Listeriosis: A model for the fine balance between immunity and morbidity

Background: Listeriosis is a severe food-borne disease caused by Listeria monocytogenes. It mostly affects immune-compromised individuals, pregnant women, and the elderly, and it is associated with huge economic losses, especially to the food industry. In the last decade, a sharp increase in listeriosis incidence was observed in several European countries. No suitable explanation was found for this increase, which occurred only in old patients and not in pregnant women. Methods: We developed a mathematical model to explore this upsurge by studying the balance between the immunized population fraction and the force of infection, and its influence on the incidence of listeriosis. Results: The model shows that the current upsurge could be the result of a decrease in exposure to the pathogen in food a few decades ago and hence decreased level of population immunity. The model also suggests that, counterintuitively, the incidence of listeriosis can be higher under reduced exposure to L. monocytogenes than under high exposure. These results rely on the accepted assumption that immunity to L. monocytogenes is long-lived (at least 20 years) or that there is a long-lived boosting effect by previous exposure to L. monocytogenes. The results are robust to wide changes in all other model parameters. Conclusions: Historical alterations in exposure to L. monocytogenes might explain current changes in incidence of listeriosis. The model may also be implied for other noncontagious infectious diseases (eg, food borne diseases or vector-borne diseases for which humans are considered dead-end hosts) for which susceptibility increases with age.