Multistate Listeria Outbreak
An overview of this virulent foodborne pathogen
By Vishnu V. Chundi, MD
Listeria monocytogenes is the bacterium that caused the recent multistate outbreak associated with consumption of ice cream produced by Blue Bell Creameries. This is just one in a long list of outbreaks associated with this bacterium. Currently there are ten cases in four states (Kansas, Texas, Oklahoma, and Arizona). The current multistate outbreak has been ongoing for several years.
Listeria causes over 2,500 illnesses a year in the U.S. with an overall mortality of ~ 17%. Outbreaks have been associated with vegetables like cabbage, fruit such as melons, meats such as cold cuts, milk products such as ice cream, and prepared foods including hummus. The bacteria grows well in salt and in colder conditions, thus allowing propagation even in cured meats and during refrigeration. There are many types of listeria but Listeria monocytogenes and, rarely, Listeria ivanovii, are pathogenic to humans. Healthy animals harbor the bacteria in their intestinal tracts. Vegetables may become contaminated through soil or manure. Unpasteurized milk products are a primary source of infections. Foods contaminated with Listeria look, smell and taste normal.
Individuals at high risk include pregnant women, who have a 20 times greater risk than healthy adults. Patients with decreased cell-mediated immunity due to hematological malignancy, solid organ or bone marrow transplantation, end-stage renal disease, diabetes, radiation therapy, or treatment with immunosuppressing medications such as steroids, anti-rejection medications, or cancer chemotherapy, are at greater risk. In fact, AIDS patients have about 300 times greater risk than those individuals with normal immune status. Neonates may become infected by their mothers.
Symptoms of infection vary from mild gastroenteritis to muscle aches and fever to diarrhea. More severe cases may present with meningitis, bacteremia and sepsis. Pregnant woman may present with fever, diarrhea, miscarriage, stillbirth, or premature delivery. Neonates may present with sepsis. Diagnosis requires a high index of suspicion. Performing a culture of the blood, cerebrospinal fluid, meconium of newborns and vaginal secretions of pregnant women will confirm infection. Ampicillin is the antibiotic of choice. Listeria is naturally resistant to cephalosporins.
Listeria outbreaks are a consequence of industrialization, of meat, milk and food production. Refrigeration and cold enhancement used in the distribution of food products are a major source of potential infections despite modern techniques. Prevention of listeriosis includes thoroughly cooking food, washing raw vegetables with clean water before eating, keeping uncooked meats separate from produce and other ready-to-eat items and avoiding consumption of unpasteurized milk.
The FDA has enacted mandatory measures for food production facilities and this has resulted in decreased contamination of meat products. Milk products, however, have not had a similar decline in infections. Physicians should be aware of the risk factors, common presenting symptoms, and signs of this deadly yet treatable infection.
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