Category Archives: listeria

Colorado Cantaloupe Listeria

Outbreak: Colorado Cantaloupe Listeria 
Product: Jensen Farms (Colorado) “Rocky Ford” cantaloupeInvestigation Start Date: 8/29/2011
Location: Multi-stateEtiology: Listeria monocytogenes
Earliest known case onset date: 7/31/2011Latest case onset date: 10/27/2011
Confirmed Case Count: 147Positive Environmental/Food Samples: 12 / 22
Hospitalizations: 143Deaths: 33

In 2011 a multistate outbreak of Listeria monocytogenes infected 147 people, including 143 hospitalizations and 33 deaths. Illnesses were associated with Colorado cantaloupes marketed as “Rocky Ford” grown at Jensen Farms.


On August 29th, 2011, two cases of Listeria monocytogenes were reported to the Colorado Department of Public Health and Environment (CDPHE). These were in addition to a L. monocytogenes case that had been reported in mid-August, bringing the total number of cases for August to three, which was unusual. By the end of August, a total of 8 cases were reported with three distinct PFGE patterns. Initial interviews using the Centers for Disease Control and Prevention (CDC) “Listeria Initiative” questionnaire implicated cantaloupe, ice cream, coleslaw, and deli meats as potential common sources of illness. While cantaloupe had not previously been associated with L. monocytogenes outbreaks, it had been added to the questionnaire after a 2000-02 FoodNet case-control study identified an increased association between illness and cantaloupe consumption.
CDPHE notified CDC of the outbreak on September 1st. CDC provided CDPHE with data from the “Listeria Initiative” database to conduct case-case studies to compare cases from the current Colorado outbreak with previously reported sporadic listeriosis cases. Because there were multiple PFGE patterns among the outbreak cases, investigators were initially unsure whether cases represented a single or multiple outbreaks.
By September 6th, there were 12 listeriosis cases and suspected food vehicles included cantaloupe, watermelon, and ham. Later the same day, the CDC informed CDPHE that Nebraska and Texas had reported L. monocytogenes cases with PFGE patterns that were indistinguishable from the Colorado cases.

From September 4th through 7th, epidemiologists collected various food items from patients’ houses for testing and the CDPHE laboratory purchased cantaloupe from three grocery stores. Only cantaloupe with an identifying produce sticker attached were purchased, as grocery store bins of cantaloupe labeled “Colorado Grown” often contained produce from multiple Colorado growers and occasionally other states. Preliminary laboratory results demonstrated that all cantaloupe from one grocery store tested positive for L. monocytogenes. On September 9th, results from the “Listeria Initiative” case-case study indicated only cantaloupe was statistically associated with illnesses. A media release warned consumers that those at high risk for L. monocytogenes infection should avoid eating cantaloupe. CDPHE informed Colorado cantaloupe growers of their concerns and arranged farm visits.
Investigators first visited Jensen Farms on September 10th. September is the end of the cantaloupe growing season in Colorado, and Jensen Farms was the only local farm still producing cantaloupes. The investigation team noted the farm changed their handling processes earlier in the year. They replaced a chlorinated water wash with new equipment that sprayed cantaloupes with municipal water and used a series of felt rollers and brushes for cleaning and drying. At the end of this process, cantaloupes were packed in boxes and refrigerated. This new process lacked a pre-cooling step and therefore did not remove “field heat” from the produce, which allowed condensation to form on the cantaloupe once boxed and refrigerated. Additionally, the new processing equipment could not be dissembled and disinfected. Investigators hypothesized L. monocytogenes colonized the equipment and was subsequently sprayed onto all processed cantaloupe. The suboptimal storage conditions further allowed the organism to multiply.

Ultimately, five PFGE patterns were identified in patient and environmental samples. Of these, all five patterns were identified from cantaloupes obtained from patient homes, four patterns were identified from cantaloupes at retail locations, three patterns were identified from environmental swabs taken at the processing facility, and two patterns were identified from cantaloupes sampled directly from the Jensen Farm’s cooler. While the initial source of contamination was never determined, all contamination of produce occurred downstream of the processing equipment. It is possible that the equipment, which was refurbished from a potato farm, was previously contaminated. Alternatively, the cantaloupes may have been contaminated prior to processing and the pathogen was amplified by the equipment. However, testing of growing fields were negative and examination of growing methods provided no evidence of their contribution to the contamination. It is also possible that contamination entered the facility via a truck that routinely carried agricultural waste between the farm and a nearby cattle ranch. The incidence of polyclonal L. monocytogenes outbreak-related strains indicates multiple niche sites, extensive and multiple contamination sources, or repeated introductions within the processing facility.
In total, there were 147 cases (40 in Colorado) in 28 states and 33 deaths (9 in Colorado). Cases were primarily older adults and very few pregnant women, as compared to other L. monocytogenes outbreaks. Investigators hypothesized pregnant women consumed cantaloupe more quickly than older adults, thereby preventing further multiplication of L. monocytogenes in their home refrigerators.

Lessons Learned

Public health increasingly identifies fresh produce as a vehicle for foodborne illness and novel pathogen-vehicle combinations. Jensen Farms’ introduction of new processing equipment and failure to follow FDA guidance about safe melon handling likely promoted L. monocytogenes contamination and colonization. In this outbreak, rapid collection of environmental specimens, coupled with the “Listeria Initiative” that facilitated rapid case-case comparisons between outbreak-related and sporadic cases, allowed for rapid identification of a food vehicle, and prompted swift intervention measures.

