Viliuisk encephalomyelitis (VE), a progressive neurological disorder with a 
fatal outcome usually in several months to 6 yrs after disease onset, is seen 
only among the Iakut people of Siberia. The acute meningoencephalitic phase of 
the disease is followed by progressive dementia, rigidity and spastic 
tetraparesis. The disease is characterized by multiple micronecrotic foci with 
marked inflammatory reactions and gliosis in the grey matter. The acute febrile 
onset, with cerebrospinal fluid (CSF) pleocytosis and increased protein in the 
CSF, the epidemiology and the inflammatory neuropathology suggest the disease is 
infectious. Studies on household spread indicate an incubation time of up to 
several years. Viliuisk encephalomyelitis was restricted to an ethically 
distinct group of Iakut people of the Middle Viliui region, but in recent 
decades, with migration from this region, it has been spreading into previously 
unaffected Iakut populations. The occurrence of multiple VE cases in households 
and introduction of the disease by migrants into new populations indicate 
horizontal transmission in a setting of long intimate contact.