Changing scenario of hepatitis A virus and hepatitis E virus exposure among the primitive tribes of Andaman and Nicobar Islands, India over the 10-year period 1989-99

The Andaman and Nicobar Islands, Union Territory of India, are home to six primitive tribes. Studies carried out earlier among these tribes revealed very high rates of hepatitis B infection. We have now studied hepatitis A and E infection among them. A total of 951 serum samples were collected from four accessible tribes (Nicobarese, Shompens, Onges and Great Andamanese) and tested for antibodies against hepatitis A and E viruses. In addition, 240 serum samples collected a decade earlier from age-stratified Nicobarese were also screened. Hepatitis A virus (HAV) infection was found to be highly endemic among all the tribes, whereas hepatitis E virus (HEV) infection was common among the Nicobarese and Shompens. The age group-wise prevalence of these infections among the Nicobarese showed different patterns, HAV prevalence rising significantly from those aged 10 years and thereafter reaching a plateau, whereas HEV prevalence was found to be more evenly distributed over all age groups, but rising somewhat after 30 years of age. Over the last decade, the prevalence of HAV among the Nicobarese has declined slightly, particularly in those aged 10 years or less whereas HEV infection has more than doubled over all age ranges. Different HEV prevalence observed among the tribes could not be attributed to differences in sanitation or water supply. This fact and the different age-wise patterns of HAV and HEV prevalences is suggestive of different modes of transmission of HEV that are not shared. The highest rates for HEV were among those tribes which reared pigs suggesting that pigs might serve as reservoir of HEV. Further studies are needed, however, to validate these findings.