Epidemiology of Viliuisk encephalomyelitis in Eastern Siberia

Abstract

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.

FIGURE 1

Map of North-East Asia showing the area of Viliuisk encephalomyelitis distribution.

FIGURE 2

Basic characteristics of the Viliuisk encephalomyelitis epidemic in Eastern Siberia. A, monthly distribution of disease onsets, average for VE cases registered in 1950–1999; B, average annual incidence rate per 100,000 population decade-by-decade within 1940–1999; C, average age of disease onset in consecutive time intervals; D, average sex-dependent annual incidence rate. The 1940–1949 interval is represented by surviving patients who were diagnosed and registered in 1951. Vertical lines represent 95% CIs.

FIGURE 3

Distance (in km) between the area of the initial Viliuisk encephalomyelitis outbreak (Lake Mastakh) and villages newly affected in subsequent decades. Vertical lines represent 95% CIs.

FIGURE 4

Location of villages in which new patients with Viliuisk encephalomyelitis were identified within each of epidemic time periods. The 1940–1949 interval is represented by surviving patients who were diagnosed and registered in 1951.

FIGURE 5

Average annual incidence rate of Viliuisk encephalomyelitis in the Mid-Viliui region (filled circles) and newly affected villages along the route of population migration to the South-West and South-East densely populated regions (triangles). The 1940–1949 interval is represented by surviving patients who were diagnosed and registered in 1951. Vertical lines represent 95% CIs.