Impact of Environment and Social Gradient on Leptospira Infection in Urban Slums

Leptospirosis, a life-threatening zoonotic disease, has become an important urban slum health problem. Epidemics of leptospirosis now occur in cities throughout the developing world, as the growth of slum settlements has produced conditions for rat-borne transmission of this disease. In this prevalence survey of more than 3,000 residents from a favela slum community in Brazil, Geographical Information System (GIS) and modeling approaches identified specific deficiencies in the sanitation infrastructure of slum environments—open sewers, refuse, and inadequate floodwater drainage—that serve as sources for Leptospira transmission. In addition to the environmental attributes of the slum environment, low socioeconomic status was found to independently contribute to the risk of infection. These findings indicate that effective prevention of leptospirosis will need to address the social factors that produce unequal health outcomes among slum residents, in addition to improving sanitation

Prospective Study of Leptospirosis Transmission in an Urban Slum Community: Role of Poor Environment in Repeated Exposures to the <i>Leptospira</i> Agent

<div><p>Background</p><p>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.</p><p>Methodology/Principal Findings</p><p>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 <i>Leptospira</i> 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 <i>Leptospira</i> 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.</p><p>Conclusions/Significance</p><p>This study found that slum residents had high risk (>3% per year) for acquiring a <i>Leptospira</i> 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.</p></div

Age-specific attack rates for A) overall <i>Leptospira</i> infection according to gender [light grey bars, male; dark grey bars, female], and B) primary and secondary <i>Leptospira</i> infection rates [open bars, primary infection; black bars, secondary infection].

<p>Whiskers indicate the 95% Confidence Interval for the rate, adjusted for the survey design. The x axes refer to age groups in years. Rates in y axes are shown as infections per 1,000 person-years</p

Multivariate risk factors for primary and secondary infection among cohort participants.

<p>Abbreviations: OR, odds ratio; CI, confidence interval.</p

<i>Leptospira</i> infection rates among 1,585 participants of the slum community cohort according to gender.

<p>Abbreviations: CI: confidence intervals adjusted according to design effect, PY: person-years of follow-up.</p>a<p>Rates expressed as infections per 1,000 person-years.</p>b<p>Primary infection was defined as an increase in the microscopic agglutination test (MAT) titer for any of the tested serovars from zero in the first test to at least 50 in the second test.</p>c<p>Secondary infection was defined as an increase in the MAT of four-fold from an initial titer ≥25.</p

Selected univariate risk factors for <i>Leptospira</i> infection among cohort participants.

<p>Abbreviations: No., number; %, percentage; IQR, interquartile range; OR, odds ratio; CI, confidence interval.</p