Viral encephalitis: causes, differential diagnosis, and management

Molecular analyses of paired oral/labial and brain sites have indicated that HSE can be the result of a primary infection, a reactivation of latent HSV, or a re-infection by a second HSV. 1 These estimates of HSE may well be underestimates as judged by the experience of individual neurological centres and detailed studies using the polymerase chain reaction (PCR). [...]a small PCR based study suggested that up to a fifth of patients with HSE may have mild or atypical disease caused by either HSV-1 or HSV-2, occurring especially in immunocompromised individuals such as those with HIV infection. 3 Table 1 Causes of viral encephalitis Herpes simplex virus (HSV-1, HSV-2) Other herpes viruses: varicella zoster virus (VZV), cytomegalovirus (CMV), Epstein-Barr virus (EBV), human herpes virus 6 (HHV6) Adenoviruses Influenza A Enteroviruses, poliovirus Measles, mumps and rubella viruses Rabies Arboviruses-for example, Japanese B encephalitis, St Louis encephalitis virus, West Nile encephalitis virus, Eastern, Western, and Venezuelan equine encephalitis virus, tick borne encephalitis viruses Bunyaviruses-for example, La Crosse strain of California virus Reoviruses-for example, Colorado tick fever virus Arenaviruses-for example, lymphocytic choriomeningitis virus A recent study in Finland also used PCR to detect various viruses in the CSF of over 3000 patients who had infections of the CNS including encephalitis, meningitis, and myelitis. 4 It was found that, rather surprisingly, varicella zoster virus (VZV), the cause of chickenpox and herpes zoster, was the most frequently detected virus at 29%, with HSV and enteroviruses accounting for 11% of cases each, and influenza A virus found in 7% of cases. The frequency and distribution of these viruses clearly varies according to the geographical region, with large differences seen between Europe, Asia, and the USA. [...]St Louis virus encephalitis, which is caused by a mosquito borne arbovirus, occurs in the midwestern and eastern states of the USA, and not in the UK, while Japanese encephalitis is a major problem in Asia, and is the most important cause of epidemic encephalitis worldwide, causing up to 15000 deaths annually. 5 Two "emerging" viral infections of the nervous system which have received much attention recently are West Nile virus encephalitis and Nipah virus encephalitis.