Oct. 19, 2011 -- Results of an investigation by state and federal health officials have shed light on how cantaloupes grown by a Colorado farm became contaminated with deadly listeria bacteria.
Investigators say dirty, corroded equipment, unsanitary conditions, and unsafe food handling practices at a packing facility used by Jensen Farms likely allowed listeria bacteria to grow and spread to melons that were washed and refrigerated there.
No listeria was found in the fields where the cantaloupes were grown.
Tainted "Rocky Ford" cantaloupes from Jensen Farms have thus far sickened at least 123 people and killed 25 across 26 states. One miscarriage has also been attributed to the contaminated melons.
"This is the deadliest food-borne outbreak in the United States in more than 25 years," says Barbara Mahon, MD, deputy branch chief of enteric diseases at the CDC. She spoke at a news briefing to announce early results of an official investigation into the cause of the outbreak.
Problems identified by investigators include:
- Water that pooled and sat under packing equipment.
- Processing equipment that was difficult to clean and sanitize.
- Refrigeration practices that likely allowed condensation to form on the melons. Experts say condensation is an ideal environment for listeria growth.
Additionally, packing equipment installed at the facility in July had been previously used to wash and pack potatoes. Officials say they aren't worried about a similar listeria outbreak in potatoes since those vegetables are rarely eaten raw.
A truck parked next to the packing area also made frequent trips to a nearby cattle farm. Listeriosis -- the illness caused by listeria bacteria -- is a common infection in livestock like cattle, sheep, and goats. The bacteria they shed can live in soil, manure, and grass.
Testing of Samples
FDA inspectors collected 39 environmental samples at the packing facility used by Jensen Farms on Sept. 10, 2011. Tests showed that 13 of those, including some taken from refrigerated melons and some from food contact surfaces, were positive for strains of listeria bacteria that were involved in the outbreak.
Conditions at the packing facility were not typical in the produce packing industry.
"I've been to a lot of produce handling facilities and again, the key issues were sanitary facility design, sanitary equipment design, and post-harvest handling. None of these were typical for a typical post-harvest handling operation of any fruit or vegetable," says James Gorny, PhD, a senior advisor for produce safety at the FDA's Center for Food Safety and Applied Nutrition in College Park, Md.
The FDA has issued a warning letter ordering the owners of Jensen Farms to correct the problems at the open-air packing facility. Officials say they are also weighing other sanctions against the farm.
The peak in reported illnesses appears to have passed. The number of new cases is now decreasing.
"Because of the long incubation period of listeria -- that's the time between when the person is exposed and when they get ill -- it's too soon to declare the outbreak over," Mahon says, "We'll need to monitor for at least another two weeks."
The rapid investigation and cooperation between state and federal officials was credited for preventing more cases and deaths.
A consumer alert about cantaloupes was issued within 10 days after the outbreak was first reported to federal officials.
"Listeria outbreak investigations can often take a month or longer, so this is a real success," Mahon says.
FDA officials say the investigation into how the contamination happened will continue so that future outbreaks can be prevented.
FDA Commissioner Margaret Hamburg, MD, says that though the response to the outbreak was "quick and effective," there are still many lessons yet to be learned.
"The tragic deaths and illnesses from this outbreak have again demonstrated the need to continually address and improve the food safety system," Hamburg says.