“It is impossible to reconstruct the exact events that triggered this outbreak,” concedes Paul S. Mead, M.D., M.P.H., staff epidemiologist, Division of Bacterial and Mycotic Diseases for the CDC.
What government officials previously described as a ceiling-mounted air conditioning unit is identified now as a ceiling-mounted refrigeration unit in the report. But the ongoing theory regarding construction dust is formalized and explained.
Mead notes, “The available data suggest that the outbreak strain [of listeria] was present in the retail frank production area prior to July 1998, and that construction to remove the hopper room refrigeration unit on the July 4 weekend resulted in a new or increased contamination of production equipment.”
Earlier presenceThe presence of listeria bacteria in the plant as early as April 1998 is a new revelation of the report. But it is believed to have been a low level.
“Apparently, the level or frequency of contamination of hot dogs packaged before July 1998 was insufficient to cause a detectable outbreak,” remarks Mead.
The notable event is when “The retail frank area was closed July 3-5, 1998, while a large refrigeration unit was removed from the ceiling of the retail frank hopper room. Because of its size, this refrigeration unit had to be cut into pieces to remove it from the building. During removal, the pieces were transported through various corridors.”
It is then that airborne dust from these pieces, contaminated with Listeria monocytogenes (LM), could have been spread through the plant.
The report states that, for the six-week period just before the removal work, 25% of retail hot dog samples tested positive for psychrophilic organisms. For the six weeks immediately after the construction, 92% of the samples tested positive, and for the nine weeks after that, 67%.
However, “The results of this culturing are nonspecific and do not prove the presence of LM.” They serve as an indicator that LM may be present.
Operation of the refrigeration unit is not claimed to have caused any contamination.
While Mead does not consider the low-level April contamination at the plant to have triggered the outbreak, he does state later in the report that “Considerable morbidity and mortality can occur even when the level of contamination measured is very low.”
And he does state that any level of contamination on food is unacceptable. “Preventing such outbreaks will depend on preventing LM from being present on ready-to-eat meats in any amount.”
Further measuresWhile a focus on sanitation has reduced listeria problems in the past, Mead continues,
“This outbreak demonstrates that further measures are needed.” Recommendations are provided for processed meat producers, the U.S. Department of Agriculture (USDA), and the CDC. These include the application of use-by dates (instead of the current sell-by dates) for ready-to-eat products.
Mead also advocates that plants use heat or irradiation after packaging to enhance food safety.
He asks the USDA to “determine if regulatory testing of freshly made product adequately assesses the potential for LM contamination.”