Purpose of Study Leptospirosis is a zoonotic infection caused by spirochetes, spread to the environment by chronically infected mammals. In the Peruvian Amazon we have found a higher rate of severe human leptospirosis in urban, as compared to rural, sites. We hypothesize that numbers of Leptospira would be higher in urban than rural environmental surface waters so that severe disease might result from a larger infectious inoculum in urban areas and also if more pathogenic Leptospira might predominate in urban rather than rural settings, explaining the distribution of severe leptospirosis.
Methods Water samples were collected in the urban and in the rural settings and the position of selected water sources was digitally mapped. Water samples were tested for the presence of leptospiral DNA by a quantitative real-time PCR assay specific for the 16s ribosomal leptospiral gene of pathogenic Leptospira. Nested PCR was used to amplify leptospiral 16S rRNA genes from water samples, which were cloned into a plasmid and later sequenced. The 16S leptospiral sequences were identified by a General Time Reversible Model phylogenetic tree analysis.
Summary of Results Urban locations were more often positive for pathogenic leptospiral DNA than rural locations, with far higher leptospiral concentrations (χ2 = 46.69, p < .001). Urban samples from gutters and river had higher bacterial counts than samples from puddles and wells (t = 2.284, p < .05). In the rural setting, the streams were more often positive than the wells (χ2 = 133.74, p < .001). Urban surface waters were far more likely to harbor highly pathogenic Leptospira (L. interrogans) (5/19 samples) than rural water sources (22/22 samples contained intermediately pathogenic Leptospira most similar to L. fainei and L. inadai).
Conclusions Surface waters in an urban slum area of the Peruvian Amazon had higher concentrations of highly pathogenic Leptospira than those in a rural village, supporting our hypothesis that both quantity and type of pathogenic Leptospira in environmental surface waters are associated with increased risk for severe leptospirosis. Quantitative DNA amplification will be useful for assessing human risk for acquiring leptospirosis from exposure to contaminated water sources in endemic and epidemic settings.