America’s most deadly incidence of foodborne illness in a century was 2011’s second-most important food safety story — the outbreak of listeriosis linked to whole cantaloupes from Jensen Farms, CO, that spread over 28 states, infecting 146 mostly elderly Americans.


In its final report on the outbreak, issued Dec. 8, the Centers for Disease Control and Prevention (CDC) said 30 had died and one pregnant woman had miscarried. To find a larger outbreak of foodborne illness, one has to go back to 1911 Boston when streptococcus from raw milk infected more than 2,000 and killed 48.

Ten days after the final CDC report, 92-year old Paul Schwarz died in Kansas City after battling listeriosis for weeks. His son said the illness was devastating — as if his father was fighting brain trauma.

Colorado accounts for only one to two percent of cantaloupe production in the U.S. The state has only a couple dozen cantaloupe growers ini a few square miles; its cantaloupe fields are strung along the Arkansas River that flows out of Colorado’s Rocky Mountains east to the flatness of Kansas.

Since the 1880s, that melon-growing region has been known for Rocky Ford cantaloupes, prized for their sweetness, which is said to be the result of the area’s hot days and cool nights.

When the 2011 growing season got underway, two adult brothers, Eric and Ryan Jensen were in charge at Jensen Farms for the first time since their father died. With a packing facility in Granada, CO and offices 12 miles away in Holly, just three miles from where the Arkansas River enters Kansas, Jensen Farms sold cantaloupes under the Rocky Ford brand.

The farm obtained a clean bill from third-party auditor Primus Labs and signed up Edinburg, TX-based Frontera Produce to distribute its cantaloupes to at least 24 states.

The first sign of trouble came on Sept. 2.  The Colorado Department of Public Health and Environment, based in Denver, issued a news alert saying seven new cases of Listeria had been reported in the previous five days, for a total of nine cases in August. Normally, Colorado sees no more than one case of Listeria a month.

Over the weekend, as the case count mounted, epidemiological evidence from the patients’ food histories indicated that cantaloupes were the prime suspect. On Monday, Sept. 12, Colorado’s chief medical officer said: “People at high risk for Listeria infection should not eat cantaloupe from the Rocky Ford growing region.”

Grocery stores began removing Rocky Ford-labeled cantaloupes from their produce departments.

Also on Sept. 12, the CDC issued its first report on what had become a multistate outbreak investigation. It was about to get worse, much worse.

Forty-eight hours later, CDPDE had the evidence.

Testing by Colorado public health labs identified Listeria monocytogenes bacteria on cantaloupes collected from grocery stores and from an ill person’s home.  Traceback work by the state led to Jensen Farms.

On Sept. 14 Jensen Farms recalled 300,000 cases of cantaloupes —  somewhere between 1.5 to 4.5 million individual melons — that had been shipped to at least 24 states from July 29 to Sept. 10. 

As more states reported illnesses linked to the outbreak, investigators said four different strains of Listeria was involved. Nearly everyone sickened was also hospitalized, and the death toll kept climbing.

State and federal investigators did most of their work at Jensen Farms during September. By the time lab results were in and inspection reports were concluded, was well into October.   

The U.S. Food and Drug Administration, CDC and their state public health partners said 13 of 39 environmental swabs collected from inside the Jensen Farms packing house were confirmed positive for Listeria monocytogenes with pulsed-field gel electrophoresis (PFGE) pattern combinations that were indistinguishable from three of the four outbreak strains.  

Twelve of the positive samples were from the processing line and one was from the packing area.

Cantaloupes collected from the firm’s cold storage during the inspection were also confirmed positive for Listeria monocytogenes with PFGE pattern combinations that were indistinguishable from two of the four outbreak strains.

While no Listeria was found in the fields, the evidence from the packing house equipment much amounted to the proverbial “smoking gun.”

The FDA said there could have been low level sporadic Listeria monocytogenes in the field   where the cantaloupe were grown, which could have been introduced into the packing facility.

The agency also noted that a truck used to haul culled cantaloupe to a cattle operation was parked adjacent to the packing facility, and could have introduced contamination into the facility.

Inspectors also observed that the packing facility’s design allowed water to pool on the floor near equipment and employee walkways, and that the packing facility floor was constructed in a manner that made it difficult to clean.

Likewise, the packing equipment was not easily cleaned and sanitized and that washing and drying equipment used for cantaloupe packing was previously used for postharvest handling of potatoes.

Jensen Farms was also faulted for having no pre-cooling step to remove field heat from the cantaloupes before cold storage. As the cantaloupes cooled, condensation may have promoted the growth of Listeria monocytogenes, the FDA noted.

CDC director Dr. Thomas Frieden said this major outbreak could have been much worse if not for the fast response by public health investigators to identify contaminated cantaloupes as the source of the epidemic.

“I will highlight that this is an example of a very rapid response from the state of Colorado,” he wrote.  “They identified in the first week a large number of listeria cases.  They immediately began an epidemiologic investigation of those cases and informed ourselves and the FDA.  They worked with doctors in the community to identify this so that when CDC was first notified of the suspected outbreak the Friday before Labor Day weekend on September 2, we were able to work through the weekend with the state.”