A case-control study was carried out among the residents of Diglipur in North Andaman, to assess the important risk factors for leptospiral infection. For selection of cases and controls, a random sample of 1014 apparently healthy persons, aged 10 years and above was drawn. Blood samples were collected tested for anti-leptospiral antibodies by Microscopic Agglutination Test (MAT) using four serovars endemic in this area, namely Australis, Canicola, Grippotyphosa and Icterohaemorrhagiae. A MAT titre of 1:50 or more to any of the serovars was considered as an evidence of leptospiral infection and such persons were defined as sero-positive cases. Persons whose sera gave negative results in MAT were taken as controls.

Of the 1014 sera samples tested by MAT, anti-leptospiral antibodies against any of the serovars of leptospires were found in 550 persons with an overall seroprevalence rate of 54.2%. The prevalence rate was significantly lower in females (c2=23.92, p=0.0000010) and was linearly increasing with age (c2 for linear trend=11.34, p=0.00076) with the highest rate in people aged 30 years and above. Among the positive sera samples,  Australis was the commonest serovar, followed by  Grippotyphosa,  Canicola and  Icterohaemorrhagiae.

People who use stream water for domestic purposes were at an increased risk of acquiring leptospiral infection, particularly Grippotyphosa and  Australis. None of the other household characteristics were found to be statistically significant. As compared to students, the risk of leptospiral infection was found to be significantly more among manual outdoor workers with an odds ratio of 1.47 (95% CI=1.01-2.13). Indoor workers had non-significant increase in the risk. (OR=1.19, 95% CI=0.80-1.77). Infection with Australis was significantly higher in families having their own agricultural land (OR=1.4, 95% CI=1.0 –1.90).

Presence of cattle in the house was found to have a significant association with infection with Grippotyphosa whereas Australis infection was significantly associated with persons who kept dogs in their houses and who used to handle domestic animals. Working in the fields was associated with Australis infection, defecation in open fields with Grippotyphosa infection and exposure to stagnant water bodies was associated with infection with all of the four serovars of leptospires. Canicola infection was common in those who had the habit of bathing in ponds and in those children playing in fields. None of the factors were significantly associated with infection with  Icterohaemorrhagiae. Other behavioural factors like walking barefoot or fishing in the fields were not associated with any significant increase in the risk of infection. Recent outdoor exposures in the form of agriculture work, jungle work, outdoor work and fishing in the fields were significantly associated with  Australis, Grippotyphosa and Canicola infection.

The analysis of association between sero-positivity to individual serovars and the probable risk factors revealed some findings, which were important for further investigation into the transmission cycles existing in the area. The independent variables for study were selected keeping in mind the known transmission cycles of the disease, which involve factors like handling of animals, exposure to wet and water logged environment and behavioural factors leading to unprotected exposure. One finding was the lack of association between sero-positivity to Icterohaemorrhagiae and all the variables studied. For the three serovars studied other than  Icterohaemorrhagiae, one or more forms of recent outdoor exposure had significant associations indicating that the water-logged environment of the area plays an important role in the transmission of infection with these serovars.. Besides this, infection with  Grippotyphosa was found to have a significant association with use of well or stream water and presence of dogs in the house, infection with  Australis had association with farming families and presence of cattle in the houses and infection with  Canicola had association with the habit of bathing in ponds.