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Viral encephalitis


- most cases are sporadic accompaniments of common infections eg mumps, measles, HSV. (UK & Europe)
- most epidemics are due to Japanese B encephalitis except in USA where most cases are due to arboviruses (arthropod-bourne)
- viruses are by far the commonest causes of encephalitis globally but in certain locations and seasons other organisms eg malaria, other protozoa, rickettsiae and fungi may cause an encephalitic syndrome


- haematogenous infection in most cases although in the case of rabies and HSV virus ascends neurons centripetally
- certain viruses exhibit tropism towards specific cell types eg limbic system in rabies, temporal lobes in HSV encephalitis

Clinical features

- prodrome of several days, may include:

  • malaise
  • myalgia
  • mild URTI
  • fever
  • rash
  • parotitis

- headache
- mental change
- drowsiness
- meningism
- may progress to: disorientation, behavioural disturbance, speech disturbance and coma
- fits common
- focal features appropriate to the part of the brain taking the brunt of the infection may appear. eg hallucinations and memory loss from temporal lobes, hemiparesis, cerebellar deficits
- +/- signs of raised ICP
- often more severe in very young, very old, and immunocompromised

Differential diagnosis

- all forms of bacterial meningitis
- malaria
- other protozoal and fungal infestations
- intracranial suppuration
- septicaemia & endocarditis
- metastatic disease
- connective tissue disease
- drug abuse


- U&E
- travel history
- blood films for malaria
- blood cultures
- CT. NB During first 2-3 days no abnormality may be evident and changes may not show for 5-6 days
- EEG: usually shows non-specific diffuse slow-wave activity +/- seizure activity. Temporal lobe focal abnormality with high-voltage spike and slow-wave complexes is highly suggestive of HSV encephalitis
- LP as soon as is deemed safe. Unfortunately this is very difficult to determine as CT scanning does not convey an accurate picture of ICP.

  • raised pressure
  • WCC: 10/ml - several thousand/ml. Usually lymphocytes but polymorphs may predominate in early stages
  • RBCs may be found if there is a necrotising element eg HSV encephalitis
  • glucose normal
  • protein raised
  • CSF and serum for viral studies
  • CSF for HSV Ag PCR


- supportive
- acyclovir 10 mg/kg tds unless HSV has been excluded
- theoretical reasons for not using steroids (may inhibit interferon synthesis) but many continue to use dexamethasone in severely ill patients

Specific encephalitides


- most common cause of sporadic encephalitis in Europe and N America. Accounts for 10-20% of cases of viral encephalitis
- 0.5/100 000/yr. Probably underestimate
- any age
- not more common in immunocompromised
- clinical picture very variable
- onset usually with a 4-10 day prodrome: malaise, pyrexia, irritability
- followed by frontal and temporal lobe disturbance:

  • personality change
  • hallucinations
  • psychiatric disturbance
  • memory loss

- increasing focal signs, seizures, deteriorating conscious level
- focal signs in as many as 87%
- in a few patients onset much more acute with evolution over only a few days
- differential diagnosis includes:

  • abscess
  • granuloma
  • vascular disease
  • other forms of viral encephalitis

- treatment: acyclovir IV for 10 days. This decreases mortality to 30%. Has been suggested that multiple courses may be necessary and that a longer initial course (ie 21 days) should be the standard


- affects approx 0.09% of cases of chickenpox. May rarely follow shingles
- in 50% cerebellar signs are prominent with ataxia, dysarthria, headache, drowsiness coming on about a week after onset of rash
- neurological signs may precede rash
- convulsions common
- +/- progression to hemiplegia, cranial nerve palsies, aphasia and coma
- patients with cerebellar form usually recover completely
- 10% of those with general form die
- management supportive


- only important as a cause of encephalitis in neonates and the immunocompromised
- no specific treatment of proven value

Human herpes virus 6

- recently identified as a possible cause of human CNS infection although aetiological link between virus and disease is not clearly established
- ubiquitous in the community and appears to be acquired early in life
- rarely associated with encephalitis
- capable of latent infection with reactivation in response to a variety of stimuli
- usual presentation is with decreased consciousness and fits
- detection of HHV-6 DNA in CSF is not specific for HHV-6 encephalitis


- acute encephalitis associated with measles usually caused by postinfectious, perivenous, demyelinating, allergic phenomenon +/- direct viral damage
- up to 1/1000 cases above age of 2 yrs. Frequency increases with age
- typical presentation: recurrence of fever on 7th or 8th day as rash is fading, with rapid development of encephalitis: convulsions, focal signs, myoclonus and coma
- no specific features on investigation
- treatment supportive
- mortality 15% and some disability in most survivors


- rare. Occurs in < 1% of cases of mumps
- no specific features
- occurs up to 2 weeks after development of parotitis. Latter may be absent in 50%
- prognosis excellent

Epidemic encephalitis

- caused by arboviruses. All zoonoses with birds and smaller vertebrates as primary hosts. Transmission is by mosquito or tick
- clinical features are common to all with some variation in incubation, progression, and severity
- specific diagnosis can only be made by rise in antibody titre
- treatment is symptomatic

Eastern equine encephalitis

- rare
- Atlantic and Gulf coasts of America
- summer and autumn
- children most affected
- 70% mortality

Western equine encephalitis

- less severe
- eastern, central and western USA & Canada and eastern S. America

St Louis encephalitis

- most cases are benign and of short duration

California and La Crosse viruses

- similar
- central and midwest USA
- children
- peak in August and September
- sudden onset
- recovery within 10 days

Central European encephalitis

- woodlands in Scandinavia through to northern Greece and former Yugoslavia
- tick-borne
- biphasic course: flu-like illnesss followed within 2 weeks by mild meningitis or encephalitis +/- muscle weakness
- management symptomatic
- recovery usually complete

Russian spring-summer encephalitis

- similar to Central European encephalitis but move severe

Japanese encephalitis

- most common arbovirus infection worldwide
- endemic in SE Asia, Japan, China, Philippines, Borneo, and large parts of Indian sub-continent
- age: < 15 yrs and elderly predominantly
- mosquito-borne
- incubation 1-2 weeks followed by abrupt onset of encephalitis often with myalgia
- +/- extra-pyramidal features
- convalescence may be prolonged and sequelae are common
- treatment symptomatic

Further reading

Caserta MT. Human herpesvirus 6 infection of the central nervous system. Current Infectious Diseases Reports, 2004, 6:316-321

© Charles Gomersall December 1999, February 2005


©Charles Gomersall, January, 2018 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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