Distribution of Listeria monocytogenes molecular subtypes among human and food isolates from New York State shows persistence of human disease--associated Listeria monocytogenes strains in retail environments.

While there is considerable information available regarding Listeria monocytogenes contamination patterns in food processing plants, our understanding of L. monocytogenes contamination and transmission in retail operations is limited. We characterized 125 food, 40 environmental, and 342 human clinical L. monocytogenes isolates collected in New York State from 1997 to 2002 using automated ribotyping and hly allelic variation. All environmental isolates were obtained from retail establishments and the majority of food isolates (98 isolates) were obtained from foods that were prepared or handled at retail. Overall, food and/or environmental isolates from 50 different retail establishments were characterized. The 125 food and 40 environmental isolates were differentiated into 29 and 10 ribotypes, respectively. For 16 retail establishments, we found evidence for persistence of one or more specific L. monocytogenes strains as indicated by isolation of the same EcoRI ribotype from food or environmental samples collected in a given establishment on different days. The human isolates were differentiated into 48 ribotypes. Statistical analyses showed that two ribotypes were significantly (P < 0.0001) more common among food isolates as compared with human isolates. However, a total of 17 ribotypes found among the human clinical isolates were also found among the food and environmental isolates. We conclude that L. monocytogenes, including subtypes that have been linked to human disease, can persist in retail environments. Implementation of Listeria control procedures in retail operations, which process and handle products that permit the growth of L. monocytogenes, are thus a critical component of a farm-to-table L. monocytogenes control program.

Prevalence, distribution, and diversity of Listeria monocytogenes in retail environments, focusing on small establishments and establishments with a history of failed inspections.

Despite growing concerns about cross-contamination of ready-to-eat foods with Listeria monocytogenes, our knowledge about the ecology and transmission of L. monocytogenes in retail establishments has remained limited. We conducted a cross-sectional study to characterize the prevalence, distribution, and subtype diversity of L. monocytogenes in 120 New York State retail deli establishments that were hypothesized to present an increased risk for environmental L. monocytogenes contamination (i.e., small establishments and establishments with a history of failed New York State Agriculture and Markets inspections). Analysis of these data along with previously reported data for 121 predominantly larger retail establishments in New York State identified establishment size, geographic location, and inspection history as significant predictors of L. monocytogenes presence and prevalence. The odds of an establishment being L. monocytogenes positive were approximately twice as high for large establishments, establishments located in New York City, or establishments with poor inspection history (as compared with establishments without these attributes), even though correlation between location and inspection history complicated interpretation of results. Within an establishment, L. monocytogenes was significantly more prevalent on nonfood contact surfaces than on food contact surfaces; prevalence was particularly high for floors and in floor drains, sinks, the dairy case, and milk crates. L. monocytogenes subtype diversity differed between sites, with lineage I isolates significantly associated with nonfood contact surfaces and lineage II isolates significantly associated with food contact surfaces. Isolates belonging to the same ribotype were often found dispersed across multiple sites within an operation.

Quantitative risk assessment of listeriosis due to consumption of raw milk.

The objectives of this study were to estimate the risk of illness for raw milk consumers due to Listeria monocytogenes in raw milk sold by permitted dealers, and the risk for people on farms who consume raw milk. Three scenarios were evaluated for raw milk sold by dealers: raw milk purchased directly from bulk tanks, from on-farm stores, and from retail. To assess the effect of mandatory testing of raw milk by regulatory agencies, the number of listeriosis cases per year was compared where no raw milk testing was done, only a screening test to issue a permit was conducted, and routine testing was conducted and milk was recalled if it was L. monocytogenes positive. The median number of listeriosis cases associated with consumption of raw milk from bulk tanks, farm stores, and retail for an intermediate-age population was 6.6 × 10(-7), 3.8 × 10(-5), and 5.1 × 10(-5) cases per year, respective ly. In populations with high susceptibility, the estimated median number of cases per year was 2.7 × 10(-7) (perinatal, i.e., pregnant women and their fetuses or newborns) and 1.4 × 10(-6) (elderly) for milk purchased from bulk tanks, 1.5 × 10(-5 ) (perinatal) and 7.8 × 10(-5) (elderly) for milk from farm stores, and 2.1 × 10(-5) (perinatal) and 1.0 × 10(-4) (elderly) for milk from retail. For raw milk consumed on farms, the median number of listeriosis cases was 1.4 × 10(-7) cases per year. A greater risk of listeriosis was associated with consumption of raw milk obtained from retail and farm stores as compared with milk obtained from bulk tanks. This was likely due to additional time-temperature combination steps in the retail and farm store models, which increased the chances for growth of L. monocytogenes in raw milk. A close relationship between prevalence of L. monocytogenes in raw milk and the values of disease incidence was observed. Hence, a reduction in the number of cases per year in all populations was observed when a raw milk-testing program was in place, especially when routine testing and recalling of milk was conducted.