME)
- Synonyms: Pug dog encephalitis, necrotizing encephalitis of the Maltese
- Breed/age: Pug, Maltese, Chihuahua, Papillion, Shih-tzu, Boston Terrier, generally young dogs (6 months to 7 years old)
- Clinical features: predominantly forebrain, seizures common
Necrotizing Leukoencephalitis (NLE)
- Synonyms: necrotizing encephalitis of the Yorkshire terrier
- Breeds: Yorkshire Terrier , Chihuahua , Shih-Tzu, slightly older dogs (1 to 10 years old)
- Clinical features: Multifocal localization (forebrain and brainstem), central vestibular disease common, chronic progressive
There are no tests to definitively diagnose these diseases other than direct brain biopsy or autopsy examination. Many reports on GME and necrotizing encephalitis suggest that these are inevitably progressive and fatal disorders. These studies are usually based on autopsy reports and therefore all examined animals have died from the disease. We have seen a large number of animals with idiopathic encephalitis respond to immunosuppression and so a final diagnosis cannot be made. GME and necrotizing encephalitis may simply reflect one of the most severe forms of immune-mediated encephalitis. There has also been some speculation that GME may represent a form of the cancer lymphoma.
What is the treatment for encephalitis?
Once an animal has been diagnosed with encephalitis, antibiotics are usually started to treat for the common infectious diseases. If the animal has seizures, anticonvulsants like phenobarbital are started. Low doses steroids may be used if there is significant inflammation in the spinal fluid or severe clinical signs. Blood and/or spinal fluid is then submitted to test for common infectious diseases. Anti-fungal medications are not routinely started at the NC State Veterinary Hospital because these medications are expensive and fungal tests are usually available in a few days. If an infectious cause is identified, antibiotics may be continued long term. However, a positive infectious disease test does not always indicate an active infection and may simply represent prior exposure. If a test is low or weak positive, it may be repeated 2-3 weeks later to see if it is on the rise.
The treatment for idiopathic encephalitis usually involves suppression of the immune system. This is in clear opposition to the treatment for infectious encephalitis and so it is imperative that infectious diseases be ruled out before starting immunosuppression. High dose therapy with the steroid prednisone is the most common treatment for idiopathic encephalitis. Treatment usually lasts 3-6 months but can be longer. In cases of severe or recurrent encephalitis, more potent immunosuppressive medications are used along with prednisone. Most of these medications are types of chemotherapy and require periodic blood tests to avoid toxicity. These medications may also be used in animals that are susceptible to the side effects of prednisone.