BACKGROUND: Viliuisk encephalomyelitis is a disorder that starts, in most cases, 
as an acute meningoencephalitis. Survivors of the acute phase develop a slowly 
progressing neurologic syndrome characterized by dementia, dysarthria, and 
spasticity. An epidemic of this disease has been spreading throughout the Yakut 
Republic of the Russian Federation. Although clinical, neuropathologic, and 
epidemiologic data suggest infectious etiology, multiple attempts at pathogen 
isolation have been unsuccessful.
METHODS: Detailed clinical, pathologic, laboratory, and epidemiologic studies 
have identified 414 patients with definite Viliuisk encephalomyelitis in 15 of 
33 administrative regions of the Yakut Republic between 1940 and 1999. All data 
are documented in a Registry.
RESULTS: The average annual Viliuisk encephalomyelitis incidence rate at the 
height of the epidemic reached 8.8 per 100,000 population and affected 
predominantly young adults. The initial outbreak occurred in a remote isolated 
area of the middle reaches of Viliui River; the disease spread to adjacent areas 
and further in the direction of more densely populated regions. The results 
suggest that intensified human migration from endemic villages led to the 
emergence of this disease in new communities. Recent social and demographic 
changes have presumably contributed to a subsequent decline in disease 
incidence.
CONCLUSIONS: Based on the largest known set of diagnostically verified Viliuisk 
encephalomyelitis cases, we demonstrate how a previously little-known disease 
that was endemic in a small indigenous population subsequently reached densely 
populated areas and produced an epidemic involving hundreds of persons.