Although outbreaks of invasive disease caused by Listeria monocytogenes have been associated with ingestion of a variety of contaminated foods (1-5), most listeriosis in the United States occurs as isolated or sporadic cases. To determine the incidence of listeriosis and identify risk factors for disease, during 1988-1990, CDC collaborated with investigators in four states to conduct active laboratory-based surveillance and special studies in a population of more than 18 million U.S. residents. This report summarizes the findings of these studies (6,7).
The study areas included Los Angeles County, the San Francisco Bay area, the Atlanta metropolitan area, four counties in Tennessee, and the state of Oklahoma. Investigators made regular calls to all hospital laboratories and completed case report forms for all residents in whom L. monocytogenes was isolated from a usually sterile site (e.g., blood, cerebrospinal fluid (CSF), or amniotic fluid).
From November 1988 through December 1990, 301 cases were identified in the surveillance areas, an annual incidence of 7.4 cases per 1 million population; 67 (23%) persons died. Of the 301 cases, 99 (33%) occurred among pregnant women or their newborns. Among the 98 persons with nonperinatal listeriosis for whom information was available, nearly all had at least one immunosuppressive condition, including corticosteroid use (31%), malignancy (29%), renal disease (24%), diabetes (24%), or acquired immunodeficiency syndrome (20%).
Dietary histories of persons with listeriosis identified through the active surveillance project were compared with those of controls matched for age and medical condition (including pregnancy). Patients with listeriosis were more likely than controls to have eaten soft cheeses (odds ratio (OR)=2.6; 95% confidence interval (CI)=1.4-4.8) or food purchased from store delicatessen counters (OR=1.6; 95% CI=1.0-2.5). Thirty-two percent of sporadic disease could be attributed to consumption of these foods. Eating undercooked chicken was also associated with increased risk in immunosuppressed persons (OR=3.3; 95% CI=1.2-9.2) (6).
Food obtained from the refrigerators of patients with listeriosis was cultured for L. monocytogenes using at least two selective enrichment methods, and isolates of L. monocytogenes from food were compared with isolates from patients using multilocus enzyme electrophoresis. Overall, 79 (64%) of 123 refrigerators contained at least one food with L. monocytogenes, and 26 (33%) of the 79 refrigerators with L. monocytogenes grew the same strain as that which caused illness in a person living in the household. Foods that were ready-to-eat and foods containing higher concentrations of L. monocytogenes (those positive by a direct-plating method) were independently associated with an increased likelihood of containing the patient-matching strain (7).
Reported by: G Anderson, MPH, Alameda County Health Dept; Contra Costa County Health Dept; San Francisco Dept of Public Health; L Mascola, MD, Los Angeles County Dept of Health Svcs; GW Rutherford, MD, State Epidemiologist, California Dept of Health Svcs. MS Rados, Vanderbilt Univ School of Medicine, Nashville; R Hutcheson, MD, State Epidemiologist, Tennessee Dept of Health and Environment. P Archer, P Zenker, MD, State Epidemiologist, Oklahoma State Dept of Health. C Harvey, MPH, Emory Univ, Atlanta; JD Smith, Georgia Dept of Human Resources. Meningitis and Special Pathogens Br, Div of Bacterial and Mycotic Diseases, National Center for Infectious Diseases, CDC.
Editorial Note: Listeriosis is a rare but serious illness in the United States. Although the potential for epidemic foodborne transmission of L. monocytogenes was first documented in 1981 (1), recent studies indicate that a substantial portion of sporadic listeriosis is foodborne (6,7) and associated with consumption of nonreheated hot dogs (8), undercooked chicken (6,8), various soft cheeses (6), and food purchased from store delicatessen counters (6).
Although contaminated food has been a major cause of both epidemic and sporadic listeriosis, most persons are at low risk for listeriosis. Persons at increased risk for listeriosis (i.e., pregnant women, the elderly, and those with immunosuppressive conditions) can decrease their risk by avoiding consumption of certain foods and following food-handling practices that also may help prevent other foodborne illnesses (see box).
Early recognition of Listeria infection, especially in pregnant women, is important to assure prompt treatment and to limit adverse outcomes. Although physicians usually practice increased diagnostic vigilance in caring for severely immunocompromised patients, pregnant women may not be routinely considered at risk for invasive bacterial disease.
Diagnosis of listeriosis is best made by routine bacterial culture of specimens from usually sterile sites such as blood or CSF. Stool culture is not reliable because many persons have enteric colonization with L. monocytogenes without invasive disease. Serologic testing is not useful in diagnosing listeriosis. Health-care providers should therefore 1) consider listeriosis in ill patients at risk for the disease, 2) obtain blood cultures and, when appropriate, CSF or amniotic cultures from ill patients at risk for listeriosis, including pregnant women with fever, 3) disseminate dietary recommendations to high-risk persons, and 4) report all cases of listeriosis to state health departments. The continued active surveillance for listeriosis in several states will assist evaluation of the impact of prevention strategies.
Additional information about listeriosis (including consumer information designed for distribution to patients) is available from CDC’s Meningitis and Special Pathogens Branch, Division of Bacterial and Mycotic Diseases, National Center for Infectious Diseases, Mailstop C-09, 1600 Clifton Road, NE, Atlanta, GA 30333.