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IFSAC Releases Foodborne Illness Source Attribution Estimates for 2021

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The model estimated percentage of foodborne Salmonella, Escherichia coli O157, and Listeria
monocytogenes illnesses in 2021 attributed to each of 17 food source categories was recently released by the Interagency Food Safety Analytics Collaboration (IFSAC) made up of members from CDC, FDA, and USDA-FSIS.

The IFSAC group was established in 2011 to improve coordination of federal food safety analytic efforts and address priorities for food safety data collection, analysis, and use. IFSAC analyzes foodborne illness outbreak data for priority pathogens and specific foods and food categories responsible for foodborne illnesses in the United States. The data are analyzed by calendar
year and released in annual reports as part of ongoing efforts to understand sources of foodborne illness in the United States. The CDC estimates that together these priority pathogens — Salmonella, E. coli O157, Campylobacter, and L. monocytogenes — cause nearly two million cases of foodborne illnesses in the U.S. each year.

Foodborne illness source attribution estimates were generated from surveillance data collected between 1998 through 2021. The dataset included 1,322 outbreaks in which the confirmed or suspected implicated food or foods could be assigned to a single food category: 987 caused or suspected to be caused by Salmonella, 275 by E. coli O157, and 60 by L. monocytogenes. These include 46 outbreaks caused by
multiple serotypes of Salmonella. IFSAC assessed which categories of foods were most responsible for Salmonella, E. coli O157, and L. monocytogenes infections. These pathogens were chosen because of the frequency or severity of the illnesses they cause, and because targeted interventions can have a major impact in reducing them. The implicated foods were divided into 17 categories for the analysis. The method used for estimation gave the greatest weight to the most recent five years of outbreak data (2016–2020). The top food categories associated with each pathogen are detailed below.

For Salmonella, over 75% of illnesses were attributed to seven food categories including chicken, fruits, pork, seeded vegetables (such as tomatoes), other produce (such nuts), beef, and turkey. Estimated Salmonella illnesses were more evenly distributed across food categories than illnesses from E. coli O157, and L. monocytogenes.

Over 80% of E. coli O157 illnesses were attributed to vegetable row crops, such as leafy greens (lettuce, spinach), celeries, broccoli, and beef. Vegetable row crops had a significantly higher estimated attribution percentage than all other categories followed by beef.

For L. monocytogenes, over 75% of illnesses were attributed to dairy (fluid milk, hard and soft cheese), fruits (melons, apples, cherries, berries, mangoes, avocados), and vegetable row crops. However, the small total number of outbreaks (60) in the data set caused this estimate to be less reliable than estimates for the other pathogens.


The attribution of Salmonella illnesses to multiple food categories suggests that multiple types of
interventions are required to reduce illnesses from these pathogens. In contrast, the majority of E. coli O157 illnesses were attributed to two food categories suggesting that interventions for E. coli O157 focusing on these two food categories may be most effective in reducing illnesses. Most
L. monocytogenes illnesses were attributed to three food categories implicated in outbreaks in recent years.


Campylobacter attribution estimates were not provided. This was due to concerns about the limitations of using outbreak data that attributes Campylobacter illnesses to foods that are not routinely consumed by the general public. For example, 90% of dairy outbreaks involved raw milk and 55% of chicken outbreaks involved chicken livers, neither of which are readily consumed by the general public. As such, IFSAC analysts are developing other methods to estimate the sources of Campylobacter infection in future publications.

The authors advise that the estimates provided should not be interpreted as suggesting that all foods in a category are equally likely to transmit pathogens and comparisons over years can be skewed by a limited number of outbreaks. These results should be used with other scientific data for decision making. Overall, the attribution estimates can help inform efforts to prioritize food safety initiatives, interventions, and policies for reducing foodborne illnesses. The estimates also allow stakeholders (i.e. scientists; federal, state, and local policy-makers; the food industry; consumer advocacy groups; and the public) to assess whether prevention-oriented measures are working at intended.

