Listeria monocytogenes outbreak in Colorado 2011


On September 2, the Centers for Disease Control and Prevention (CDC) was notified of seven cases of listeriosis in Colorado. Nine days later, a total of 15 cases, including one death, were reported from 4 states. All infected persons were hospitalized, and the majority were over 60 years old. The number of cases continued to rise and spread to other states in the following weeks. The multistate investigation revealed that cantaloupes grown at Jensen Farms and marketed as “Rocky Ford” were the source of the outbreak. By the end of the outbreak, there were 147 confirmed cases across 28 states, including 33 deaths and one related miscarriage (Figure 1).

Figure 1: Number of people infected with L. monocytogenes during the 2011 outbreak. Source: Center for Disease Control and Prevention, Case Count Maps (2012).


Listeriosis is an uncommon but serious infection caused by Listeria monocytogenes. Listeria is ubiquitous in the environment and is transmitted through the consumption of contaminated food products. Outbreaks are most often associated with raw produce, deli meats, and unpasteurized cheese. The elderly, immunocompromised individuals, pregnant women, and newborns are at high risk for an invasive type of infection when the bacterium enters the bloodstream. Neonatal infections may also occur if the bacterium penetrates the placental barrier. Severe symptoms include sepsis, encephalitis, and meningitis. In healthy individuals, listeriosis causes fever and diarrhea that is usually self-limiting.

Source of the outbreak

Whole cantaloupes grown at Jensen Farms in Colorado were identified to be the source of the outbreak. Among patients with information on food consumption, 93% reported eating cantaloupes in the month before the outbreak. The following traceback investigations indicated that the contaminated cantaloupes all came from Jensen Farms and were shipped to at least 24 states between July and September. Moreover, five Listeria subtypes were isolated from Jensen Farms’ packing facility. Subtyping is a powerful method to investigate bacterial transmission by looking at similarities between different isolates of the same bacterial species. In other words, if bacteria isolated from different samples are of the same subtype, it is more likely that they come from a common source. Indeed, the five subtypes from Jensen Farms corresponded to the isolates from the infected persons, the leftover cantaloupes in their homes, and the retail locations where they purchased the melons. Surprisingly, no outbreak-associated subtypes were found in the soil samples collected from where the cantaloupes were grown. The suspected sources of the outbreak were a truck that routinely travels between the processing facility and a nearby cattle farm, low-level contamination of incoming cantaloupes, and persisting bacteria on the processing equipment.

Since the outbreak was linked to whole cantaloupes rather than pre-cut products, there must be substantial numbers of L. monocytogenes that survive and persist on the rind of the fruit. Indeed, follow-up research indicates that the outbreak-associated strains are able to survive and grow on the rind, which somehow supports the growth of Listeria better than the flesh or extracts. This could be explained by the intricate netting surface that promotes bacterial adherence and proliferation. For the pathogens to enter human bodies, they may be introduced to the inside of the fruit where they multiply quickly due to sufficient nutrients and optimum pH levels (6.1-7.1). 

Cause of the outbreak

Although the exact cause of contamination remains unknown, the FDA investigation report reveals several problems that might have led to the outbreak (Figure 2). In particular, post-harvest processing and sanitation practices in the packing facility were of major concern. To start with, the cooling and drainage system was poorly designed, which allowed water to collect near the processing area and the employee walkways. The facility floor and packing equipment were also difficult to clean, which gave Listeria a chance to persist in biofilms. Additionally, Jensen Farms upgraded their hydro cooler about a year before the outbreak, as recommended by the auditor. However, the old equipment was replaced by a second-handed machine for processing potatoes, not cantaloupes. Not only was the equipment not suitable for decontaminating melons, but the washing solution also contained no antimicrobial compound of any kind. Finally, the cantaloupes were not pre-cooled to optimal temperature before cold storage. Consequently, energy stored in the fruits may raise the temperature of the storage unit, thus promoting Listeria growth on the rind.



Figure 2: Possible causes of the outbreak. Source: Yuan Zhuang (2021).

Measurement taken to end the outbreak

The public health authorities responded immediately as soon as the first few cases had been reported. The FDA worked closely with the CDC, the firms involved, and public health authorities in states where illnesses occurred to determine the exact source of contamination. Initial interviews of infected persons suggested that the outbreak was most likely associated with eating “Rocky Ford” cantaloupes. This hypothesis was tested and confirmed by laboratory analysis of samples taken at different locations. In response, Jensen Farms issued a voluntary recall of all their whole cantaloupes. The processing facility was also temporarily shut down to prevent additional food contamination. Furthermore, several companies in neighbouring states initiated recalls of products containing cantaloupes produced by Jensen Farms. Meanwhile, the CDC issued a warming that people at high risk for listeriosis, including older adults, persons with weakened immune systems, and pregnant women, should avoid eating cantaloupes from the Jensen Farms.


According to the FDA, the outbreak could have been prevented if Jensen Farms had adhered to FDA guidelines during operation. However, the agency also recognized that the complex procedures in the Food Safety Modernization Act (FSMA), which was signed into law in January 2011, could be difficult to implement in real-life practice. Therefore, after the outbreak, the FDA proposed two rules that address problems found at Jensen Farms. The first rule aims to prevent microbial contamination through better sanitation controls. The second rule emphasizes the development of a hazard-reduction plan that includes four aspects: identification of potential food contamination, actions needed to prevent them, monitoring procedures, and steps required to correct the problem.


Centers for Disease Control and Prevention. (2011, November 2). Clinical Features/Signs and Symptoms. Retrieved November 15, 2021, from

Centers for Disease Control and Prevention. (2012, August 27). Multistate Outbreak of Listeriosis Linked to Whole Cantaloupes from Jensen Farms, Colorado. Retrieved November 15, 2021, from

House Committee on Energy and Commerce. (2012, January 10). Report on the Investigation of the Outbreak of Listeria monocytogenes in Cantaloupe at Jense Farms.

Johnston, L. M., Jaykus, L. A., Moll, D., Martinez, M. C., Anciso, J., Mora, B., & Moe, C. L. (2005). A field study of the microbiological quality of fresh produce. Journal of food protection68(9), 1840–1847. doi: 10.4315/0362-028x-68.9.1840

Martinez, M. R., Osborne, J., Jayeola, V. O., Katic, V., & Kathariou, S. (2016). Capacity of Listeria monocytogenes Strains from the 2011 Cantaloupe Outbreak To Adhere, Survive, and Grow on Cantaloupe. Journal of food protection79(5), 757–763. doi: 10.4315/0362-028X.JFP-15-498

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Sandora, T. J., Gerner-Smidt, P., & McAdam, A. J. (2014). What’s your subtype? The epidemiologic utility of bacterial whole-genome sequencing. Clinical chemistry, 60(4), 586-588. doi:10.1373/clinchem.2013.217141

Schlech, W. F., III, & Acheson, D. (2000). Foodborne Listeriosis. Clinical Infectious Diseases, 31(3), 770–775. doi: 10.1086/314008

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