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Viral infections 

Viral infections

Viral infections

Jeremy Farrar

, Bridgit Wills

, Menno De Jong

, and David A Warrell


A relevant case history from Neurological Case Histories: Case Histories in Acute Neurology and the Neurology of General Medicine has been added to this chapter.

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date: 23 April 2017


Enteroviruses are responsible for 80 to 90% and mumps for 10 to 20% of diagnosed cases of viral meningitis, with many other viruses sometimes incriminated with considerable geographical and seasonal variation.

Clinical features and prognosis—typical presentation is with sudden onset of fever, headache, change in conscious level, and (occasionally) a stiff neck and vomiting. The specific cause may be suggested by characteristic signs outside the nervous system, e.g. swelling in the parotid region (mumps). Prognosis is excellent.


Japanese encephalitis is the commonest cause of encephalitis in Asia: other causes—with considerable geographical and seasonal variation—include dengue viruses, Enteroviruses (EV71) rabies, Nipah virus, herpes simplex, West Nile virus, and mumps.

Clinical features and prognosis—most patients present with the symptoms of meningitis followed by altered consciousness, convulsions, and sometimes focal neurological signs, signs of raised intracranial pressure, or psychiatric symptoms. Some manifestations suggest particular viruses, e.g. temporal lobe features in herpes simplex encephalitis; hydrophobia in rabies; Parkinsonian and extrapyramidal features in Japanese encephalitis. Mortality and morbidity vary according to cause, but are high, e.g. mortality 10 to 40% in Japanese encephalitis, with neurological sequelae in 5 to 75% of survivors.


Viral ‘paralytic’ myelitis is classically caused by poliovirus, which has now been virtually eliminated from the Americas: other causes—with considerable geographical and seasonal variation—include Japanese encephalitis and various coxsackieviruses, echoviruses, enteroviruses and flaviruses.

Clinical features—following a nonspecific episode of influenza-like symptoms, poliomyelitis typically presents with features of viral meningitis preceding or accompanying the development of lower motor neurone (flaccid) paralysis. Respiratory and bulbar paralysis is life-threatening. Mortality in adults is more than 20%.


The most important investigation is lumbar puncture to allow examination of the cerebrospinal fluid (see Chapter 24.11.1 for discussion), with typical findings of (1) pleocytosis—ranging from tens to thousands of cells/µl, with lymphocytes and other mononuclear cells usually predominating; (2) modest increase in protein concentration; (3) normal glucose concentration. Some viruses can be isolated from the cerebrospinal fluid, and viruses can sometimes be cultured from distant sites, but polymerase chain reaction (PCR) technology is now used routinely for diagnosis of viral central nervous system infection.


Aside from supportive care, aciclovir is effective in treating herpes simplex encephalitis, and hyperimmune plasma reduces mortality of Argentine haemorrhagic fever (Junin virus) and Congo Crimean haemorrhagic fever, but there is no effective specific treatment for most viral infections of the central nervous system.


Prophylactic vaccination is available against poliomyelitis, measles, Japanese encephalitis, and rabies. Postexposure rabies vaccination is effective in preventing rabies encephalitis. Hyperimmune immunoglobulin has been used for prophylaxis of measles, herpes zoster virus, HSV-2, vaccinia, rabies, and some other infections in high-risk groups.

Other neurological disorders in which viruses play a role

These include (1) Reye’s syndrome—an acute encephalopathy affecting children aged 2 to 16 years, associated with use of salicylates during the preceding viral illness. (2) Subacute sclerosing panencephalitis—caused by measles virus; typically presents with very gradual onset of altered behaviour, mild intellectual deterioration, and loss of energy and interest; periodic involuntary movements then appear; further progression is marked by intellectual deterioration, rigidity, spasticity, and increasing helplessness; there is no effective treatment; 40% of patients die within a year. (3) Progressive multifocal leucoencephalopathy—caused by opportunistic infection by papovaviruses, most commonly JC virus and the simian virus SV40; onset is usually with progressive evidence of a focal lesion of one cerebral hemisphere, before gradual development of more widespread signs; there is no effective treatment; most patients die within 6 to 12 months.

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