Salmonella Survival and Transport in the Environment: Implications for Produce Safety and Human Health Risk Pubblico
Lee, Debbie (Spring 2018)
Abstract
Fresh produce can become contaminated in the field through contact with microbes in irrigation water and soil. In Georgia, many growers rely on surface water to irrigate crops even though Salmonella is regularly detected in surface water in this region. To identify key pathways of produce contamination and the risks of surface water irrigation, field studies were conducted in southern Georgia. Salmonella was regularly detected in 42.8% of surface water irrigation pond samples (N=507) in 2012-2013 (Chapter 1) and 33.3% of irrigation pond samples (n=24) in 2014 (Chapter 2). We determined that Salmonella could be detected not only in surface water irrigation ponds, but also in well water (20%; n=5), irrigation systems (26.2%; n=65), and even on produce (3%; n=65) from commercial farms (Chapter 2). These results indicate that irrigation with surface water may pose a risk to produce safety. A high level of genetic diversity (11 serovars, 17 pulsed-field gel electrophoresis patterns) was detected on these produce farms, suggesting the role of nonpoint source contamination. Salmonella could be detected in soil in southern Georgia and we demonstrated through field experiments that Salmonella in soil could be transported onto crops via splash transfer (Chapter 3). Compared to dry soils (7% soil moisture), wet soils (15% soil moisture) were associated with higher levels of Salmonella on produce (mean: 0.24 CFU/g vs. 0.04 CFU/g; p=0.04) and in splash water (at 5 cm heights: 21.14 CFU/100ml vs. 1.14 CFU/100ml; p=0.07). These results indicate that produce safety may be compromised by heavy rainfall events in rainy periods. Salmonellosis risk in southern Georgia was elevated by 13% following wet periods (95% CI: 6-19%). Extreme precipitation events were associated with an 11% increase in risk (95% CI: 5-18%) when they occurred in moderate rainfall or wet periods (Chapter 4). These results highlight the importance of characterizing the relationship between climatic changes, environmental reservoirs of Salmonella, and the pathways of environmental exposure to Salmonella. The findings also underscore the need for comprehensive risk assessments that can estimate the produce safety risks of irrigation and rainfall events in areas where environmental pathogens are prevalent.
Table of Contents
TABLE OF CONTENTS
PAGE
INTRODUCTION
1
CHAPTER ONE
Evaluation of Grower-Friendly, Science-Based Sampling Approaches for the Detection of Salmonella in Ponds Used for Irrigation of Fresh Produce
11
CHAPTER TWO
Salmonella Diversity and Distribution in Irrigation Ponds, Irrigation Systems, and Produce on Farms in Southern Georgia
48
CHAPTER THREE
Salmonella Survival in Soil and Transfer onto Produce via Splash Events
97
CHAPTER FOUR
Precipitation and Salmonellosis Incidence in Georgia: Interactions Between Antecedent Rainfall Conditions and Extreme Rainfall Events
132
CONCLUSION
158
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