Exotic and emerging viral encephalitides.

PURPOSE OF REVIEW

The exotic and emerging viral encephalitides are caused by animal or human viruses and characterised by sudden unexpected outbreaks of neurological disease, usually in tropical and sub-tropical regions, but sometimes spreading to temperate areas. Although a wide range of viruses come within this label, as this review highlights, there are common research questions as to the origin and spread of the viruses, the contribution of viral and host factors to the clinical presentations and outcome, and the possibilities for treatment and vaccination.

RECENT FINDINGS

During 2002, North America experienced the largest ever outbreak of West Nile encephalitis; a poliomyelitis-like flaccid paralysis due to West Nile virus was recognised, and transfusion-related infections were documented. Globally, Japanese encephalitis virus is the most important emerging viral encephalitis; interferon alpha was not effective against Japanese encephalitis in a double-blind placebo-controlled trial, but new chimeric vaccines are in development. Recent work suggests Japanese encephalitis virus originated in the Indonesia-Malaysia region, and spread from there. The origin of Nipah virus, which caused an encephalitis outbreak in Malaysia in 1998, is not known, but flying foxes have been identified as a natural host. Enterovirus 71 continues to cause large outbreaks of hand foot and mouth disease across Asia, associated with neurological and systemic complications; recent work has focused on the pathogenesis of these complications.

SUMMARY

Disease surveillance remains important for the early recognition and containment of encephalitis outbreaks. Detailed clinical and laboratory studies will help answer the key questions, but there is a need to ensure the results translate to real benefits for the communities affected by these diseases.