Multiyear Outbreaks – When Unmitigated Pathogen Hazards Persist

Whole genome sequencing (WGS) is a powerful tool used to identify related illnesses occurring over long periods of time. This technology is also used in outbreak traceback investigations to link contaminated products and processing environments to past illnesses. Most recently, reports of illness from Listeria monocytogenes have been linked to Hispanic-style cheese in an ongoing outbreak with illness association going as far back as 2014. This article takes a look at how regulatory officials attribute illness to a specific food and provides examples of multiyear outbreaks resulting from unmitigated pathogen hazards.

When illnesses occur, healthcare officials are usually first to collect data from patients that can eventually be used to identify outbreaks. The CDC, FDA, and USDA, along with state and local health officials, monitor this data to help identify outbreaks. They also collect and analyze three types of information to identify the food attributed to an outbreak: epidemiological information, laboratory analyses of food and/or samples taken from food production environments, and traceback investigation findings. While WGS plays an important role, the FDA will not publicly name a specific product until there is sufficient evidence from all the information available to implicate a product as a cause of illnesses or adverse event.

When a specific food has been attributed to an outbreak, past illnesses epidemiologically linked to that food type can be matched to the outbreak when WGS indicates a pathogen strain match. Some of the past multiyear outbreaks are shown in the following table.

In all the multiyear outbreaks, the outbreak strain could be found in the implicated food and its production environment. The strain was then linked with WGS to past incidents of illnesses caused by the same outbreak strain that identified the implicated food product during epidemiological investigations. Unmitigated pathogen hazards found in the production environment are described in each case. The 2010-2015 ice cream outbreak investigation found harborage of the L. monocytogenes strain in drains that likely spread throughout the plant and into the manufacturing equipment. The 2010-2015 soft cheeses outbreak found the L. monocytogenes strain in environmental samples collected from the manufacturing facility in 2015 and 2010. The 2005-2017 infant formula S. Agona strain was found on non-food contact surfaces in the plant, possibly re-released into the environment during construction in 2017. However, multiple cases involving the same S. Agona strain were reported in the time between the 2005 and 2017 outbreak. The 2017-2019 hard-boiled egg outbreak strain of L. monocytogenes was found in environmental samples collected at the firm’s processing facility during inspections conducted in February 2019 and December 2019. The strain of L. monocytogenes associated with the 2018-2023 peaches, plums, and nectarines outbreak was found in environmental samples collected from the packaging facility.

Environmental pathogens can be an issue for ready-to-eat food producers. FDA indicates that surveillance programs should be able to detect the presence of pathogens such as L. monocytogenes when present in the environment. The finding of these organisms is to be expected, however, documented corrective and preventive actions that eliminate harborage sites must follow with ongoing verification that the pathogen contamination was actually mitigated.

WGS plays an important role in identifying the source of multiyear outbreaks, preventing future illness, and saving lives. As investigators continue to collect environmental samples, more pathogen strains will be added to their database likely resulting in quicker illness/food source attribution. This makes it increasingly important to have appropriate preventive controls in place that are verified to be effective as part of a food safety plan.