BACKGROUND: Viliuisk encephalomyelitis (VE) is an endemic neurological disease 
in Northeast Siberia and generally considered to be a chronic encephalomyelitis 
of unknown origin actually spreading in the Sakha (Yakutian) Republic.
METHODOLOGY AND PRINCIPLE FINDINGS: In search for the pathophysiology and 
causative agent of VE, we performed a cross-sectional study on clinical, 
serological and neuroimaging data on chronic VE patients during two medical 
expeditions to three villages within the Viliuiski river basin in the Republic 
of Sakha in 2000 and to the capital Yakutsk in 2006. The severity of the core 
clinical picture with predominant sensory ataxia, gait apraxia, lower limb 
spasticity, cognitive impairment and bladder dysfunction correlated with the 
degree of MRI findings showing enlargement of inner ventricular spaces as in 
communicating hydrocephalus. Laboratory studies revealed transient eosinophilia 
during the preceding acute meningitis-like phase, but no ongoing inflammatory 
process in the CSF. We found immune reactions against Toxocara canis in the 
majority of chronic VE patients but rarely in controls (P = 0.025; Fisher's 
exact test). Histological analysis of subacute to subchronic VE brain samples 
showed eosinophilic infiltrations with no signs of persistent Toxocara canis 
infection.
CONCLUSIONS AND SIGNIFICANCE: Our data showed that pressure by the communicating 
hydrocephalus as a mechanical factor is the major pathogenic mechanism in 
chronic VE, most likely triggered by eosinophilic meningitis. There are no signs 
for an ongoing inflammatory process in chronic VE. The past eosinophilic 
reaction in VE might be caused by Toxocara ssp. infection and might therefore 
represent the first hint for an initial cause leading to the development of 
chronic VE. Our data provide a framework for future studies and potential 
therapeutic interventions for this enigmatic epidemic neurological disease 
potentially spreading in Sakha Republic.