In the United States, Listeria monocytogenes is an important foodborne pathogen that contributes to an estimated 1600 foodborne illnesses, 1500 hospitalizations, and 260 deaths each year (Scallan et al., 2011). Invasive infections caused by L. monocytogenes (listeriosis) disproportionally affect pregnant females and their newborns, older adults, and individuals with comorbidities (Painter and Slutsker, 2007; Goulet et al., 2008, 2012). In Europe, listeriosis incidence has increased among males 75 years of age and older and females older than 25 years (EFSA, 2016). In contrast, in the United States, no significant changes have been observed in the overall listeriosis incidence rate for the period 2006–2016 (Crim et al., 2014, 2015; Marder et al., 2017); however, recent trends in incidence in U.S. subgroups have not been evaluated.

Using surveillance data on foodborne illnesses reported by 10 state health departments in the United States to the Foodborne Diseases Active Surveillance Network (FoodNet), we examined the trends in listeriosis incidence by age, sex, race/ethnicity, and pregnancy status from 2008 to 2016. We also characterize the demographics of listeriosis incidence and mortality in the catchment areas and evaluate changes over the past decade in selected populations at an increased risk for listeriosis. Our study follows the work of Pouillot et al. (2012), which identified significantly higher incidence rate ratios (RRs) in the U.S. Hispanic population compared with non-Hispanic, and expands the analysis to include examination of the black and Asian populations.

A total of 1122 cases of nonoutbreak-associated invasive listeriosis recorded in the FoodNet catchment area from 2008 to 2016 met our criteria of inclusion; 153 (14%) were pregnancy-associated cases and 969 (86%) were nonpregnancy-associated cases. Estimated crude listeriosis incidence rates are available in Supplementary Table 1 and the numbers of listeriosis cases and person-year in each category in Supplementary Table 2.

To our knowledge, this study is the first reporting higher rates of listeriosis in the non-Hispanic black and Asian populations compared with the non-Hispanic white population. Among females of childbearing age (15–44 years), we identified race/ethnicity to be a significant risk factor for listeriosis with non-Hispanic Asian and Hispanic females having higher rates compared with non-Hispanic white females. Non-Hispanic black females of childbearing age (15–44 years) were not at significantly increased risk compared with non-Hispanic white females.

We found that among females of childbearing age (15–44 years), pregnant females had a significantly higher incidence of listeriosis with an RR of 91.0 (CI: 61.5–135) compared with nonpregnant females in this age group. The incidence in pregnancy-associated cases decreased significantly in the 2011–2013 period compared with the 2008–2010 period; although the incidence was also lower in the 2014–2016 period compared with the 2008–2010 period, the difference was not significant and it is unclear whether this decrease represents a longer-term trend.

In addition, our study found that U.S. males, older adults, and pregnant females have higher incidences of listeriosis comparable to those previously estimated (Pouillot et al., 2012; Silk et al., 2012). Although the FoodNet population is thought to be generally representative of the U.S. population, the U.S. Hispanic population is undersampled within the catchment area, and therefore, this may influence the findings pertaining to the Hispanic population (CDC, 2011).

With respect to the overall nonpregnant population, our analysis shows no significant change in listeriosis incidence over the three periods. Our findings differ from recently published European incidence rates, which showed an increasing rate of listeriosis among males older than 75 years and females 25 years of age or older for the 2008–2015 period (EFSA, 2016). In contrast to the FoodNet 2008–2016 findings, the preliminary 2017 FoodNet data, which was not included in this study, show a 26% increase in listeriosis cases dispersed across the population compared with the 2014–2016 period (Marder et al., 2018). As this is the first year showing an increase and the data are not finalized, this change will require further study.

With respect to our subgroup analyses, Pouillot et al. (2012) found U.S. Hispanics to be at significantly higher risk compared with non-Hispanics (RR: 1.8, CI: 1.3–2.5) using FoodNet data from 2004 to 2009, but evaluated the effect of Hispanic ethnicity only, not race (Pouillot et al., 2012; Silk et al., 2012). In addition, Pouillot et al. (2012) found that pregnant females had increased incidence (RR: 114.6, CI: 68.9–205.1) compared with females of childbearing age and Hispanic females had an independently elevated incidence (RR: 1.9) compared with non-Hispanic females, but that study did not evaluate race. Where comparable, the incidence RRs in our study for the Hispanic populations are not significantly different from those found by Pouillot et al. (2012).

Census data projections indicate increases in population sizes and proportions for both older adults and Hispanics over time (Census Bureau, 2014). Pohl et al. (2017) examined the effects of changing U.S. population demographics on listeriosis incidence rates in the United States and found that if exposure and infectivity were constant, U.S. listeriosis rates would be expected to increase from 0.25 in 2010 to 0.32 per 100,000 in 2030. The calculations showed that achieving a steady state in listeriosis incidence from 2010 to 2020 required a 12% reduction in exposure to L. monocytogenes across the entire population or a larger reduction in a specific population, such as among older adults or the immunocompromised, assuming unchanged infectivity (Pohl et al., 2017).

An increase in the U.S. incidence rate was not observed from 2008 to 2016, suggesting a reduction in exposure (Pohl et al., 2017). Results from the recent multiyear interagency Market Basket Survey, examining 27,389 ready-to-eat foods in the United States, also provide supportive evidence of reduction in exposure to foods associated commonly with L. monocytogenes contamination for the five food categories where comparison was possible (Luchansky et al., 2017).

The significant decrease we observed in the rate of listeriosis among the nonpregnant population in 2014–2016 versus 2008–2010 and among pregnant females in 2011–2013 versus 2008–2010 after controlling for age, race, and sex and the lack of significant interaction between time and age, race, or sex is consistent with a diffuse reduction across the population rather than a more marked decrease in a single susceptible population (Pohl et al., 2017).

Hispanic-type soft cheese has been implicated in numerous listeriosis outbreaks (Linnan et al., 1988; MacDonald et al., 2005; Cartwright et al., 2013; Jackson et al., 2018), due to lack of pasteurization or inadequate hygiene practices postpasteurization (Jackson et al., 2018), and is suspected as a contributor to the increased risk of listeriosis among the Hispanic population (Silk et al., 2012). Gillespie et al. (2010) found an association between listeriosis and “neighborhood deprivation.”

Whether the differences in listeriosis rates among the race/ethnicity groups are due to differing consumption patterns that include more foods with higher contamination levels, or differences in food handling practices, rates of comorbidities, care-seeking behaviors, or other undetermined causes is not currently known. The finding in the literature is inconclusive and this is an area where further research is needed. No suspected food source of Listeria has been identified that is more commonly consumed by the non-Hispanic black and Asian populations. Further studies on demographics, consumption patterns, and L. monocytogenes contamination in the food supply may shed light on the increased risk in these populations and on additional targeted prevention strategies.

We found that advanced age significantly increases the risk of death in listeriosis cases (all causes). Given that mortality, comorbidities, and immunodeficiencies naturally increase with age, this finding is not surprising, but more research into the underlying causes of this association could help disentangle the effects of confounding factors. Without more complete information on comorbidities among the cases, we were unable to evaluate their effect on mortality.

This study provides information for public health risk managers. Our results show a relatively stable incidence rate of listeriosis in the United States among various subgroups for the period 2008–2016, despite active efforts to reduce L. monocytogenes contamination in the food supply. The findings suggest, however, that interventions targeted for specific susceptible subgroups may help reduce the risk of listeriosis as the demographics of the U.S. population continue to change.