The Andaman islands were known to be endemic for leptospirosis during the early 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 1980s leptospirosis reappeared among the settler population
and several outbreaks have been reported with high case fatality rates. Besides settlers, these
islands are the home of six primitive tribes of which two are still hostile. These tribes have
ample exposure to environment conducive for transmission of leptospirosis. Since no
information about the level of endemicity of the disease among the tribes is available, a
seroprevalence study was carried out among all the accessible tribes of the islands. A total of
1557 serum samples from four of the tribes were collected and examined for presence of
antileptospiral antibodies using Microscopic Agglutination Test (MAT) 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 seropositivity rates in the other tribes were 16·4%
among Nicobarese, 22·2% among 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 with
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 seroprevalence rates continued to rise
beyond 40 years. In all the tribes, seroprevalence rates were found to be significantly higher
among the males. The commonest serogroups encountered were Australis followed by
Grippotyphosa, Icterohaemorrhagiae, Pomona and Canicola.