Leptospiral infection among primitive tribes of Andaman and Nicobar Islands

The Andaman islands were known to be endemic for leptospirosis during the e arly part of the century. Later, for about six decades no information about the status of the disease in these islands was available. In the late 1980 s leptospirosis reappeared among the settler population and several outbrea ks have been reported with high case fatality rates. Besides settlers, thes e islands are the home of six primitive tribes of which two are still hosti le. These tribes have ample exposure to environment conducive for transmiss ion of leptospirosis. Since no information about the level of endemicity of the disease among the tribes is available, a seroprevalence study was carr ied out among all the accessible tribes of the islands. A total of 1557 ser um samples from four of the tribes were collected and examined for presence of antileptospiral antibodies using Microscopic Agglutination Test (MAT)I employing 10 serogroups as antigens. An overall seropositivity rate of 19.1 % was observed with the highest rate of 53.5% among the Shompens. The serop ositivity rates in the other tribes were 16.4% among Nicobarese, 22.2% amon g the Onges and 14.8% among the Great Andamanese. All of the tribes except the Onges showed a similar pattern of change in the seroprevalence rates wi th age. The prevalence rates were rising from low values among children to reach a peak in those aged 21-40 years and then declined. Among Onges the s eroprevalence rates continued to rise beyond 40 years. In all the tribes, s eroprevalence rates were found to be significantly higher among the males. The commonest serogroups encountered were Australis followed by Grippotypho sa, Icterohaemorrhagiae, Pomona and Canicola.