Listeria Soft Cheese

Outbreak: Listeria Soft Cheese 
Product: Soft CheeseInvestigation Start Date: 06/27/2013
Location: Multi-StateEtiology: Listeria monocytogenes
Earliest known case onset date: 5/14/2013Latest case onset date: 6/16/2013
Confirmed / Presumptive Case Count: 5 / 0Positive Samples (Food / Environmental): 18 / 1

Five outbreak cases of listeriosis were identified in Minnesota, Indiana, Illinois, and Ohio. The use of the CDC Listeria Initiative questionnaire to identify a soft cheese signal, along with re-interviewing cases (or their proxies) and calling restaurants to get specific details on cheese brand and cheese type were critical in identifying the outbreak vehicle. Environmental isolates in the PulseNet national database that were collected from the Crave Brothers cheese production plant (and indistinguishable from outbreak case isolates) were also a key supporting clue. These clues led to the testing of Crave Brothers cheese samples and detection of the outbreak stain in this product.

This investigation resulted in perhaps the fastest identification of a commercially distributed food vehicle of a listeriosis outbreak; the cluster was detected in Minnesota on June 27 and public health interventions were implemented on July 3 (6 days later). This was a remarkable accomplishment, especially with only 5 cases to work with. The rapidity of the investigation can be attributed primarily to the urgency displayed by the lead investigator and the aggressive acquisition of details on brand and type of soft cheese consumed, in collaboration with epidemiologists in the other affected states.

The MN CoE has created a detailed case study about this investigation that can be used for training public health practitioners in outbreak investigation. Public Health students love it too!


A multi-state outbreak of listerosis associated with soft cheese from Crave Brothers.


On June 27, 2013, the Minnesota Department of Health (MDH) Public Health Laboratory (PHL) determined that two clinical Listeria monocytogenes (LM) isolates submitted through routine surveillance had indistinguishable PFGE patterns. A review of the national PulseNet database revealed three additional LM isolates with the outbreak PFGE pattern in Indiana, Illinois, and Ohio. A multi-state investigation was initiated.


Five cases from three states were ultimately identified in this outbreak (Minnesota, 2; Indiana, 1; Illinois, 1; and Ohio, 1). In addition, subsequent whole genome sequencing efforts identified another likely case in Texas, but exposure information was not available. Four (80%) cases were female and the median age of the cases was 55 years (range, 31 to 67 years). All five cases were hospitalized and one died. Listeria Initiative case report forms were completed for all five cases and all five reported consuming a variety of soft cheeses. Potato salad was reported by both Minnesota cases; however, no other cases reported consuming it. Epidemiologists conducted a case-case comparison study to compare outbreak case food consumption rates against estimated background consumption rates using non-outbreak associated LM cases. LM cases with the outbreak PFGE pattern were significantly more likely than sporadic LM cases to have consumed any soft cheese (odds ratio, 16.8; 95% confidence interval, 1.2–∞) and yogurt (odds ratio, 7.5; 95% confidence interval, 1.3–∞). Review of the initial interviews revealed that cases reported different brands of yogurt, making this an unlikely source of illness.

The Minnesota cases were re-interviewed to get additional specific product details on the soft cheese exposures. The first Minnesota case reported consuming a cheese plate at a restaurant in North Dakota. The investigator called the restaurant which reported that the cheese plate included Crave Brothers Les Frères cheese. The Illinois case reported consuming Crave Brothers Les Frères cheese purchased from a grocery store. The Indiana case reported consuming a cheese plate at a restaurant, and a review for the restaurant menu indicated that Crave Brothers Les Frères was served on the cheese plate. The Ohio case was re-interviewed and also reported consuming a cheese plate at a restaurant which included Crave Brothers Petit Frère cheese. PulseNet identified non-human isolates in PulseNet with the outbreak PFGE pattern that had been collected from the Crave Brothers cheese production facility in 2010 and 2011.

On July 1, Crave Brothers cheese was collected for testing from two locations of the grocery chain where MN case #2 had purchased the product, and submitted to Minnesota Department of Agriculture (MDA) for LM testing. In addition, leftover blue cheese, Irish cheddar, parmesan cheese, and an unknown hard cheese were collected for testing from MN case #2’s home. By July 3, Enzyme-linked Fluorescent Assay results indicated that a wedge of Crave Brothers Petite Frère with Truffles from one grocery store location in Minnesota, two wedges of Crave Brothers Les Frères from another location of the same grocery store chain in Minnesota, and a wedge of blue cheese collected from MN Case #2’s home all tested positive for LM. A traceback of the Crave Brothers cheese consumed by the cases revealed that the cheeses had different distributors, which indicated that the source of the contamination was the Crave Brothers plant.
A press release was issued warning consumers not to eat Crave Brothers Les Frères, Petit Frère, and Petit Frère with truffles and Crave Brothers issued a voluntary nationwide recall of these cheeses.
The spouse of the second Minnesota case called investigators to report that he thought they had purchased Crave Brothers Petit Frère. He did not have a receipt but had used a credit card to make the purchase. The investigator contacted the grocery store and gave them the credit card transaction numbers, the date of the transaction, and the dollar amount of the transaction. Using this information the store was able to reprint the original receipt with included Crave Brothers Petit Frère.


This was a multi-state outbreak of LM infections associated with Crave Brothers cheese. The use of the Listeria initiative questionnaire along with re-interviewing cases (or their proxies) and calling restaurants to get specific details on cheese brand/type were critical in identifying the outbreak vehicle.
Environmental isolates in the PulseNet national database that were collected from the Crave Brothers cheese production plant were also a key supporting clue. This investigation resulted in perhaps the fastest identification of a commercially distributed food vehicle of a listeriosis outbreak.