Spatiotemporal Determinants of Urban Leptospirosis Transmission: Four-Year Prospective Cohort Study of Slum Residents in Brazil.

BACKGROUND: Rat-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 FINDINGS: We 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/SIGNIFICANCE: The 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.

Hypertension in a Brazilian urban slum population.

Low- and middle-income countries account for the majority of hypertension disease burden. However, little is known about the distribution of this illness within subpopulations of these countries, particularly among those who live in urban informal settlements. A cross-sectional hypertension survey was conducted in 2003 among 5649 adult residents of a slum settlement in the city of Salvador, Brazil. Hypertension was defined as either an elevated arterial systolic (≥140 mmHg) or diastolic (≥90 mmHg) blood pressure. Sex-specific multivariable models of systolic blood pressure were constructed to identify factors associated with elevated blood pressure. The prevalence of hypertension in the population 18 years and older was 21% (1162/5649). Men had 1.2 times the risk of hypertension compared with women (95% confidence intervals (CI), 1.05, 1.36). Increasing age and lack of any schooling, particularly for women, were also significantly associated with elevated blood pressure (p < 0.05). There was also a direct association between men who were black and an elevated blood pressure. Among those who were hypertensive, 65.5% were aware of their condition, and only 36.3% of those aware were actively using anti-hypertensive medications. Men were less likely to be aware of their diagnosis or to use medications (p < 0.01 for both) than women. The prevalence of hypertension in this slum community was lower than reported frequencies in the non-slum population of Brazil and Salvador, yet both disease awareness and treatment frequency were low. Further research on hypertension and other chronic non-communicable diseases in slum populations is urgently needed to guide prevention and treatment efforts in this growing population.

Influence of household rat infestation on leptospira transmission in the urban slum environment.

BACKGROUND: The Norway rat (Rattus norvegicus) is the principal reservoir for leptospirosis in many urban settings. Few studies have identified markers for rat infestation in slum environments while none have evaluated the association between household rat infestation and Leptospira infection in humans or the use of infestation markers as a predictive model to stratify risk for leptospirosis. METHODOLOGY/PRINCIPAL FINDINGS: We enrolled a cohort of 2,003 urban slum residents from Salvador, Brazil in 2004, and followed the cohort during four annual serosurveys to identify serologic evidence for Leptospira infection. In 2007, we performed rodent infestation and environmental surveys of 80 case households, in which resided at least one individual with Leptospira infection, and 109 control households. In the case-control study, signs of rodent infestation were identified in 78% and 42% of the households, respectively. Regression modeling identified the presence of R. norvegicus feces (OR, 4.95; 95% CI, 2.13-11.47), rodent burrows (2.80; 1.06-7.36), access to water (2.79; 1.28-6.09), and un-plastered walls (2.71; 1.21-6.04) as independent risk factors associated with Leptospira infection in a household. We developed a predictive model for infection, based on assigning scores to each of the rodent infestation risk factors. Receiver operating characteristic curve analysis found that the prediction score produced a good/excellent fit based on an area under the curve of 0.78 (0.71-0.84). CONCLUSIONS/SIGNIFICANCE: Our study found that a high proportion of slum households were infested with R. norvegicus and that rat infestation was significantly associated with the risk of Leptospira infection, indicating that high level transmission occurs among slum households. We developed an easily applicable prediction score based on rat infestation markers, which identified households with highest infection risk. The use of the prediction score in community-based screening may therefore be an effective risk stratification strategy for targeting control measures in slum settings of high leptospirosis transmission.

Prospective study of leptospirosis transmission in an urban slum community: role of poor environment in repeated exposures to the Leptospira agent.

BACKGROUND: Leptospirosis has emerged as an urban health problem as slum settlements have rapidly spread worldwide and created conditions for rat-borne transmission. Prospective studies have not been performed to determine the disease burden, identify risk factors for infection and provide information needed to guide interventions in these marginalized communities. METHODOLOGY/PRINCIPAL FINDINGS: We enrolled and followed a cohort of 2,003 residents from a slum community in the city of Salvador, Brazil. Baseline and one-year serosurveys were performed to identify primary and secondary Leptospira infections, defined as respectively, seroconversion and four-fold rise in microscopic agglutination titers. We used multinomial logistic regression models to evaluate risk exposures for acquiring primary and secondary infection. A total of 51 Leptospira infections were identified among 1,585 (79%) participants who completed the one-year follow-up protocol. The crude infection rate was 37.8 per 1,000 person-years. The secondary infection rate was 2.3 times higher than that of primary infection rate (71.7 and 31.1 infections per 1,000 person-years, respectively). Male gender (OR 2.88; 95% CI 1.40-5.91) and lower per capita household income (OR 0.54; 95% CI, 0.30-0.98 for an increase of $1 per person per day) were independent risk factors for primary infection. In contrast, the 15-34 year age group (OR 10.82, 95% CI 1.38-85.08), and proximity of residence to an open sewer (OR 0.95; 0.91-0.99 for an increase of 1 m distance) were significant risk factors for secondary infection. CONCLUSIONS/SIGNIFICANCE: This study found that slum residents had high risk (>3% per year) for acquiring a Leptospira infection. Re-infection is a frequent event and occurs in regions of slum settlements that are in proximity to open sewers. Effective prevention of leptospirosis will therefore require interventions that address the infrastructure deficiencies that contribute to repeated exposures among slum inhabitants.

Knowledge, attitudes, and practices related to Leptospirosis among urban slum residents in Brazil.

Leptospirosis disproportionately affects residents of urban slums. To understand the knowledge, attitudes, and practices regarding leptospirosis, we conducted a cross-sectional study among residents of an urban slum community in Salvador, Brazil. Of the 257 residents who were interviewed, 225 (90%) were aware of leptospirosis and more than two-thirds of respondents correctly identified the modes of disease transmission and ways to reduce exposure. However, study participants who performed risk activities such as cleaning open sewers had limited access to protective clothing such as boots (33%) or gloves (35%). Almost all respondents performed at least one activity to prevent household rat infestation, which often included use of an illegal poison. Our findings support the need for interventions targeted at the individual and household levels to reduce risk of leptospirosis until large-scale structural interventions are available to residents of urban slum communities.

Impact of environment and social gradient on Leptospira infection in urban slums.

BACKGROUND: Leptospirosis 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 FINDINGS: We 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. CONCLUSIONS: Deficiencies 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.