Multistate Outbreak of Listeriosis associated with cantaloupe
Abstract & Commentary
By Dean L. Winslow, MD, Clinical Professor of Medicine and Pediatrics Division of Infectious Diseases and Geographic Medicine, Stanford University School of Medicine, Associate Editor of Infectious Disease Alert. Dr. Winslow is a consultant for Siemens Diagnostic.
SYNOPSIS: A total of 147 outbreak-related cases of listeriosis were identified in 28 states. Epidemiologic study confirmed that whole cantaloupe produced by a single Colorado farm was the outbreak source.
Source: McCollum JT, et al. Multistate outbreak of listeriosis associated with cantaloupe. New Eng Jrl Med 2013;369:944-53.
During 2011 the Centers for Disease Control and Prevention and Colorado Department of Public Health and Environment (CDPHE) investigated a nation-wide listeriosis outbreak. Multistate epidemiologic, trace-back, and environmental investigations were conducted, and outbreak-related cases (as shown by pulse-field gel electrophoresis) were compared to previously-reported sporadic cases of listeriosis identified by the Listeria Initiative (an enhanced surveillance system that routinely collects detailed information about U.S. cases of listeriosis).
147 outbreak-related cases were identified in 28 states. 86% of patients were 60 years of age or older (median age 77). Seven infections occurred in pregnant women (4) and newborns (3) and one related miscarriage was reported. 88% of patients had one or more potentially immunosuppressing conditions. 99% of patients were hospitalized and 22% died. 84% of patients had positive blood cultures and 5% of patients had positive CSF cultures.
Patients with outbreak-related illness were more likely than sporadic listeriosis cases to have eaten cantaloupe (odds ratio 8.5). Cantaloupe and environmental samples collected during the investigation yielded isolates matching the five outbreak-related subtypes. Unsanitary conditions identified at the processing facility operated by a farm in Colorado likely resulted in contamination of the cantaloupes with L. monocytogenes.
Listeria monocytogenes is an interesting organism, unusual in that it can continue to grow in cold conditions (in contrast to most bacteria which grow optimally at warm temperatures). Listeria is a facultative intracellular bacterium and generally causes disease in elderly individuals, pregnant women, neonates, and individuals with defects in cellular immunity. Because it is so unusual even a few cases should prompt an investigation into the source of infection.
Many outbreaks of Listeria have occurred over the years and have been related to ingestion of things like cabbage stored over the winter in cellars, in Mexican-style cheese (queso fresco), other unpasteurized dairy products, hot dogs, ham and cold cuts. Contamination generally is felt to occur during harvesting or processing of food and then storage at refrigerator temperatures actually results in amplification of the original inoculum of Listeria.
In this particular outbreak, the investigation revealed that the implicated farm had recently switched from a cantaloupe cleaning process that used recirculating, chlorinated, chilled-water wash to one where they used non-recirculated municipal water without chlorination and a series of brush and felt rollers to mechanically clean and dry the cantaloupe.