Updated 19 May 2015


Encephalitis refers to inflammation of the brain.



Encephalitis refers to inflammation of the brain. This may be a result of primary bacterial or viral infection, or an allergic reaction at a later stage to such an infection or to a vaccination.

This article will deal with viral encephalitis.


Viruses causing primary encephalitis may cause epidemics of the disease. These include the arboviruses, polio viruses, echoviruses or coxsackie viruses. The disease may also be sporadic, implying isolated cases, caused by viruses such as the herpes simplex virus, varicella zoster (causing chicken pox) or mumps virus.

Mosquitoes can carry arboviruses and cause encephalitis in particular areas during warm weather.

The parainfectious or post-infectious forms of encephalitis typically develop five to 10 days after the onset of a viral illness or vaccination.

Vary rarely, encephalitis occurs as a late consequence of viral infections, as may occur with measles.

Symptoms and signs

These may take three forms:

  • There may be no nervous system symptoms at all, with only fever and malaise present
  • The symptoms may be those of meningitis – fever, headache, vomiting, malaise and stiff neck and back
  • The symptoms may involve alteration in consciousness, personality changes, seizures and limb weakness.


Viral encephalitis must be distinguished from other infectious and non-infectious causes. The main distinction to be made is between viral and bacterial causes.

Diagnosis is usually based on examining the CSF, obtained using a procedure called a lumbar puncture. This often-uncomfortable investigation involves passing a needle into the space between the vertebrae in the spine and removing enough CSF to examine under a microscope and send for biochemical testing.

The appearance and composition of the CSF is different in viral and bacterial encephalitis.


Generally, viruses do not respond to specific treatment. However, the herpes simplex virus, which can cause encephalitis, does respond well to treatment with acyclovir.

The patient is kept well hydrated and pain and fever are controlled.


Even very sick patients may recover completely. The acute phase of the illness lasts from a few days to a week. Recovery can be abrupt or gradual.

The disease can be complicated by salt imbalances, or problems with temperature and respiration resulting from involvement of certain parts of the brain.

Permanent neurological damage is more likely to occur in very young children, but at the same time, young children show improvement over a longer period than adults with similar infections.

When to see your doctor

Seek medical help immediately at the first signs or symptoms of encephalitis. This is particularly important if you know of cases in your area or at your child’s school.

Reviewed by Dr Andrew Rose-Innes, MD, Department of Neurology, Yale University School of Medicine, New Haven, 2007

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