Viral Encephalitis

Viral encephalitis is the commonest cause of encephalitis. The general features and causes of encephalitis are described in a separate section. Here we discuss the various viral encephalitides.

HSV Encephalitis:


Clinical findings combined with MRI features and isolation of virus by PCR or pathology

Findings on investigations:


  • Low density in: Insular regions, hippocampus, cingulated gyrus


  • T2 high signal intensity, especially in the temporal lobes or orbitofrontal lobe
  • May cause Rhombencephalitis

Electroencephalography, EEG:

  • Temporal lobe disease
  • Periodic lateralised epileptiform discharges (PLEDs),
  • Temporal spikes at 2-3Hz

CSF PCR: false negatives can occur
Lumbar puncture & CSF analysis:

  • White cell counts: high,
    • Lymphocytes or monocytes, can be very high
  • RCC: raised in HSV encephalitis
  • Opening pressure: Normal or mildly elevated 100-350 mmH2O
  • Glucose normal or slightly decreased
  • Protein: normal or slightly increased 0.2-0.8 g/L (20-80 mg/dL)
  • Oligoclonal bands & electrophoresis abnormalities may occur

Pathology, Biopsy:

  • Perivascular lymphocytes
  • Lymphocytic infiltrate
  • Microglial proliferation
  • Intranuclear & cytoplasmic inclusions, Cowdry A inclusions (large with halo)



  • Acyclovir I.V. for at least 3 weeks


West Nile Virus encephalitis, WNV meningoencephalitis & WMV neuropathy WNV:

Clinical features:

  • –      Meningeal symptoms: fever, headache, neckstiffness, rash
  • Encephalopathy, seizures may occur
  • Myoclonus, tremour, rigidity, postural instability, bradykinesia, Cranial nerve palsies, vertigo, cerebellar ataxia
  • Flaccid paralysis syndrome may occur (with or without encephalitis)
  • Optic neuritis & chorioretinitis


  • Clinical features plus CSF antibody tests or isolation of the virus

Findings on Investigations:

+CSF IgG & IgM for West nile virus:

  • Microspheric immunoassay (MIA): if serology is positive to exclude a heterologous response to other flaviviruses


  • False negatives occur

+Serology: IgG & IgM for West nile virus: not 100% specific, negative test doesn’t rule out infection
CSF analysis:

  • CSF: lymphocytic pleocytosis (rarely normal), raised protein, normal glucose,

CT: is usually normal

  • leptomeningeal or pervascular enchancement. T2: normal initially, high signal in caudate nuclei, thalami, brainstem, spinal cord,
  • MRI brachial plexus: Thickened brachial plexus on T2 in WNV neuropathy

EEG: non-specific features of encephalopathy

  • normal SNAPs, reduced CMAPs



Rabies encephalitis:

Clinical features:

  • Prodrome: headache, fever, malaise, anxiety
  • Dysphagia,
  • Hydrophobia: laryngeal spasm on swallowing liquids
  • Seizures

Pathology, biopsy:

  • Negri bodies: eosinophilic intracytoplasmic inclusions in pyramidal cells & Purkinje cells
  • Babes nodules: microglial nodules


  • Clean the wound
  • Human rabies Ig
  • Human diploid cell vaccine