Acute encephalitis syndrome (AES) is caused by a wide range of viruses and bacteria. Japanese encephalitis (JE) is considered as a main viral aetiology of patients with AES in Assam. In the year 2006, West Nile (WN) virus emerged as another cause of encephalitis in this part of the country. In the year 2008-2010, we investigated 550 patients with AES admitted to nine Government and private hospitals representing 14 districts of Assam. Two hundred and fifty nine patients were identified as positive for JE virus and 56 patients for WN virus. Recent infection of Dengue (28 cases) and Chikungunya (10 cases) have also been reported
for the first time in this region. But the etiologic agents in a large number of cases still remain unidentified.