What is encephalitis?

• Encephalitis means brain (“encephalo-”) inflammation (“-itis”).
• Often, the meninges which are the covering of the brain and spinal cord, also become inflamed (“meningitis” or “meningoencephalitis” when both meninges and brain are affected).
• There are two main categories of encephalitis: sterile/immune-mediated and infectious.
  

Immune-mediated encephalitis occurs when the immune system mounts an inappropriate attack towards the brain as though it has an infection. We do not know why the immune system makes this mistake, but it is likely due to a combination of hereditary factors and environmental stimulation of the immune system.

There are several different forms of immune-mediated encephalitis: granulomatous meningoencephalitis (GME), necrotizing meningoencephalitis (NME), necrotizing leukoencephalitis (NLE), eosinophilic meningoencephalitis (EME) and steroid-responsive meningitis-arteritis (SRMA). GME, NME and NLE are also referred to as meningoencephalitis of unknown etiology (MUE) when definitive differentiation between these forms is not possible.

There are multiple different types of infections which may affect the central nervous system: algal, bacterial, fungal, parasitic, protozoal, rickettsial and viral. The frequency and likelihood of these infections depends on geographic location and species.

What are the signs that your pet may have encephalitis?

Symptoms of encephalitis are non-specific and depend on the part(s) of the brain which is affected.

• Signs of forebrain disease may include seizure, disorientation/confusion, dull mentation, pacing, circling, getting stuck in corners, staring into space and blindness.
• Signs of brainstem disease may include dull mentation, loss of balance, head tilt, limb weakness, abnormal eye position or movement, circling and facial asymmetry.
• Signs of cerebellar disease may include wide-based stance, goose-stepping gait, tremors and loss of balance. Head and/or neck pain is also common in animals with encephalitis.
• Systemic signs such as fever are relatively uncommon for most forms of encephalitis except in the case of SRMA.

Signs tend to be rapid in onset and progressive, but sometimes more chronic symptoms are reported.

What other diseases have similar signs?

Any other disease process affecting the brain may have similar signs. The most common diseases to rule out are congenital malformation and tumor. Other diseases which may be considered include metabolic diseases, neurodegenerative diseases and stroke. In animals with seizure as the only presenting sign, idiopathic epilepsy must also be considered as a possibility.

How can encephalitis be differentiated from these other diseases?

A combination of patient history, neurologic exam findings, and diagnostic test results is used to make a diagnosis of encephalitis.

• Magnetic resonance imaging (MRI) of the brain is the test of choice to evaluate structural changes to the brain. Some animals with encephalitis may have a normal MRI.
• Cerebrospinal fluid (CSF – the fluid around the brain and spinal cord) often contains increased numbers of white blood cells and protein, consistent with inflammation. In some cases, CSF may be normal.
• Infectious disease screening is often performed to help differentiate infectious and immune-mediated causes.
• Definitive diagnosis is achieved with brain biopsy, but this is rarely performed as often a good working diagnosis may be made with the previously mentioned tests.