Encephalitis

Encephalitis is predominantly a viral disease (Table 3.17.1), with herpes simplex infection being the most common in the UK (Fig. 3.17.1). Cerebral dysfunction presents as behavioural disturbance, fits and diminished consciousness. If progressive, then localized neurological signs, coma and death may occur. Diagnosis of the condition is clinical with confirmation from imaging and electroencephalography. Virus may also be detectable in the cerebrospinal fluid (CSF) but may not be cultivable. Brain biopsy offers the definitive means of diagnosis but is only used in specialist centres. Herpes simplex virus (HSV) encephalitis has a 70% mortality unless treated with aciclovir. Rabies is fatal but can be treated with postexposure prophylaxis, as the long incubation period allows time for an adequate immune response.

Table 3.17.1 CAUSES OF ENCEPHALITIS

Cause	Features
Viruses
Herpes simplex virus (herpesvirus)	Bitemporal localization detectable by imaging; HSV2 common in neonates, HSV2 in adults
Mumps virus (herpesvirus)	Meningoencephalitis may precede parotitis
Eastern and Western equine encephalitis virus (toga/bunyavirus)	Mosquito-borne in parts of North America
Rabies virus (rhabdovirus)	Incubation period of weeks to months after mammal bite; fatal if not treated
Tick-borne flaviviruses	Forested areas of Scandinavia; vaccine available
Japanese B virus (flavivirus)	Southeast Asia; vaccine available
Poliovirus and enteroviruses	Most commonly causes meningitis but may cause meningoencephalitis
Rubella and measles viruses	Subacute panencephalitis with high mortality
JC virus (papovavirus)	Progressive multifocal leukoencephalopathy in the immunocompromised
Post-viral infection/post-vaccination	Immune mediated; occurs with measles, influenza and others; good prognosis
Protozoa/fungi
Toxoplasma gondii	Immunocompromised and newborn
Cryptococcus neoformans	Common in HIV infection
Plasmodium falciparum	Cerebral malaria
Trypanosomas spp.	Sleeping sickness; central Africa
Prions	Creutzfeldt–Jakob disease