White Paper: Review on Epidemiology of Foodborne Listeriosis

Objectives

Scientific literature databases, U.S. government publications from CDC, FDA, and USDA, relevant government publications from Canada, the European Union, Australia, and other countries will be searched for relevant information on epidemiology of foodborne listeriosis and interventions for controlling this pathogen. Reports of human illness and outbreaks and on levels of contamination of meat and other foods will be gathered to demonstrate trends in recent years and identify foods and environments where contamination problems persist. Since RTE deli items have been implicated as a major source of human foodborne listeriosis, factors affecting the risk for significant levels of L. monocytogenes in RTE meat and poultry deli products will be emphasized.

Information on the presence and persistence of L. monocytogenes on meat and poultry products and interventions for its control in different environments will be presented in narrative form. In addition, data on outbreaks, reported contamination levels in different foods, possible routes of transmission, and important bacterial strains will be tabulated or presented graphically, as appropriate.

Data from American and international studies of listeriosis will be summarized and examined to determine potential transmission/contamination pathways and identify effective interventions to address these routes of contamination of RTE deli meats. Gaps and weaknesses in food attribution data for listeriosis will be noted. Such information will point out future research needs and approaches to further reduce human listeriosis cases and outbreaks.

Listeria monocytogenes is an important foodborne pathogen not because it causes large numbers of symptomatic cases but because of its relatively high case-fatality rate. About 94 percent of listeriosis cases are hospitalized and about 16 percent die. Despite the widespread occurrence of L. monocytogenes in the environment, relatively few exposed people become ill. Average annual incidence in the U.S. is estimated to be 1,591 cases. This estimate is based on the actual number of cases identified in a year multiplied by factors to correct for underreporting and underdiagnosis. Some outbreaks of listeriosis resemble other foodborne illness with symptoms of gastroenteritis and fever occurring after a median incubation period of 24 hours. Listeria concentrations in implicated food may be quite high (104 – 109cfu/g) and cases often do not have well-established risk factors. In other outbreaks, most victims are elderly, immunocompromised, or pregnant and Listeria is invasive causing bacteremia, meningitis, or illness or death in the fetus or newborn infant. In these cases, median incubation periods are 2, 9, and 27.5 days, respectively. I n one case associated with the 2011 cantaloupe outbreak, an 88-year old woman lost vision in one eye due to invasive listeriosis.

Although the incidence of cases of L. monocytogenes per 100,000 population at FoodNet sites declined significantly since the baseline years of 1996-1998 (when it ranged from 0.43 to 0.53), there has been little progress during the past 10 years (2002-2011) when incidence in the U.S. fluctuated between 0.26 and 0.32 with an incidence of 0.28 in 2011. Preliminary data for 2012 indicate an incidence of 0.25. We have not yet reached the Healthy People 2010 goal for listeriosis of 0.24 or the 2020 goal of 0.20.

Listeriosis appears to be primarily a foodborne infection and is particularly a problem on foods that are not cooked just prior to consumption, including RTE meats, soft cheeses and unpasteurized dairy products, as well as sprouts, salad vegetables, and fruit. Thermal processing of milk and meat will destroy L. monocytogenes but post-processing contamination does occur. Since this pathogen grows during refrigeration, simply keeping foods cold does not ensure their safety.

L. monocytogenes, that contaminate foods, may originate in soils from farms and pastures , in slaughtering facilities, in food processing plants, and in slicers and other equipment in delicatessens. Elimination of Listeria from many environmental sources can be challenging because of the resistant, persistent biofilms formed by these pathogens.

Several high profile outbreaks, with high fatality rates, in the 1980s, were attributed to cole slaw, milk, and Mexican style cheese. In the 1990s, there were several outbreaks and recalls of meat products due to the presence of this pathogen. RTE deli meats were identified as vehicles of infection in 31% of the outbreaks (1998-2011) with known etiology listed by Centers for Disease Control and Prevention (CDC), with dairy products accounting for another 41.4 percent. However, only one of the outbreaks, tabulated by CDC, occurring after 2006, was associated with meat. More recent outbreaks (2010-2012) in the U.S. and other countries have been attributed to: soft cheese in Australia, imported ricotta cheese in the U.S., hog head cheese (an RTE meat) in Louisiana, packaged sliced ham in Switzerland, chopped celery in Texas, and cantaloupe in the U.S.