Survey of Listeria monocytogenes contamination of various environmental samples and associated health risks

Listeria monocytogenes is the causative agent of listeriosis, one of the most important foodborne infections. Pre-harvest contamination of ready to eat crops constitutes a potential source of Listeria infections. The aim of this study was to investigate the presence of Listeria spp. and L. monocytogenes via molecular methods in wastewater effluent, sewage sludge (SS) and livestock manure as potential sources of Listeria. The presence of L. monocytogenes in wastewater-irrigated soil and crops was also investigated. Quantitative microbial risk assessment (QMRA) was performed for analysis of Listeria infections for SS workers and farmers. The results of survey showed no presence of L. monocytogenes in wastewater effluent and wastewater-irrigated soil and crop samples, whereas L. monocytogenes was detected in SS (50%) and manure (8%) samples. QMRA analysis using exponential model showed that the likelihood of Listeria infections from direct ingestion of SS to be very low. In contrast, Beta-Poisson dose-response model showed an annual risk of Listeria infections higher than the proposed value of 10(-4) per person per year (pppy) by EPA, with the highest risk (4.41 x 10(-1) pppy) for SS workers. Based on our results, the agricultural use of wastewater doesn't seem to present risk of Listeria infections for consumers of wastewater-irrigated crops. However, direct ingestion of SS may pose a risk for exposed individuals. The estimated risk values could be further improved by obtaining accurate insights into the prevalence of Listeria infections especially at national or regional level. Further studies are also needed to determine inactivation rate of Listeria in SS-amended soils in arid and semi-arid areas.