My Earth Week series would not be balanced, without a post describing research on environmental conditions that affect millions of humans, particularly in the developing world. Approximately one billion people across the globe reside in urban slums, which were defined in 2002 by a United Nations Expert Group as settlements with the following characteristics: inadequate access to safe water and to sanitation, poor structural quality of housing, overcrowding, and insecure residential status, i.e. individuals live as squatters, and have no legal title to the land (Riley et al., 2007). In general, chronic non-communicable and communicable diseases go unrecognized and untreated in urban slum inhabitants, until they manifest as kidney failure, heart attack, stroke, pulmonary failure, or multidrug-resistant tuberculosis. In Salvador, a large Brazilian city where 60% of the inhabitants reside in slums (favelas), severe leptospirosis was diagnosed almost exclusively in slum-dwellers (95% of cases). This disease, which can cause acute, potentially fatal, kidney failure or lung hemorrhage, is entirely preventable (Riley et al., 2007).
A favela in Salvador, Brazil, showing proximity of houses to an open sewer (from Riley et al., 2007)
Leptospirosis is caused by a spirochete, which is excreted in urine by rats and other infected mammals, and transmitted in contaminated water supplies. Both subsistence farmers and urban slum-dwellers in developing countries are at a particularly high risk for contracting leptospirosis, which in its severe kidney (Weil’s disease) and lung (severe pulmonary hemorrhage syndrome, SPHS) forms is associated with 10%, and over 50% case fatality, respectively (McBride et al., 2005). In a recent article in PloS Neglected Tropical Diseases, Reis and colleagues reported on how the urban environment and social gradient affect the incidence of infection with the Leptospira spirochete, using study populations in the Pau da Lima favela community in Salvador, Brazil. The researchers chose a study site within this favela that included four valleys within an area of 0.46 square kilometers, and 1079 sample households were selected at random. Reis and colleagues focused on sewage systems, refuse collection services, rat infestation, microgeographic features, and socioeconomic status within their study site, and collected blood samples from the research subjects to look for serological evidence of prior Leptospira infection.
Panel C shows the distribution of high risk (dark red-orange) and low risk (yellow) for Leptospira infection with the study site. Panel E shows the locations of open sewer systems (tan lines) and open rainwater drainage systems (blue lines) on the same study site map. (from Reis et al., 2008 )
Of the 3,171 individuals tested, 489 (15.4%) had detectable levels of anti-Leptospira antibodies in the serological analyses, and the risk of acquiring these antibodies was correlated with residence at the bottoms of valleys within the study site. Open sewers are most often located at the bottoms of valleys, and the closer a household was to an open sewer, the greater the risk of Leptospira infection for its inhabitants. Living close to an open refuse deposit, and more frequent sightings of rats, were also associated with higher frequencies of Leptospira infection in the favela residents. These correlations, and the figures included in the paper, reminded me of Dr. John Snow’s maps of London cholera deaths and water pumps, described in Edward Tufte’s wonderful book, The Visual Display of Quantitative Information.
The risk of acquiring Leptospira antibodies (f), as associated with the continuous variables of A. per capita daily household income, and C. distance in meters to the nearest open sewer. (from Reis et al., 2008 )
The authors identified and confirmed several risk factors, associated with the infrastructure of the urban slum environment, for Leptospira infection. Since rats are the main reservoir of the spirochete in this setting, proximity to open sewers and refuse deposits, where rats have home ranges, was positively correlated with Leptospira antibody titers in the study population. Independent of these environmental factors, the researchers found that both low per capita household income and black race were risk factors for spirochete infection, indicating a role for social determinants in transmission of this disease organism. Reis and colleagues concluded that factors related to climate, geography, and urban poverty interact to influence patterns of Leptospira infection in the Salvador population, and suggest that this is likely to be the case in many other urban slums. Importantly, the researchers point out that many of the transmission factors could be addressed readily by improving sanitation in this Brazilian community, and by considering the social determinants that lead to health outcome inequalities. Recent analyses by other investigators have indicated that the stresses of low socioeconomic status and racial prejudice contribute significantly to poor health outcomes, though measurements of corticosteroid levels and other physiological stress indicators were not the focus of this study.
McBride, A.J.A., Athanazio, D.A., Reis, M.G., and Ko, A.I. (2005). Leptospirosis. Current Opinion in Infectious Disease 18, 376-386.
Riley, L.W., Ko, A.I., Unger, A., and Reis, M.G. (2007). Slum health: Diseases of neglected populations. BMC International Health and Human Rights 7:2 (doi:10.1186/1742-698X-7-2)
Reis, R.B., Ribeiro, G.S., Felzemburgh, R.D., Santana, F.S., Mohr, S., Melendez, A.X., Queiroz, A., Santos, A.C., Ravines, R.R., Tassinari, W.S., Carvalho, M.S., Reis, M.G., Ko, A.I., Gurtler, R.E. (2008). Impact of Environment and Social Gradient on Leptospira Infection in Urban Slums. PLoS Neglected Tropical Diseases, 2(4), e228. DOI: 10.1371/journal.pntd.0000228