Investigators found Listeria monocytogenes matching an outbreak strain in cantaloupe at a Denver-area store, and on equipment and cantaloupe at the Jensen Farms packing facility in Colorado, the U.S. Food and Drug Administration said Monday.

Four deaths among 35 infections have now been confirmed in 10 states in the cantaloupe-caused Listeria outbreak, according to the Centers for Disease Control and Prevention’s latest update. Those numbers are likely to climb; tests are pending for several more listeriosis deaths and illnesses suspected to be part of the outbreak.

Jensen Farms, located in the Holly-Granada area on the east end of Colorado’s Arkansas River, announced that it was recalling its cantaloupe on September 14 because of possible Listeria contamination.  It now appears, however, that both major retailers and Jensen, with its Edinburg, TX-based distributor Frontera Produce, began the recall two days earlier as soon as Colorado officials issued the first public warning about Rocky Ford-area cantaloupes.

All Jensen Farms cantaloupes shipped between July 29 and September 10 are included in the recall.

Amy Gates, Frontera’s executive vice president, told The Packer, the produce industry’s news service, that they shut down operations, stopped the harvest, packing and distribution — even calling back trucks on the road — on September 12.  

At least 10 of the 17 or more states where Jensen Farms Rocky Ford-brand cantaloupes were distributed have reported outbreak-linked listeriosis cases.  According to the CDC, Colorado has confirmed 12 illnesses, Oklahoma six, Nebraska four, Texas three and California, Illinois, Indiana, Montana and West Virginia each have reported one case.

Of 28 case patients with available information, all had symptoms so severe they required hospitalization. 

The four deaths officially connected to the outbreak were two people from New Mexico and patients from Colorado and Oklahoma. New Mexico is investigating two more deaths, and Missouri one death, that may also be connected to the outbreak.  

The federal “root-cause” investigation and environmental assessment includes on-site FDA and state of Colorado microbiologists, environmental health specialists, veterinarians and investigative officers, according to the agency.

“It is very unusual to find leftover product testing positive for the outbreak strain of a bacteria,” noted Bill Marler, food safety attorney and publisher of Food Safety News. “Finding it in cantaloupe and processing equipment makes in impossible for Jensen Farms and Frontera to argue otherwise.

“The local and state health departments, the CDC and FDA did their jobs. Now is the grim task of counting the sick and dead.”

The FDA said on-scene experts will continue to analyze the evidence to try to determine  how the contamination occurred and, if possible, to identify controls to help prevent contamination in the future. The agency said the investigation’s findings should help regulators develop Listeria policy and produce safety guidelines.

This is the first outbreak of Listeria involving cantaloupe. In the U.S., Listeria was found in sprouts in 2009 and in celery in 2010. Canada saw a Listeria outbreak linked to raw cabbage in 1983. But Listeria, which thrives in damp places and can be a persistent problem if it takes hold in processing plants, is still rarely found in fresh produce. More common carriers are hot dogs, deli meats, unpasteurized milk and soft cheeses.

Although the suspect Jensen Farms cantaloupes have been recalled, they were widely distributed and may still be in some stores or homes, the public health regulators said.

“Even if some of the cantaloupe has been eaten without (consumers) becoming ill, dispose of the rest of the cantaloupe immediately. Listeria bacteria can grow in the cantaloupe at room and refrigerator temperatures,” the CDC cautioned.

The FDA advises anyone in a high risk group — older adults, pregnant women, and those with compromised immune systems — to be especially sure to avoid the recalled cantaloupe.  Listeriosis risk increases with age, and the deadly bacteria can pass through to a fetus to cause stillbirth. Listeria is one of the most lethal foodborne pathogens.

“Do not try to wash the harmful bacteria off the cantaloupe as contamination may be both on the inside and outside of the cantaloupe. Cutting, slicing and dicing may also transfer harmful bacteria from the fruit’s surface to the fruit’s flesh,” the FDA said.

Although the state and federal warnings about Listeria-tainted cantaloupe from Colorado initially referred to melons from the Rocky Ford area, Colorado’s chief medical officer, Chris Urbina, has since said that the matching strains taken from Jensen Farms cantaloupe indicate “it’s the only farm” connected to the outbreak.

Rocky Ford cantaloupe are popular because the hot days and cool nights experienced along Colorado’s Arkansas River produce a sweeter tasting fruit, causing its demand over such a large area of the United States. Grown for more than 100 years, Rocky Ford cantaloupe were once delivered far from the growing area by Santa Fe’s fast passenger trains.

Outbreak map from the Centers for Disease Control and Prevention

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Epi Curve: Persons infected with the outbreak strain of Listeria monocytogenes, by date of illness onset, as of September 19. Illnesses that occurred after August 28 might not be reported yet.  — Centers for Disease Control and Prevention