Viliuisk encephalomyelitis (VE) is a unique disease occurring in the Yakut 
(Sakha) population of Eastern Siberia. VE is always fatal, with some patients 
dying during the acute encephalitic phase of illness; those surviving the acute 
phase develop progressive dementia, rigidity and spastic quadriparesis as part 
of a more prolonged pan-encephalitic syndrome. The disease is characterized 
neuropathologically by multiple widespread micronecrotic foci with marked 
inflammatory reactions and subsequent gliosis throughout the cerebral cortex, 
basal ganglia, cerebellum and brain stem. The acute febrile onset with 
cerebrospinal fluid pleocytosis and increased protein and neuropathology showing 
inflammatory reactions suggest that VE is an infectious disease, but the 
causative agent has not been identified. Initially detected in a small mixed 
Yakut-Evenk population of the mid-Viliui region, the disease subsequently spread 
south to densely populated areas around the capital city of Yakutsk. The 
occurrence of secondary VE cases in households and the introduction of the 
disease by migrants into new populations indicate that the disease is 
horizontally transmitted in a setting of a long intra-household contact. 
Although there has been a recent decline in the number of cases, increasing 
travel may result in further spread of this fatal disease to susceptible 
individuals in other regions of the world.