Foodborne Illnesses / Listeria /

The Incidence of Listeria Infections

How common are Listeria infections?

Listeria bacteria are found widely in the environment in soil, including in decaying vegetation and water, and may be part of the fecal flora of a large number of mammals, including healthy human adults.

According to the United States Food and Drug Administration (FDA), some healthy adults may be asymptomatic intestinal carriers of the bacteria. Another authority notes that the “organism has been isolated from the stool of approximately 5% of healthy adults.”

According to the Centers for Disease Control and Prevention’s (CDC) National Center for Zoonotic, Vector-Borne, and Enteric Diseases, listeriosis was added to the list of nationally notifiable diseases in 2001:

To improve surveillance, the Council of State and Territorial Epidemiologists has recommended that all L. monocytogenes isolates be forwarded to state public health laboratories for subtyping through the National Molecular Subtyping Network for Foodborne Disease Surveillance (PulseNet). All states have regulations requiring health care providers to report listeriosis cases, and public health officials should try to interview all persons with listeriosis promptly using a standard questionnaire about high risk foods. To reach this goal, FoodNet conducts active laboratory- and population-based surveillance.

The following table shows selected data from the CDC’s Annual Surveillance Summaries of reported Listeria cases. Increases over time could be explained in part by increased surveillance and reporting.

Table 1. Selected data from the CDC’s Annual Surveillance Summaries of reported Listeria cases

Despite these numbers, CDC’s Technical Information website estimates that there were more than 1,600 cases of Listeria infection annually in the United States based on data through 2008. Although the nature and degree of underreporting is subject to dispute, all sources agree that the confirmed cases represent just the tip of the iceberg.

The Listeria Initiative and PulseNet conduct nationwide surveillance to rapidly detect and respond to outbreaks; the Foodborne Diseases Active Surveillance Network (FoodNet) conducts active, sentinel population-based surveillance to track incidence trends; and the Foodborne Disease Outbreak Surveillance System (FDOSS) receives reports of investigated outbreaks to track foods and settings associated with outbreaks. In a CDC report summarizing data on 2009-2011 listeriosis cases and outbreaks reported to U.S. surveillance systems:

Nationwide, 1,651 cases of listeriosis occurring during 2009–2011 were reported. The case-fatality rate was 21%. Most cases occurred among adults aged ≥65 years (950 [58%]), and 14% (227) were pregnancy-associated. At least 74% of nonpregnant patients aged <65 years had an immunocompromising condition, most commonly immunosuppressive therapy or malignancy. The average annual incidence was 0.29 cases per 100,000 population. Compared with the overall population, incidence was markedly higher among adults aged ≥65 years (1.3; relative rate [RR]: 4.4) and pregnant women (3.0; RR: 10.1). Twelve reported outbreaks affected 224 patients in 38 states. Five outbreak investigations implicated soft cheeses made from pasteurized milk that were likely contaminated during cheese-making (four implicated Mexican-style cheese, and one implicated two other types of cheese). Two outbreaks were linked to raw produce.