A Case-Control Study of Environmental and Occupational Risks of Leptospirosis in Sri Lanka

Sri Lanka has one of the highest incidences of human leptospirosis worldwide. Outbreaks of this zoonotic infection are related to the monsoons and flooding. The study investigates risk factors associated with environmental, animal and occupational exposure while acknowledging the potential bias due to hanta viral infections in the study samples. Data were obtained from structured interviews with 483 patients (276 cases and 207 controls). Risk exposures were studied for the entire population and for two stratified occupational groups: non-paddy workers and paddy workers. A higher odds ratio (OR) of leptospirosis transmission for paddy workers was observed compared to non-paddy workers (OR 1.905, 95% CI 1.274-2.856). Rat exposure was not associated with a significant higher risk for any of the groups. Instead, cattle and household animals seemed to be important for transmission of leptospirosis to humans, especially among non-paddy workers (OR 10.655, 95% CI 1.213-93.582). Leptospirosis in paddy workers was associated with environmental factors linked to contamination and wetness in paddy fields. Interestingly, abandoned paddy fields were found to have a protective effect against transmission to paddy workers (OR 0.421, 95% CI 0.237-0.748). Keeping animals on these dryer fields may act as a boundary for contamination of paddy fields with infectious animal urine. This finding may be considered as a public health intervention targeting leptospirosis among paddy workers.