Understanding the mechanisms of transmission of human leptospirosis in slums settlements in Salvador, Brazil

Global health need
Leptospirosis, a spirochetal zoonosis, has emerged as an important health problem as slum settlements have expanded rapidly worldwide and created conditions for rat-borne transmission. In Brazil, 12,000 individuals are annually infected with leptospirosis, with a death rate of about 12%. In the city of Salvador, Brazil, where transmission mostly occurs in peridomestic environments, the government-led expansion of closed sewer networks has resulted in a significant decrease of infection rates for leptospirosis. However, due to political neglect and the uncontrolled expansions of slums, construction of conventional governmentally led closed sewer systems (GOV-INT) is increasingly problematic. To address this issue, innovative and more economical methods that require of community residents involvement (COM-INT) have emerged as suitable alternatives to improve sanitation in slum settlements. However, assessing the real impact of sanitation intervention, such as GOV-INT and COM-INT, is problematic because of methodological issues due to confounding factors and difficulties with randomization of the population to obtain large enough large sample sizes.
Current knowledge gaps
To the best of our knowledge, no study has been carried out to assess the potentially positive effects of COM-INT on the reduction of leptospirosis risk. Based on preliminary descriptive statistics from an ongoing study in Salvador, to which this project directly contributes, it has been conjectured that GOV-INT and COM-IN are equally effective in the reduction of Leptospirosis infections. In this context, the use of spatio-temporal statistical methods are crucial in order to infer the actual impact of COM-INT by taking account of both measured and unmeasured risk factors of Leptospirosis infection.
Study aims
1. Determine the mechanisms by which COM-INT and GOV-INT reduce direct human contact with sewage and environmental pathogen load in urban slums.
2. Determine the contribution of COM-INT and GOV-INT on a decline in severe disease incidence in a large urban center.
Study design/methodology
As part of a Wellcome-Trust project, this study will be split into the following two parts (each referring to the above aims in turn)
1. The main cause of Leptospira infection is exposure to contaminated sewage and run-off. Hence, to pursue the first study aim, statistical methods that combine information from environmental reservoirs and resident's movement will be fitted to data from an ongoing study in the community of Marechal Rodon in Salvador, Brazil. This study site consists of 9 areas, 3 with GOV-INT, 3 with COM-INT and 3 with no intervention. A total of 2,582 soils samples will be collected and tested for Leptospira DNA, before and after the interventions. These data will be integrated with human mobility data collected on 150 individuals using multifunctional sensor nodes to quantity the exposure to contamination sources
2. A secondary spatio-temporal geostatistical analysis will be conducted to assess the effectiveness of COM-INT and GOV-INT, using data collected since 1996 from a hospital-based surveillance for leptospirosis that covers the population of Salvador since. To account for confounding effects, information on socio-economic and environmental factors will also be incorporated into the spatio-temporal model.