BackgroundLeptospirosis has become an urban health problem as slum settlements have expanded worldwide. Efforts to identify interventions for urban leptospirosis have been hampered by the lack of population-based information on Leptospira transmission determinants. The aim of the study was to estimate the prevalence of Leptospira infection and identify risk factors for infection in the urban slum setting.Methods and FindingsWe performed a community-based survey of 3,171 slum residents from Salvador, Brazil. Leptospira agglutinating antibodies were measured as a marker for prior infection. Poisson regression models evaluated the association between the presence of Leptospira antibodies and environmental attributes obtained from Geographical Information System surveys and indicators of socioeconomic status and exposures for individuals. Overall prevalence of Leptospira antibodies was 15.4% (95% confidence interval [CI], 14.0–16.8). Households of subjects with Leptospira antibodies clustered in squatter areas at the bottom of valleys. The risk of acquiring Leptospira antibodies was associated with household environmental factors such as residence in flood-risk regions with open sewers (prevalence ratio [PR] 1.42, 95% CI 1.14–1.75) and proximity to accumulated refuse (1.43, 1.04–1.88), sighting rats (1.32, 1.10–1.58), and the presence of chickens (1.26, 1.05–1.51). Furthermore, low income and black race (1.25, 1.03–1.50) were independent risk factors. An increase of US$1 per day in per capita household income was associated with an 11% (95% CI 5%–18%) decrease in infection risk.ConclusionsDeficiencies in the sanitation infrastructure where slum inhabitants reside were found to be environmental sources of Leptospira transmission. Even after controlling for environmental factors, differences in socioeconomic status contributed to the risk of Leptospira infection, indicating that effective prevention of leptospirosis may need to address the social factors that produce unequal health outcomes among slum residents, in addition to improving sanitation.

BackgroundRat-borne leptospirosis is an emerging zoonotic disease in urban slum settlements for which there are no adequate control measures. The challenge in elucidating risk factors and informing approaches for prevention is the complex and heterogeneous environment within slums, which vary at fine spatial scales and influence transmission of the bacterial agent.Methodology/Principal FindingsWe performed a prospective study of 2,003 slum residents in the city of Salvador, Brazil during a four-year period (2003–2007) and used a spatiotemporal modelling approach to delineate the dynamics of leptospiral transmission. Household interviews and Geographical Information System surveys were performed annually to evaluate risk exposures and environmental transmission sources. We completed annual serosurveys to ascertain leptospiral infection based on serological evidence. Among the 1,730 (86%) individuals who completed at least one year of follow-up, the infection rate was 35.4 (95% CI, 30.7–40.6) per 1,000 annual follow-up events. Male gender, illiteracy, and age were independently associated with infection risk. Environmental risk factors included rat infestation (OR 1.46, 95% CI, 1.00–2.16), contact with mud (OR 1.57, 95% CI 1.17–2.17) and lower household elevation (OR 0.92 per 10m increase in elevation, 95% CI 0.82–1.04). The spatial distribution of infection risk was highly heterogeneous and varied across small scales. Fixed effects in the spatiotemporal model accounted for the majority of the spatial variation in risk, but there was a significant residual component that was best explained by the spatial random effect. Although infection risk varied between years, the spatial distribution of risk associated with fixed and random effects did not vary temporally. Specific “hot-spots” consistently had higher transmission risk during study years.Conclusions/SignificanceThe risk for leptospiral infection in urban slums is determined in large part by structural features, both social and environmental. Our findings indicate that topographic factors such as household elevation and inadequate drainage increase risk by promoting contact with mud and suggest that the soil-water interface serves as the environmental reservoir for spillover transmission. The use of a spatiotemporal approach allowed the identification of geographic outliers with unexplained risk patterns. This approach, in addition to guiding targeted community-based interventions and identifying new hypotheses, may have general applicability towards addressing environmentally-transmitted diseases that have emerged in complex urban slum settings.

Leishmaniasis constitutes a spectrum of human diseases caused by protozoa belonging to the genus Leishmania, divided into two subgenera, Leishmania, which is present in both the Old World and the New World, and Viannia, which is restricted to the New World. Productive infections may either be asymptomatic or lead to different clinical syndromes involving internal organs, skin, and/or mucosal surfaces (3). Leishmania (Viannia) braziliensis is endemic in Latin America and causes at least three distinct clinical types of American tegumentary leishmaniasis (ATL), localized cutaneous leishmaniasis (CL), mucosal leishmaniasis (ML), and disseminated leishmaniasis (DL) (3,4,6,9,29). The most striking feature that differentiates CL from either ML or DL is the degree of metastasis of lesions from the original inoculation site. CL lesions are restricted to the entry site of the parasites, whereas ML is defined by spreading to mucosal surfaces of the upper digestive and airways tracts. DL is characterized by wide dissemination to distant cutaneous sites. Some individuals with DL develop hundreds of lesions throughout their body surfaces.Many reports cite ML as the most common metastatic form of disease caused by L. (V.) braziliensis. It was previously more common, but ML currently develops in 4 to 5% of individuals with CL (2, 7, 17). Recently, in our study area, DL has emerged as the most prevalent metastatic form of L. (V.) braziliensis infection (26). The present study is based on the hypotheses that different strains of L. (V.) braziliensis are prone to cause the different forms of ATL, and it is the spread of a DL-predisposing strain that is responsible for the increasing incidence of this new cutaneous syndrome.Individuals living in the region of Corte de Pedra in the state of Bahia, Northeast Brazil, can be afflicted by any of the three forms of ATL due to L. (V.) braziliensis. However, DL has only recently been recognized in Corte de Pedra (6, 9). Careful longitudinal study of this form of leishmaniasis has revealed a recent increase in the absolute numbers of DL cases, an increase in the relative proportion of DL cases compared to the total number ATL cases, and a pattern of geographic spread of the disease across a wider region of Corte de Pedra over the past 2 decades (16,26,29). Indeed, the geographic distribution of DL cases differs from that of ML in Corte de Pedra, which led us to investigate the regional risks of these two forms of L. (V.) braziliensis disease (26).One of the first clues that different parasite strains may contribute to the different ATL forms was in vitro evidence showing that the total antigen from L. (V.) braziliensis isolated from a DL patient caused peripheral blood mononuclear cells to release larger amounts of the proinflammatory cytokines gamma interferon and tumor necrosis factor alpha than the total antigen of a parasite from a subject with CL (14). Parallel investigations revealed that parasites isolated from ATL patients could be distinguished according to randomly amplified poly...