Recently, after a long day at the hospital, I stopped at the grocery store to pick up something for a quick dinner. I drifted to the frozen food case in the organic food section, but pulled up short when I saw empty shelves. A paper sign announced that Amy’s Kitchen, a manufacturer of organic and natural frozen foods, had become the latest company to recall its products because of concern about Listeria monocytogenes contamination.
According to information posted on the Food and Drug Administration website, this facultative, anaerobic gram-positive bacillus has been the impetus behind 10 national recalls of food products between April 1 and May 8, 2015 alone. Implicated food products have ranged from gourmet ice cream to soybean sprouts to frozen vegetables. Unlike some other bacterial causes of food-borne illness, Listeria organisms can thrive at cold temperatures. Historically, outbreaks of disease have been linked to a variety of foods, including raw produce, contaminated ready-to-eat foods such as deli meats and prepared salads, and unpasteurized milk and milk products.
Clinical manifestations of listeriosis range from febrile gastroenteritis to bacteremia and meningitis, with severe disease seen primarily in immunocompromised individuals and adults 65 and older.
Pregnant women are especially susceptible, with incidence rates 13 times higher than in the general population. Probably as a result of food choices, Hispanic women are disproportionately affected, with rates up to 24 times higher. Maternal infection may be asymptomatic or may manifest with flulike symptoms that include fever, myalgias, headache, and backache, with or without a preceding diarrhea illness. Even mild maternal illness may result in adverse pregnancy outcomes such as fetal loss, premature labor, and severe neonatal infection.
While medical students and residents are still taught to think of Listeria infection as one of the “big three” causes of neonatal sepsis along with group B streptococcus and Escherichia coli, many pediatricians have never seen a case of this rare, but potentially devastating disease. As with group B streptococcus, both early-onset and late-onset disease occur. Sepsis is the most common presentation of disease in the first week of life, while meningitis predominates in late-onset disease. Pneumonia and myocarditis are occasionally seen. Granulomatosis infantisepticum is an uncommon manifestation of severe, disseminated Listeria infection. Granuloma can occur in nearly every organ, although involvement of the liver and skin is most common.
In 2002, investigators from the Centers for Disease Control and Prevention and the American College of Obstetricians and Gynecologists surveyed more than 400 pregnant women from across the United States about their knowledge of the transmission, risk factors, symptoms, and prevention of listeriosis (Infect. Dis. Obstet. Gyn. 2005;13:11-15). A year later, the Minnesota Department of Health surveyed an additional 286 pregnant women from their state using the same survey instrument.
More than 80% of survey respondents had never heard of the disease, and knowledge about prevention strategies was therefore predictably limited. Only 33% of respondents in the national survey and 17% of respondents in the Minnesota survey knew, for example, that infection could be prevented by avoiding delicatessen meats and soft cheeses. Investigators concluded that “timely and appropriate education” of pregnant women about listeriosis could reduce cases of perinatal infection.
Data from the CDC suggest we have more work to do. The Listeria Initiative is an enhanced national surveillance system that collects laboratory, clinical, and food exposure data about listeriosis cases in the United States. Between 2009 and 2011, 14% of the 1,651 invasive Listeria infections reported were classified as pregnancy associated. Morbidity and mortality were significant, with 40 fetal losses and 6 neonatal deaths (MMWR 2013;62:448-52).
The CDC offers some common sense tips for preventing listeriosis and other food-borne illness. Raw fruits and vegetables should be thoroughly rinsed with tap water and dried with a clean cloth or paper towel before being eaten or cooked. Even foods that are typically peeled first should be washed, and firm produce, such as cantaloupe, should be scrubbed with a produce brush to reduce surface contamination. Uncooked meats and poultry should never come in contact with other food. Hands, knives, cutting boards, and other food preparation surfaces should be washed thoroughly after uncooked food is handled.
Pregnant women and others at increased risk for listeriosis should not eat hot dogs or deli meats unless they are cooked to steaming. Soft cheeses, including feta, brie, Camembert, queso blanco, or anything blue veined, should be avoided unless the label clearly states that the product has been made with pasteurized milk. Even then, it might not be safe. Pasteurized Mexican-style cheeses, such as queso fresco, have been linked to Listeria infections, likely as a result of contamination during the cheese-making process.