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Investigation Announcement: Multistate Outbreak of Listeriosis Linked to Rocky Ford Cantaloupes

September 12, 2011

Introduction

CDC is collaborating with public health officials in several states, including Colorado, and the U.S. Food and Drug Administration (FDA) to investigate a multistate outbreak of listeriosis. Listeriosis is a serious infection usually caused by eating food contaminated with the bacterium Listeria monocytogenes. Investigators are using DNA analysis of Listeria isolated from patients to identify cases of illness that may be part of this outbreak. The Listeria bacteria are obtained from diagnostic testing; pulsed-field gel electrophoresis (PFGE) is used to determine DNA fingerprint patterns. Investigators are using data from PulseNet, the national subtyping network made up of state and local public health laboratories and federal food regulatory laboratories that performs molecular surveillance of foodborne infections. 

A total of 15 persons infected with the outbreak strain of Listeria monocytogenes have been reported from 4 states. All illnesses started on or after August 15, 2011. The number of infected persons identified in each state is as follows:  Colorado (11), Nebraska (1), Oklahoma (1), and Texas (2). Listeriosis illnesses in several other states are currently being investigated by state and local health departments to determine if these illnesses are part of this outbreak.

Among persons for whom information is available, illnesses began on or after August 15, 2011. Ages range from 38 to 96 years, with a median age of 84 years old. Most ill persons are over 60 years old or have health conditions that weaken the immune system. Seventy-three percent of ill persons are female. All 15 (100%) patients were hospitalized, and one death has been reported.

The outbreak can be visually described with a chart showing the number of persons who became ill each day. This chart is called an epidemic curve or epi curve. Illnesses that occurred after August 26, 2011, might not be reported yet due to the time it takes between when a person becomes ill and when the illness is reported. Please see the description of the steps in a foodborne outbreak investigation for more details.

About 800 cases of Listeria infection are diagnosed each year in the United States, along with 3 or 4 outbreaks of Listeria-associated foodborne illness. The typical foods that cause these outbreaks have been deli meats, hot dogs, and Mexican-style soft cheeses made with unpasteurized milk. Produce is not often identified as a source, but sprouts caused an outbreak in 2009, and celery caused an outbreak in 2010.

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Investigation of the Outbreak

Ongoing collaborative investigations by local, state, and federal public health and regulatory agencies indicate the likely source of the outbreak is a type of cantaloupe, called Rocky Ford cantaloupes, which are grown in the Rocky Ford region of southeastern Colorado. These cantaloupes were harvested in August and September, distributed widely in the United States, and are currently available in grocery stores.  Ill persons were interviewed about exposures during the month before becoming ill; investigators compared their responses to persons with listeriosis reported through the CDC Listeria Initiative, whose illnesses were not part of this outbreak. Preliminary results strongly suggest that illnesses are linked to consumption of cantaloupes.  Several ill persons who remembered the type of cantaloupe said they were Rocky Ford cantaloupes.  Product traceback information indicated these cantaloupes were marketed as cantaloupes harvested in the Rocky Ford region.

Laboratory testing by the Colorado Department of Public Health and Environment identified Listeria monocytogenes bacteria on cantaloupe collected from grocery stores and from an ill person’s home. Product traceback information from Colorado State officials indicated these cantaloupes were harvested in the Rocky Ford region.  FDA is working closely with CDC, the firms involved, and public health authorities in states where illnesses occurred to determine the exact source of contamination.

On September 9, the Colorado Department of Public Health and Environment [PDF - 2 pages] advised persons in Colorado at high risk for severe listeriosis to avoid eating cantaloupes.  CDC now advises persons throughout the mainland United States and at high risk for listeriosis, including older adults, persons with weakened immune systems, and pregnant women, to not eat cantaloupes marketed as coming from the Rocky Ford region of Colorado.

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Clinical Features/Signs and Symptoms

Listeriosis is a serious infection caused by eating food contaminated with the bacterium Listeria monocytogenes. The disease primarily affects older adults, persons with weakened immune systems, pregnant women and newborns.  Rarely, persons without these risk factors can also be affected.

A person with listeriosis usually has fever and muscle aches, often preceded by diarrhea or other gastrointestinal symptoms. Almost everyone who is diagnosed with listeriosis has "invasive" infection, in which the bacteria spread from the intestines to the blood stream or other body sites.

The symptoms vary with the infected person:

  • Persons other than pregnant women: Symptoms, in addition to fever and muscle aches, can include headache, stiff neck, confusion, loss of balance, and convulsions.
  • Pregnant women: Pregnant women typically experience only a mild, flu-like illness. However, infections during pregnancy can lead to miscarriage, stillbirth, premature delivery, or life-threatening infection of the newborn.

More general information about Listeriosis can be found at the CDC's Listeriosis webpage.

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Advice to Consumers

Contaminated cantaloupes may still be in grocery stores and in consumers' homes.

  • CDC recommends that persons at high risk for listeriosis, including older adults, persons with weakened immune systems, and pregnant women, do not eat cantaloupes marketed as coming from the Rocky Ford region of Colorado.
  • Consumers who have cantaloupes in their homes can check the label or inquire at the store where they purchased it to determine if the fruit was marketed as coming from the Rocky Ford region of Colorado.  
  • Listeriosis primarily affects older adults, persons with weakened immune systems, pregnant women, and newborns. Persons who think they might have become ill from eating possibly contaminated cantaloupes should consult their doctor immediately.
  • Cantaloupes marketed as coming from the Rocky Ford region should be disposed of in a closed plastic bag placed in a sealed trash can. This will prevent people or animals from eating them.

Food items other than cantaloupes can also carry Listeria bacteria. People at high risk for listeriosis and those who prepare their meals can take steps to lower the risk.

  • Rinse raw produce, such as fruits and vegetables, thoroughly under running tap water before eating. Dry the produce with a clean cloth or paper towel before cutting them up.
  • Thoroughly cook raw meat and poultry.
  • Heat hot dogs, deli meats, and cold cuts until they are steaming hot just before serving.
  • Do not drink raw (unpasteurized) milk and do not eat fresh soft cheeses that have unpasteurized milk in them, especially Mexican style cheeses like queso fresco.
  • Be sure that your refrigerator is at or below 40 degrees F, and your freezer at or below 0 degrees F by using a refrigerator thermometer.
  • Follow general food safety guidelines for preparing food, such as those at FoodSafety.gov.

General Melon Safety Advice:

  • Consumers and food preparers should wash their hands before and after handling any whole melon, such as cantaloupe, watermelon, or honeydew.
  • Wash the melons and dry them with a clean cloth or paper towel before cutting.
  • Cut melon should be promptly consumed or refrigerated at or less than 40 degrees F (32-34 degrees F is optimal for storage of cut melon).
  • Cut melons left at room temperature for more than 4 hours should be discarded.

More information about Listeriosis and recommendations to reduce risk of getting Listeriosis from food can be found at the CDC's Listeriosis webpage.

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CDC's Role in Food Safety

CDC leads federal efforts to gather data on foodborne illnesses, investigate foodborne outbreaks and monitor the effectiveness of prevention and control efforts. CDC is not a food safety regulatory agency but works closely with the food safety regulatory agencies, in particular, with FDA and U.S. Department of Agriculture's Food Safety and Inspection Service. CDC also plays a key role in building state and local health department epidemiology, laboratory and environmental health capacity to support foodborne disease surveillance and outbreak response. Notably, CDC data can be used to help document the effectiveness of regulatory interventions.

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