Environmental and Behavioural Determinants of Leptospirosis Transmission: A Systematic Review

Abstract

Background

Leptospirosis is one of the most widespread zoonotic diseases, which is of global medical and veterinary importance, and also a re-emerging infectious disease. The main tracks of transmission are known; however, the relative importance of each of the components and the respective environmental risk factors are unclear. We aimed to assess and specify quantitative evidence of environmental risks of leptospirosis transmission.

Methods/findings

A database of pre-selected studies, with publication dates from 1970 until 2008, was provided by an expert group. The database has been updated until 2015 using a text mining algorithm. Study selection was based on stringent quality criteria. A descriptive data analysis was performed to calculate the medians of the log transformed odds ratios. From a selection of 2723 unique publications containing information on leptospirosis, 428 papers dealing with risk factors were identified. Of these, 53 fulfilled the quality criteria, allowing us to identify trends in different geo-climatic regions. Water associated exposures were, with few exceptions, associated with an increased leptospirosis risk. In resource poor countries, floods and rainfall were of particular importance, whereas recreational water activities were more relevant in developed countries. Rodents were associated with increased leptospirosis risk, but the variation among studies was high, which might be partly explained by differences in exposure definition. Livestock contact was commonly associated with increased risk; however, several studies found no association. The median odds ratios associated with dog and cat contacts were close to unity. Sanitation and behavioural risk factors were almost always strongly associated with leptospirosis, although their impact was rarely investigated in Europe or North America.

Conclusion

This review confirms the complex environmental transmission pathways of leptospirosis, as previously established. Although, floods appeared to be among the most important drivers on islands and in Asia, the consistent pattern observed for exposure to rodents and behavioural and sanitation related risk factors indicate potential areas for intervention.

Author Summary

Leptospirosis is a bacterial disease passed from animals to people—either through direct contact with animals or indirectly via the environment. The disease can be found worldwide but is more important in tropical and subtropical countries. Due to their sheer genetic diversity, virtually all mammals can be infected by leptospires. People can be infected from a broad range of animals including livestock, pets and rodents. Transmission pathways differ among animal species as well as across rural and urban landscapes and environmental conditions which influence survival of pathogenic leptospira in surface water and moist soil. Consequently, spatial and temporal patterns of disease incidence are complex; however, a better understanding is crucial for the planning and implementation of effective interventions. We systematically reviewed the scientific literature to identify regional risk factors and assess their importance. We found considerable heterogeneity among studies indicating that epidemiological patterns are highly setting specific. Floods and heavy rain appeared to be among the most important drivers on islands and in Asia. Exposure to rodents as well as behaviour and sanitation are often important risk factors indicating potential areas for interventions.