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What is Encephalitis?

What is Encephalitis?

Encephalitis is an acute inflammation of the brain. When I say encephalitis, it doesn’t include meninges affection. When meninges are affected, it’s called meningo-encephalitis.

Incidence of 4-7 per 100,000 persons per year.

The cause of encephalitis is usually infectious in nature. Although bacteria, fungi, protozoa can produce encephalitis, most cases are viral in origin. There is over 100 viruses worldwide that can cause encephalitis, the most important of them are the following:

  1. Herpes Simplex Virus (HSV)
  2. Varicella Zoster Virus (VZV)
  3. Rabies
  4. HIV
  5. H5N1 encephalitis
  6. ArBoVirus encephalitis (Arthropod-Borne viruses) are subdivided into:
  • Flaviviridae (Japanese encephalitis – St. Louis encephalitis – West Nile virus)
  • Togaviridae (Eastern equine encephalitis – Western equine encephalitis)
  • Bunyaviridae (La Crosse encephalitis)

Parasitic infection as toxoplasmosis, malaria, PAM (caused by Nagleria fowleri) or GAE (caused by Acanthamoeba castellanii) can cause encephalitis in immuno-compromised patients.

Bacteria like streptococci, pneumococci, staphylococci can cause ceribritis (inflammation of cerebral cortex). Bartonella henselae (causing cat-scratch disease) or Borrelia burgdorferi (causing Lyme disease) may cause encephalitis too.

Cryptococcus neoformans is a fungus notorious for causing fungal encephalitis in the immuno-compromised.


Let’s know how causative organisms (specially viruses) can lead to inflammation of the brain:

  1. The virus replicates outside the CNS.
  2. Enter CNS by hematogenous spread or along neural pathways (eg, rabies virus, HSV, VZV).
  3. Once across BBB (Blood Brain Barrier), the virus enters neural cells, with resultant disruption in cell functioning, perivascular congestion, hemorrhage, and a diffuse inflammatory response.

Regional tropism: It was found that some viruses have more affinity to a particular part of the brain, causing inflammation in that specific part only! That occurs due to neuron cell membrane receptors distribution that are  found only in specific portions of the brain, with more intense focal pathology in these areas. A classic example of regional tropism is the HSV predilection for the inferior and medial temporal lobes.

Clinical Picture

  • Adult patients: acute onset fever, headache, confusion, seizures.
  • In severe cases: nausea, vomiting, sensitivity to bright light and coma.
  • Infants may present irritability, poor appetite, fever, body stiffness and bulging fontanelle.


  • Neurological examinations: reveals a confused patient with stiff neck, due to the irritation of meninges.
  • Detection of antibodies in the CSF against a specific viral agent or by PCR.
  • Serological tests may show high antibody titre against the causative antigen.
  • Examination of the CSF obtained by lumbar puncture usually reveals increased amounts of protein and WBCs with normal glucose.
  • Bleeding is also uncommon except in patients with herpes simplex type 1 encephalitis.
  • In patients with herpes simplex encephalitis, EEG may show sharp waves in one or both of the temporal lobes.


  • Acyclovir (anti-viral) for HSV
  • In advanced cases, supportive treatment, such as mechanical ventilation, is usually needed.
  • Corticosteroids are used to reduce brain swelling and inflammation.
  • Sedatives may be needed for irritability or restlessness.
  • For Mycoplasma infection, parentral tetracycline is given.
  • Encephalitis due to Toxoplasma is treated by giving a combination of pyrimethamine and sulphadimidine.


  • There is no vaccine for most encephalitides until now.
  • A live, attenuated vaccine against Japanese encephalitis virus has been developed and tested in China and appeared to be safe and effective.

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