Epidemiology

Listeria monocytogenes is an important cause of zoonoses, especially in herd animals. It is widespread in nature, being found commonly in soil, decaying vegetation, and as part of the fecal flora of many mammals (Schuchat et al. 1991; Bille et al. 2003). The organism has been isolated from the stool of approximately 5% of healthy adults (Schlech et al. 1983; Schuchat et al. 1991) with higher rates of recovery reported from household contacts of patients with clinical infection (Schuchat et al. 1993). Many foods are contaminated with L. monocytogenes and recovery rates of 1-70% or more are common from raw vegetables, raw milk, fish, poultry, and meats, including fresh or processed chicken and beef available at supermarkets or deli counters (Farber and Peterkin 1991). Ingestion of L. monocytogenes must be a very common occurrence.

Two active surveillance studies performed in 1980-1982 and 1986 by the Centers for Disease Control and Prevention (CDC) indicated annual infection rates of 7.4 per million population, accounting for approximately 1,850 cases a year in the United States, with 425 deaths (Ciesielski et al. 1988; Gellin et al. 1994). By 1993, after food industry regulations were instituted to minimize the risk of food-borne listeriosis, the annual incidence had declined to 4.4 cases per million, or 1,092 cases, with 248 deaths (Tappero et al. 1995). A similar decline in incidence of human liste-riosis was seen in France, following control measures to decrease food contamination (Goulet et al. 2001). Following more recent risk assessment for L. monocytogenes in deli meats, regulatory and industry changes have been designed to prevent future contamination of ready-to-eat meat and poultry (Gottlieb et al. 2006).

The highest infection rates are seen in infants < 1 month and in adults > 60 years of age (Ciesielski et al. 1988; Tappero et al. 1995). Pregnant women account for about 30% of all cases and 60% of cases in the 10- to 40-year age group. Almost 70% of nonperinatal infections occur in those with hematologic malignancy, the acquired immunodeficiency syndrome (AIDS), bone marrow or solid organ transplants, or in those receiving corticosteroid therapy (Mylonakis et al. 1988; Blatt and Zajac 1991; Bucholz and Mascola 2001; Safdar et al. 2002; Siegman-Igra et al. 2002) but seemingly healthy persons may develop invasive disease, particularly those over 60 years of age.

Subsequent to the 1983 report (Schlech et al. 1983) of a widespread outbreak of food-borne human listeriosis due to contaminated coleslaw, a number of other food-borne outbreaks resulting in invasive disease (bacteremia, meningitis) have been documented, with vehicles including milk (Fleming et al. 1985), soft cheeses (Linnan et al. 1988; MacDonald et al. 2005), butter (Lyytikaiinen et al. 2000), as well as smoked trout (Miettinen et al. 1999), ready-to-eat pork products (Goulet et al. 1998), hot dogs, and deli-ready turkey (Frye et al. 2002; Olsen et al. 2005; Gottlieb et al. 2006). A 2002 outbreak due to contaminated turkey deli meat involved 54 patients in 9 states and resulted in the recall of more than 30 million pounds of food products, one of the largest meat recalls in US history (Gottlieb et al. 2006). Sporadic cases have been traced to contaminated cheese (Schwartz et al. 1989), turkey franks (Centers for Disease Control and Prevention 1989), and alfalfa tablets (Farber et al. 1990). The importance of food as a source of sporadic listeriosis is illustrated by two CDC studies in which 11% of all refrigerator food samples were contaminated, 64% of patients had at least one contaminated food, and, in 33% of instances, the patient and food isolates had identical strains (Pinner et al. 1992; Schuchat et al. 1992). Delicatessen-style ready-to-eat meats, especially chicken, had the highest rates of contamination. Cases were more likely than were controls to have eaten soft cheeses or deli-counter meats, and 32% of sporadic cases could be attributed to these foods.

Human listeriosis is typically acquired through ingestion of contaminated food but other modes of transmission occur. These include transmission from mother to child transplacentally or through an infected birth canal and cross-infection in neonatal nurseries (Farber et al. 1992; Colodner et al. 2003). Contaminated mineral oil used for bathing infants was the source of one outbreak (Schuchat et al. 1991). Localized cutaneous infections have occurred in veterinarians and farmers after direct contact with aborted calves and infected poultry.

The CDC has established PulseNet (http://www.cdc.gov/pulsenet/), a network of public health and food regulatory laboratories that use pulsed-field gel electrophoresis to subtype food-borne pathogens in order to promptly detect disease clusters that may have a common source (Swaminathan et al. 2001). This system has proved effective in the early detection of listeriosis outbreaks (Centers for Disease Control and Prevention 2002).

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