Date of Award

January 2015

Document Type

Open Access Thesis

Degree Name

Master of Public Health (MPH)

Department

School of Public Health

First Advisor

Albert Ko

Abstract

Introduction

The ecology of Leptospira, the spirochete agent for leptospirosis, in its environmental reservoir is poorly understood. We quantified the presence of pathogenic leptospires in soil and sewage from a high-risk slum community for leptospirosis, and characterized its persistence in this environment.

Methods

We collected soil and sewage samples from three sites within a slum community in the city of Salvador, Brazil, and measured the concentration of leptospires using real-time quantitative PCR targeting lipL32, a gene exclusively present in pathogenic Leptospira. Additionally, we investigated the persistence of L. interrogans serovar Copenhageni, the agent for urban leptospirosis, in soil and sewage by performing time-course experiments with in situ mesocosms spiked with live pathogen.

Results

Among 70 soil samples obtained from 41 sites, lipL32 was detected in 22 (31%), with a mean concentration of 11.6 (95% CI: 4.6, 29.2) genome-equivalents (GEq) per g for PCR-positive samples. LipL32 was also detected in seven of the eight sewage samples, with a mean of 0.55 (95% CI: 0.38, 0.81) GEq per mL among PCR-positive samples. Specific attributes of the surrounding environment were not found to be significantly associated with sample positivity. The concentration of lipL32 GEq/g, however, was positively correlated with soil moisture (r2 = 0.50) among PCR-positive samples. In experiments of spiked mesocosms, surface-soil concentrations of Leptospira decayed rapidly over the course of six days, while bacteria situated 5-10cm below the surface persisted at 100-fold higher concentrations during the same time period.

Conclusions

Pathogenic Leptospira are widely distributed in the urban slum environment where endemic transmission of leptospirosis occurs. Specific features of the microenvironment, such as soil moisture, promote abundance and persistence. Yet the overall pathogen load in soil and sewage is low, suggesting that during environmental exposures the infecting inoculum dose may be small. Control measures should therefore emphasize reducing the frequency and duration of contact with contaminated environment among slum residents, in addition to decontaminating and removing transmission sources.

Comments

This is an Open Access Thesis.

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