He said there has not been a direct link of a Listeria illness or death associated with a federally inspected meat plant in the U.S. since 2003. He said FSIS regulatory testing for Listeria monocytogenes in ready-to-eat meat and poultry products has found decreasing percentages of Lm positives from 1990 to 2015 – dropping from 4.5% in tests in 1990 to 0.5% in 2015.

“The process control for Listeria starts with looking for it,” Mr. Butts said. “Starting with dry cleaning, rinsing, then foaming and rinsing, including looking for growth niches. Then there is sanitizing, and checking effectiveness. Then we’re into production, spelled by breaks to verify control of Listeria during the production cycle.

“There are two types of sampling. There’s aggressive process control sampling, and we can find positives with process control sampling. And if the positives are verified, that indicates process control failure.”

Butts explained how a “seek-and-destroy” investigation is carried out. It begins with a normal cleaning and sanitation process. Assembly of equipment is observed, followed by a post-assembly sanitizer application. Next is normal setup and startup activities, and the operation is stopped before product is placed on the line. Equipment is disassembled and any remaining machine components are disassembled. Then all disassembled line components are cleaned and flooded or heat-sanitized.

“These questions are asked,” Mr. Butts said. “Are all components being adequately sanitized (chemical or heat)? Are GMPs being followed? After disassembling, any suspect areas are inspected and swabbed. Evidence of any unacceptable organic buildup is looked for. Is disassembly and cleaning acceptable? If the aerobic plate count shows the suspect area is a growth niche for Listeria and if it is found, then the suspect area is a harborage site.

“Best practices include having clean, dry floors with no cracks in plants; one tool or no tool equipment disassembly; cleaning ‘out-of-place’ for small parts, equipment subassemblies and hand tools; the importance of cleaning critical air handling systems; cleaning interstitial spaces above processing areas, where mechanical systems often are kept; the physical separation of raw and ready-to-eat products; and the importance of ‘decluttering’ meat and poultry plants."