A dirty packing facility at a Colorado farm was the likely source of an outbreak of listeria in cantaloupes that killed 25 people in the US since August, health authorities said Wednesday.
The foodborne outbreak was the deadliest in a quarter century in the United States, and also sickened 123 people and caused one pregnant woman to miscarry, the Centers for Disease Control and Prevention said.
Although the investigation is ongoing into how the whole melons became contaminated, the Food and Drug Administration has ruled out the soil, water, or animal feces in the cantaloupe field as potential sources of the listeria.
Instead, a packing facility that was difficult to clean, pooled water on the ground and a truck parked nearby that may have been infected with listeria from cattle feces were all named as potential sources for the problem.
"The firm was packing the cantaloupes under unsanitary conditions under which they could become adulterated," said Sherri McGarry, senior advisor at the FDA's Office of Foods.
The source of the outbreak was traced to family-owned Jensen Farms, which issued a recall of its cantaloupe on September 14. None were shipped outside the United States.
"There is no reason to believe that these factors are indicative of practices throughout the industry. I would say that they are fairly unique," McGarry told reporters.
The FDA pointed to several "factors as those that most likely contributed to the introduction, spread, and growth of Listeria monocytogenes in the cantaloupes," it said in a statement.
"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," it said.
An examination of the packing facility showed that there was pooled water on the floor near the equipment and employee walkways.
"The packing facility floor was constructed in a manner that made it difficult to clean," the FDA statement said.
"The packing equipment was not easily cleaned and sanitized; washing and drying equipment used for cantaloupe packing was previously used for postharvest handling of another raw agricultural commodity."
Some of the equipment had previously been used for potatoes, and while listeria can originate in rotting vegetables and animal feces, concerns about a listeria outbreak in potatoes are unfounded because they are usually cooked during preparation, experts said.
Melons were not allowed to pre-cool from the hot field after picking, a process that may have allowed condensation to form once they went into cold storage "that promoted the growth of Listeria monocytogenes," the FDA added.
Four distinct strains of listeria monocytogenes -- a bacterium that causes listeriosis and is among the most virulent foodborne pathogens -- have been traced to the Colorado farm.
The FDA sent the farm a warning letter saying authorities "may take further action if you do not promptly correct these violations."
Listeriosis is particularly dangerous to the elderly, those with weakened immune systems and pregnant women because it can cause miscarriage or stillbirth.
Listeriosis can cause diarrhea, fever, muscle aches and other flu-like symptoms. In most people, the bacteria spreads from the intestine to the bloodstream, but it can be treated with antibiotics.
The last major deadly US outbreak of listeria was in 1985 when the bacteria was found in Mexican-style cheese in southern California, causing 29 deaths, the CDC said.
"This is the deadliest food-borne outbreak in the United States in more than 25 years," said Barbara Mahon, deputy chief of Foodborne and Diarrheal Diseases Branch at the Centers for Disease Control and Prevention.
The peak in illness came in late August to mid-September, and although all the infected melons are believed to be off the market, the outbreak is not yet finished because it can take a person up to two months to get sick.
"Because of the long incubation period of listeria -- that is the time between a person is exposed and when they get ill -- it is too soon to declare the outbreak over," said Mahon, adding that monitoring would continue for two more weeks.