Assessment of alimentary exposure to Listeria monocytogenes

Survey data on the frequency of foodborne occurrence and dietary exposure to Listeria monocytogenes were used to estimate the mininmal mean per person annual rate of exposure in the United States during the late 1980s. The estimate was restricted to ready-to-eat (RTE) foods because proper cooking was assumed to be listericidal. The mean amount of each food type per L. monocytogenes occurrence was calculated in about 100 sources, and dietary intake data were used to calculate the mean number of occurrences of L. monocytogenes consumption per person per year. The mean number of occurrences consumed annually per person was determined to be 10 to 100 for RTE food values of 2 to 20% of the total dietary intake, respectively. The frequency of foodborne listeriosis (≈ 10−5) was consistent with the estimated exposure rate only if the susceptible population was unexpectedly small or extremely high doses were necessary for infection. Because little evidence is available to support a high rate of unreported non-severe infections, this study was concerned only with severe listeriosis cases. Published frequencies of L. monocytogenes concentrations in food were used to convert occurrences to colony forming units (CFU). Low L. monocytogenes concentrations (≈ 1 CFUg) were too frequent to be responsible for listeriosis in susceptible subjects, would have caused listeriosis only with extremely low probability in a one-cell threshold infection model. The probability of exposure to a higher dose (≥ 103 CFU) was large enough to account for the observed rate of listeriosis.