encephalitis

introduction

inflammation of the brain parenchyma which may or may not be associated with meningitis
the most common cause in in immunocompetent adults in Western society is caused by herpes simplex virus, while varicella may be life threatening in immunocompromised patients and thus patients with suspected encephalitis should generally by started on high dose iv antivirals ASAP.

aetiology

Herpes simplex virus (HSV)
varicella-zoster virus (chickenpox/shingles) - mainly in immunocompromised patients (eg. AIDS, immunosuppressants)
cytomegalovirus (CMV) - HIV-associated CMV encephalitis
arboviruses (generally account for ~10% of cases but can be much higher in an epidemic):
- in Australia:
  - Murray Valley encephalitis (MVE)
    - Murray Valley encephalitis virus - north-western Victoria and other states, particularly after floods (eg. La Nina years such as 1951, 1974, 2011) as host is water birds.
    - high subclinical rate with only 1 in 500 becoming ill.
    - some people may experience a mild form of the disease with symptoms such as fever, headaches, neck stiffness, photophobia, nausea and vomiting
    - only a very small number of these cases go on to develop MVE
    - incubation period thought to be 1-4wks
    - Dx via serologic testing
    - no specific Rx
  - Kunjin virus - rarely symptomatic
  - NB. Barmah Forest virus (BFV) and Ross River virus (RRV) generally cause a polyarthritis rather than encephalitis although usually cause fever and headaches
  - Japanese encephalitis virus (JEV)
    - caused by JEV, a flavivirus;
    - the most significant etiology of arboviral encephalitis worldwide
    - mainly rural Asia including Indonesia and PNG but in the 2020's has arrived in Australia, especially in SE NSW associated with piggeries
    - may cause Guillain-Barré syndrome
- arboviral encephaitis viruses not in Australia:
  - California Encephalitis due to La Crosse virus
  - CBRNE–Venezuelan Equine Encephalitis
  - Eastern Equine Encephalitis (EEE)
  - St. Louis Encephalitis
  - Venezuelan Equine Encephalitis - togavirus
  - West Nile Encephalitis (WNE)
    - migrated to UK in 2025
  - Western Equine Encephalitis (WEE)
other zoonoses:
- Henipavirus encephalitis (Hendra virus (HeV) / Nipah virus (NiV))
  - first recognised in 1998 among pig farmers in Malaysia
  - generally spread by urine of flying foxes and mortality rate of horses is 75% and humans 60%
  - potential for spread from humans via close body fluid contact
  - flu-like illness + encephalitis
  - outbreaks in north-eastern Australia
  - Jan 2026 outbreak in India
  - post-exposure Rx is available for humans
- rabies and related lyssaviruses
toxins:
- Chamki Bukhar - encephalitis in children associated with hypoglycaemia from eating lychees in India, thought to be due to a toxin in the fruit