Emerging and re-emerging epidemic encephalitis: a tale of two viruses

Two major epidemics of viral encephalitis occurred in Asia in 1997 and 1998 . The first was a re-emergence of neurovirulent strains of enterovirus 71, which caused severe encephalomyelitis in children in Malaysia, Taiwan and J apan, on a background of hand, foot and mouth disease. Necropsy studies of patients who died of enterovirus 71 infection showed severe perivascular cu ffing, parenchymal inflammation and neuronophagia in the spinal cord, brain stem and diencephalon, and in focal areas in the cerebellum and cerebrum. A lthough no viral inclusions were detected, immunohistochemistry showed vira l antigen in the neuronal cytoplasm. Inflammation was often more extensive than neuronal infection, suggesting that other factors, in addition to dire ct viral cytolysis, may be involved in tissue damage. The second epidemic o f viral encephalitis was the result of a novel paramyxovirus called Nipah, which mainly involved pig handlers in Malaysia and Singapore. Pathological evidence suggested that the endothelium of small blood vessels in the centr al nervous system was particularly susceptible to infection. This led to di sseminated endothelial damage and syncytium formation, vasculitis, thrombos is, ischaemia and microinfarction. However, there was also evidence of neur onal infection by the virus and this may also have contributed to the neuro logical dysfunction in Nipah encephalitis. Some patients who seemed to reco ver from the acute symptoms have been re-admitted with clinical findings su ggestive of relapsing encephalitis. As these two epidemics indicate, the em ergence and reemergence of viral encephalitides continue to pose considerab le challenges to thp neuropathologist, in establishing the diagnosis unrave lling the pathogenesis of the neurological disease.