Risk factors in the transmission of leptospiral infection

An unmatched case control study was conducted to study the various risk factors for acquiring leptospiral infection in Diglipur tehsil of North Andaman. A random sample of 1014 persons residing in various villages of Diglipur was inducted into the study. Serum samples were collected from them and tested for anti-leptospiral antibodies using microscopic agglutination test (MAT) using Leptospira grippotyphosa, L. australis, L. canicola and L. icterohaemorrhagiae antigens. Persons with a titre of 1:50 or more were considered as the cases (550) and the seronegatives as controls (464). Information about 30 variables relating to household characteristics, occupation, contact with animals and behavioural factors was collected by interviewing the subjects. The prevalences of these variables in both the groups were calculated and the odds ratio with 95 per cent confidence intervals were computed. The seroprevalence rate was found to increase linearly with age and it was significantly higher in males. None of the risk factors studied had any association with seropositivity to serovar L. icterohaemorrhagiae. For the other serovars, some form of recent exposure to outdoor environment had significant association. Other factors which had association with infection with specific serovars included use of well or stream water and presence of dogs in the house for infection with L. grippotyphosa, farming families and presence of cattle in the houses for infection with L. australis and the habit of bathing in ponds for infection with L. canicola. These observed associations can be taken as clues of the transmission cycles and would help in guiding further investigations for understanding the epidemiology of leptospirosis in these islands.