According to a new study recently published in the International Journal of Food Microbiology, over 90 percent of listeriosis cases in the U.S. come from deli meat, followed by ready-to-eat (RTE) salads at just less than 5 percent.

The study, titled “Quantitative risk assessment model to investigate the public health impact of varying Listeria monocytogenes allowable levels in different food commodities: A retrospective analysis” was led by Fernando Sampedro of the Environmental Health Sciences Division, School of Public Health at the University of Minnesota.

The study found the total number of estimated listeriosis cases in the U.S. was between 1,044 and 2,089, and the probability of infection in the U.S. susceptible population — individuals such as elderly, pregnant women, and neonates and people with underlying disease conditions or comorbidities — to be 10–10,000× higher than the general population. This highly susceptible population makes up 46.9 percent to 80.1 percent of total listeriosis cases.

The main objectives of this study were to first, perform a retrospective analysis of the changes in prevalence and concentration levels of Listeria monocytogenes in various commodities (RTE salads, deli meats, soft and semi-soft cheese, RTE seafood, and frozen vegetables) over the last 30 years.  And second, to estimate the net public health effect of removing lots with certain contamination levels from the market.

The study found that after deli meat at more than 90 percent and RTE salads at just less than 5 percent, soft and semi-soft cheese and RTE seafood accounted for 0.5 to 1.0 percent of listeriosis cases. Lastly, frozen vegetables accounted for 0.2 to 0.3 percent of cases.

The study suggests that introducing lot-by-lot testing and defining allowable quantitative regulatory limits for low-risk RTE commodities may reduce the public health impact of L. monocytogenes and improve the availability of enumeration data. The researchers hope the study provides guidance to national authorities as they formulate effective risk management strategies that account for differences in risk levels to optimize the use of threshold values in implementing preventive controls for L. monocytogenes.