While there was a lot of speculation as to what caused the Listeria contamination of Jensen Farms’ cantaloupe harvest, FDA’s recent testing has mostly ruled out gross contamination in the farm fields as the problem. Environmental testing on cantaloupes remaining in the field tested negative for Listeria monocytogenes. FDA, however, found conclusively that equipment, flooring, standing water and drains in the packing shed were contaminated with the same strains of Listeria that infected consumers who became sickened and/or died.

Some consumer watchdog groups and blogs were quick to point out close-by CAFOs (concentrated animal feeding operations) were responsible for the Listeria contamination either through runoff or other means of transport (e.g., using waste as fertilizer). The animal population, according to Food and Water Watch’s 2007 numbers, in Prowers County, CO was calculated to be 44,017 cattle and 137,766 hogs. The human population in 2010 was 12,551, according to the US Census Bureau.

Listeria monocytogenes are not the first bacteria that come to mind if a contamination is due to animal causes, said Dana Johnson in an FE interview. Johnson is brewery and produce specialist at Birko Corp., a Henderson, CO company specializing in germicidal chemicals for the food and beverage industry. For example, an E. coli O157:H7 contamination would likely originate from cattle. Birko was called in to consult on the use of proper chemical treatments to prevent Listeria contaminations. Unfortunately, part of the problem is that HACCP food safety procedures are not yet mandatory for the produce industry, says Johnson.

While FDA did not rule out that Listeria may have originated from the fields, its mode of entry to the packing plant was not on the cantaloupes, according to Johnson. Johnson recommends foaming entryways to any packing facility or food plant because very low residual levels of Listeria can be found most anywhere in the world on the ground, and they can enter a facility on workers’ shoes. Johnson’s company has many years of experience in the protein industry, and he thinks a multiple hurdle approach to pathogen reduction should be followed in the produce industry just as it is in the meat industry.

Once Listeria monocytogenes bacteria enter a cool, damp environment, they thrive and don’t have a lot of competition from other bacteria that prefer warmer temperatures to multiply, says Johnson. Another potential mode of entry, according to FDA, was a truck used to transport culled cantaloupes to livestock farms where the melons were used for feed.

In an exclusive FE interview, Mary Root, Prowers County land use coordinator, pointed out the nearest location of any feed yards is at least three miles from the fields or the packing facility located in Grenada, and on the other side of the road. The farm fields are located a few miles away from the packing facility, and cantaloupes are transported via truck between the farm and packing facility. In addition, Root reports the locality has been under drought conditions all summer and into the fall, so there was no runoff nor was there flooding from the Arkansas River.

On September 10, 2011, FDA and Colorado state officials conducted environmental tests of the growing environment and the packing facility. Samples were collected in the growing fields because known reservoirs of Listeria include ruminant animals and decaying vegetation. These samples included soil, wild animal excreta, perimeter and furrow drag swabs, agricultural water, pond water and cantaloupes. All environmental samples collected in the growing fields tested negative for Listeria monocytogenes.

Certain aspects of the packing facility, including the location of a refrigeration unit drain line, allowed water to pool on the floor in areas adjacent to packing facility equipment, according to FDA. Wet environments are known to be potential reservoirs for Listeria, and the pooling of water in close proximity to packing equipment, including conveyors, may have extended and spread the pathogen to food contact surfaces.

Of the 39 environmental samples collected from within the facility, 13 tested positive for Listeria with pulsed-field gel electrophoresis (PFGE) pattern combinations that were indistinguishable from three of the four outbreak strains collected from affected patients, according to FDA. Cantaloupes collected from the producer’s cold storage during the inspection were also confirmed positive for Listeria with PFGE pattern combinations indistinguishable from two of the four outbreak strains.

As a result of the isolation of three of the four outbreak strains of Listeria monocytogenes in the environment of the packing facility and whole cantaloupes collected from cold storage, and the fact this is the first documented Listeriosis outbreak associated with fresh, whole cantaloupes in the US, FDA is making several recommendations for good practices. Producers should:

• Assess produce facility and equipment design to ensure adequately cleanable surfaces and eliminate opportunities for the introduction, growth and spread of Listeria monocytogenes and other pathogens. 
• Assess and minimize opportunities for introduction of Listeria monocytogenes and other pathogens in packing facilities. 
• Implement cleaning and sanitizing procedures.
• Verify the efficacy of cleaning and sanitizing procedures.
• Periodically evaluate the processes and equipment used in packing facilities to assure they do not contribute to fresh produce contamination. 

For more information, read FDA’s Environmental Assessment: Factors Potentially Contributing to the Contamination of Fresh Whole Cantaloupe Implicated in a Multi-State Outbreak of Listeriosis.