Epidemiological Investigation of an Outbreak of Acute Encephalitis Syndrome (AES) in Malda District of West Bengal, India

Background: An unusual outbreak of acute encephalitis syndrome (AES) with high case fatality was reported from Kaliachak- I, II and III Blocks of Malda District of West Bengal in the month of June 2014 affecting 72 children with 34 deaths. The purpose of this study was to investigate the outbreak in the light of epidemiological as well as etiological determinants. Methods: The investigating team collected clinical and epidemiological data from the cases admitted at Malda Medical College and at the Kaliachak BPHC. Different clinical samples, (serum, CSF etc) collected from cases as well as control population were screened for different pathological, biochemical and microbiological parameters. Additionally, the CSF specimens were also processed for the isolation of viruses by inoculating in the chorio-allantoic membrane (CAM) of embryonated chick eggs and intracerebral inoculation of suckling mice. Statistical methods included calculation of proportions (percentages), different test of significance (t-test, chi square etc). Results: All children were from age group of 9 months to 10 years (median=3, mean=3.73, SD=1.98) and belonged to low socioeconomic background of litchi growing belt of Malda. Most of the cases were male (65% approx). Case Fatality Rate was 47.2%. The main presenting features were sudden onset of convulsions (100%) in the early hours of dawn followed by rapid progression to unconsciousness (100%) and decerebrate rigidity (47%). Fever was present in around one third of cases. Hypoglycaemia and leucocytosis were two predominant features. Clinical samples subjected to molecular and serological testing, were all found negative for known viruses causing acute encephalitis. 3 out of 4 CSF samples produced demonstrable pocks in Chorio allantoic membrane of the embryonated eggs although the pock count varied from 4- 22 per CAM. Significantly low blood glucose level was found in the controls from litchi belt areas as compared to the controls of non-litchi belt areas of Malda. Conclusion: The evidence gathered so far pointed towards a viral etiology although the causative virus remained unidentified. Hypoglycaemia probably induced by litchi fruit might have aggravated the encephalitis rather than actually causing it.