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Listeriosis Outbreak: Labs Pinpoint Source of Illness

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November 17, 2011

An outbreak of cantaloupe-borne listeriosis that began in late August and expanded to 28 states is the deadliest foodborne outbreak in the United States in over 25 years. The Centers for Disease Control (CDC) and public health laboratories have coordinated throughout the outbreak to identify the source and limit the number of cases.

To date (November 1, 2011), the outbreak has caused 139 infections, 29 deaths, and one miscarriage (For the latest statistics, see the CDC web site.). The outbreak was traced to Rocky Ford-brand cantaloupe grown at Jensen Farms in Colorado. It is the first listeriosis outbreak associated with melons. Earlier this month, the CDC announced that the root cause of the cantaloupe contamination had been traced to unsanitary packing facilities at Jensen Farms.

Although the initial cases in this outbreak first came to light in August, there may be more cases yet to be reported. Listeria is different from other foodborne pathogens because there can be a long incubation period between eating Listeria-contaminated food and developing illness. This incubation period can last from 3 to 70 days but is usually 3 weeks. Unlike some other bacteria, Listeria is able to survive and grow on cantaloupes regardless of whether the fruit is stored at room or cold (refrigerator) temperatures.

Infection with the bacteria Listeria monocytogenes is called listeriosis. The bacteria are commonly found in soil and water, and people can become infected by consuming contaminated food. Most people who are infected experience mild gastrointestinal illness, but the infection can be invasive, with the bacteria spreading from the intestines to the bloodstream or other parts of the body. Symptoms may include fever and muscle aches, sometimes preceeded by diarrhea and other gastrointestinal symptoms. Headache, stiff neck, confusion and convulsions are also possible. The infection can be serious for the very young, older adults, and people who are immunocompromised, leading to septicemia, meningitis or even death. Infection in pregnant women may lead to miscarriage or stillbirth.

Unlike some other foodborne illnesses, listeriosis is not tested using a stool culture because the test lacks sensitivity, requires special media, and is only indicated for unusual situations. Rather, infections are confirmed through clinical laboratory testing that identifies the bacteria in a blood culture, CSF analysis, or culture of amniotic fluid. Any Listeria bacteria isolated from these cultures are sent to public health laboratories for further studies.

To track the source of the recent outbreak, Colorado state health officials performed laboratory testing of cantaloupes from the homes of ill people and in grocery stores. In public health laboratories, the bacteria isolated from those food samples were matched to the bacteria found in clinical specimens from ill patients using a molecular test that "fingerprints" DNA. The technique, pulsed-field gel electrophoresis (PFGE), allows laboratories to rapidly identify common sources of foodborne illness by comparing the molecular patterns from microorganisms isolated by laboratory testing.

In foodborne illness outbreaks, public health laboratories enter their PFGE results into a database called PulseNet. The database is used by local and state public health agencies, federal food safety regulatory laboratories, and the CDC to compare DNA fingerprints and quickly identify if illnesses have a common source. In the recent listeriosis outbreak, four different strains of Listeria monocytogenes were identified. Product traceback information from Colorado state officials identified Jensen Farms' Rocky Ford-brand cantaloupes as the common food eaten by several listeriosis patients.

After several cantaloupe recalls since the beginning of the outbreak in August, the CDC expects that all the contaminated cantaloupes have been removed from the market place. However, they continue to recommend that consumers not eat cantaloupe with stickers identifying Rocky Ford-brand cantaloupe from Jensen Farm and to throw away cantaloupe of unknown origin.

The CDC provides steps to take if you suspect that you may have or have had contaminated cantaloupe in your home. If you have eaten cantaloupe but have no symptoms and are not ill, there is no reason to be tested. However, people with symptoms of listeriosis should immediately consult a health care provider, especially if they are in the high risk group of people over age 50, pregnant women, or the immunocompromised and have eaten cantaloupe in past 2 months.

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Listeria monocytogenes. ARUP Consult. Available online at through Accessed October 2011.

Multistate Outbreak of Listeriosis Linked to Whole Cantaloupes from Jensen Farms, Colorado. Centers for Disease Control (October 25, 2011). Available online at through Accessed October 2011.

Centers for Disease Control. Investigation Update: Multistate Outbreak of Listeriosis Linked to Whole Cantaloupes from Jensen Farms, Colorado (October 27, 2011). Available online at through Accessed October 2011.

State of Colorado. Frequently Asked Questions About the Listeria Outbreak Connected to Cantaloupe (Updated September 29, 2011). Available online at through Accessed October 2011.

Listeriosis (Listeria infection). Centers for Disease Control (February 9, 2011). Available online at through Accessed October 2011.

Deadly Listeria Outbreak Claims more Lives. John Gever. MedPage Today (September 23, 2011). Available online at through Accessed October 2011.

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FDA confirms Listeria monocytogenes on Jensen Farms’ Rocky Ford-brand cantaloupes. U.S. Food and Drug Administration (September 19 2011). Available online at through Accessed October 2011.

Results Of The FDA-Led Root Cause Investigation Of The Multi-State Listeria Outbreak Related To Jensen Farms Cantaloupe Moderator: Douglas Karas (October 19, 2011). PDF available for download at through Accessed October 2011.

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