Serosurvey, Diglipur: 1994: The first serosurvey of leptospirosis was conducted in a limited sample of population in North Andaman in 1994. Blood samples were collected from 58 healthy subjects residing in various villages of North Andaman. The subjects were aged between 3 and 70 years. The samples were tested for anti-leptospiral antibodies using 19 strains of leptospires representing 16 serogroups. 32 of the samples (55.2%) showed presence of anti-leptospiral antibodies. .

Serosurvey, Diglipur 1997: The next major survey was conducted in 1997 again in North Andaman. In this survey a sample of 1064 subjects aged 10 years and above residing in various villages of North Andaman were included. 555 of them had detectable level of antileptospiral antibodies. The seroprevalence rates showed a linear increase with age. It was 50.4% in the age group of 10-19 years, 52% in the age group of 20—29 years, 61.6% in 30—39 years and 70% in those aged 40 years and above. Males and females had almost equal prevalence rates. The common infecting serogroups were Grippotyphosa and Australis.

Sero-prevalence among school children: The survey was carried out in the rural schools of Andaman and Nicobar Islands. Three to four schools were randomly selected from each of the seven tehsils depending upon population and a random sample of students were selected from each of the selected schools. A total of 1364 students were inducted into the study. The selected students were interviewed for their habits and household environment using a preset schedule. Serum samples were collected and tested for anti-leptospiral antibodies using Microscopic Agglutination Test (MAT). The test was performed using a panel of antigens representing eight commonly circulating serogroups viz Australis, Autumnalis, Canicola, Grippotyphosa, Icterohaemorrhagiae, Javanica, Pomona and Sejroe. All the serum samples were screened at doubling dilutions starting at an initial dilution of 1:25 up to the end titre. A titre ≥1:100 to any one of the serovars used in MAT was considered indicative of past leptospiral infection.

Anti-leptospiral antibodies were detected in 299 out of the 1364 samples with an overall sero-positivity rate of 21.9%. Sero-prevalence of leptospirosis was significantly higher in Andaman district (26.7%) compared to that among the tribal students from Nicobar district (4.5%). In Andaman district, the sero-prevalence was the highest among the students of Diglipur tehsil followed by those in Ferrargunj, Mayabunder, Rangat and Port Blair tehsils. For both the districts, the sero-prevalence among males was not different from that of females. In both the districts, seroprevalence rate increased with age.

Serosurvey among tribes: A total of 1557 serum samples from four tribes viz. Great Andamanese, Onges, Nicobarese and Shompens 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 are 16.4% among Nicobarese, 22.2% among the Onges and 14.8% among the Great Andamanese. In all the tribes, seroprevalence rates were found to be significantly higher among the males and it increased with age. The Shompens had a very high seroprevalence compared to other tribes. The seroprevalence observed among them is greater than that observed in some of the settlers in the endemic areas. In contrast, the other tribes had lower prevalence rates. The difference in the environments of the islands, where these tribes live might be an important reason for this observed difference in the seroprevalence. The Shompen live in the jungles of Great Nicobar. This island has many streams and one of the two rivers of Andaman Nicobar Islands. The soil of the jungles is always wet and in some places water logged. The Shompen walk in these areas for hunting and gathering forest produce. In contrast, Car Nicobar, Little Andaman and Strait Islands are flat sandy areas with soil having low water retaining capacity. So the exposure of the other tribes to wet and water-logged environment is less frequent than that of the Shompen and consequently, they have a lower infection rates and seroprevalence.

Serosurvey, Diglipur: 1994: The first serosurvey of leptospirosis was conducted in a limited sample of population in North Andaman in 1994. Blood samples were collected from 58 healthy subjects residing in various villages of North Andaman. The subjects were aged between 3 and 70 years. The samples were tested for anti-leptospiral antibodies using 19 strains of leptospires representing 16 serogroups. 32 of the samples (55.2%) showed presence of anti-leptospiral antibodies. .

Serosurvey, Diglipur 1997: The next major survey was conducted in 1997 again in North Andaman. In this survey a sample of 1064 subjects aged 10 years and above residing in various villages of North Andaman were included. 555 of them had detectable level of antileptospiral antibodies. The seroprevalence rates showed a linear increase with age. It was 50.4% in the age group of 10-19 years, 52% in the age group of 20—29 years, 61.6% in 30—39 years and 70% in those aged 40 years and above. Males and females had almost equal prevalence rates. The common infecting serogroups were Grippotyphosa and Australis.

Sero-prevalence among school children: The survey was carried out in the rural schools of Andaman and Nicobar Islands. Three to four schools were randomly selected from each of the seven tehsils depending upon population and a random sample of students were selected from each of the selected schools. A total of 1364 students were inducted into the study. The selected students were interviewed for their habits and household environment using a preset schedule. Serum samples were collected and tested for anti-leptospiral antibodies using Microscopic Agglutination Test (MAT). The test was performed using a panel of antigens representing eight commonly circulating serogroups viz Australis, Autumnalis, Canicola, Grippotyphosa, Icterohaemorrhagiae, Javanica, Pomona and Sejroe. All the serum samples were screened at doubling dilutions starting at an initial dilution of 1:25 up to the end titre. A titre ≥1:100 to any one of the serovars used in MAT was considered indicative of past leptospiral infection.

Anti-leptospiral antibodies were detected in 299 out of the 1364 samples with an overall sero-positivity rate of 21.9%. Sero-prevalence of leptospirosis was significantly higher in Andaman district (26.7%) compared to that among the tribal students from Nicobar district (4.5%). In Andaman district, the sero-prevalence was the highest among the students of Diglipur tehsil followed by those in Ferrargunj, Mayabunder, Rangat and Port Blair tehsils. For both the districts, the sero-prevalence among males was not different from that of females. In both the districts, seroprevalence rate increased with age.

Serosurvey among tribes: A total of 1557 serum samples from four tribes viz. Great Andamanese, Onges, Nicobarese and Shompens 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 are 16.4% among Nicobarese, 22.2% among the Onges and 14.8% among the Great Andamanese. In all the tribes, seroprevalence rates were found to be significantly higher among the males and it increased with age. The Shompens had a very high seroprevalence compared to other tribes. The seroprevalence observed among them is greater than that observed in some of the settlers in the endemic areas. In contrast, the other tribes had lower prevalence rates. The difference in the environments of the islands, where these tribes live might be an important reason for this observed difference in the seroprevalence. The Shompen live in the jungles of Great Nicobar. This island has many streams and one of the two rivers of Andaman Nicobar Islands. The soil of the jungles is always wet and in some places water logged. The Shompen walk in these areas for hunting and gathering forest produce. In contrast, Car Nicobar, Little Andaman and Strait Islands are flat sandy areas with soil having low water retaining capacity. So the exposure of the other tribes to wet and water-logged environment is less frequent than that of the Shompen and consequently, they have a lower infection rates and seroprevalence.