Salmonella Contaminated Dry Pet Food Causes Outbreak and Nationwide Recall

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An outbreak caused by Salmonella Kiambu has been linked to dry pet food products distributed nationally. Seven cases of illness affecting mainly children aged one year or less have been reported from multiple states since January 2023. Retail product collected for routine testing by the South Carolina State Department of Agriculture was found positive for Salmonella and determined to be closely related to the outbreak strain utilizing whole genome sequencing. This finding led to a nationwide recall of several brands of dry kibble produced at the manufacturing facility with a best by date before 10/31/2024. While outbreaks associated with dry kibble products are infrequent, this new outbreak highlights the challenges associated with dry pet food production and the persistence of Salmonella in low moisture products.

Salmonella is readily found in the ingredients (e.g. rendered animal byproducts and grains) used to make pet food kibble. However, thermal extrusion processes used to make kibble are generally lethal to this organism. Liquid water and/or steam is added to the dry ingredients that are pushed through the barrel of an extruder under intense heat and pressure before exiting through a shaping die. The extrusion process is often validated as a process preventive control to reduce vegetative pathogens by a minimum of 5 logs. Product is then dried resulting in a shelf stable product with a water activity (aw) <0.60.

The low aw prevents pathogen outgrowth but not pathogen persistence if reintroduced after the extrusion step. Product can be re-contaminated from the environment or from the addition of ingredients such as fat and flavorings that are sprayed on the dried kibble to enhance palatability. Post-extrusion contamination likely led to the last major Salmonella outbreak involving kibble that occurred between 2006-2007. During the investigation of this previous outbreak, the same Salmonella serotype was found in the pet food manufacturing environment as well as in two of the 25 brands of pet food produced at the facility. The only positive environmental sample was found in the enrobing/flavoring room where product was coated with a palatant after extrusion. Because the extrusion process utilized time/temperature parameters validated to kill Salmonella, it was thought that cross-contamination of the extruded kibble occurred from contaminated ingredients or environmental contamination during the enrobing/flavoring process.

Supply chain controls can mitigate hazards associated with ingredients added after the extrusion process. Hygiene and sanitation controls can mitigate hazards introduced post extrusion when verified with a robust environmental monitoring program. Pathogen testing should include line samples and environmental samples including air filters, equipment, vents, and drains. Guidance is available from FDA for the safe production of pet food. Deibel Laboratories can provide full service solutions from environmental assessments of facilities to finished product testing for pathogens and indicators as well as validation of nutritional labels for pet foods.

The FDA Releases 2022 Cantaloupe Outbreak Investigation Report

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By Ryan Maus

In August 2022, a CDC epidemiological investigation and traceback data identified a multistate cluster of Salmonella Typhimurium illnesses in the upper Midwest indicating melons as a potential source of exposures. The outbreak was traced to cantaloupe and resulted in eighty-seven illnesses and thirty-two hospitalizations across eleven states. The following describes FDA’s investigational findings and provides their recommendations to growers of melons and similar produce.

The FDA utilized traceback data to identify a common packinghouse of suspect cantaloupes associated with the outbreak. However, there was no convergence to a single shipment of products. Three farms in southern Indiana that supplied the common packinghouse were identified as potential sources of cantaloupe. Around the growing locations other commodities were grown such as grain, oilseed, and beans; interspersed with various vegetable crops, including melons; and several poultry feeding operations. Heavy rains had also occurred in the growing region in late July resulting in floods.

Sampling of the packing house, the three farms, and the surrounding lands resulted in numerous findings of Salmonella, but only one of the farms produced an isolate genetically related to the outbreak strain (i.e., having an intermediate level of differences in genomes but not a genetic match). The results suggest that Salmonella presence is a reoccurring issue that may impact the safety of melons grown in this region.

FDA provides the following recommendations/requirements to producers of melons and similar produce.

  • Review current conditions and practices to determine whether they are adequate or if additional
  • prevention measures are warranted.
  • Understand previous land use to identify and address potential sources of pathogens that may
    affect their farming operations.
  • Assess risks that may be posed by adjacent and nearby land uses, especially as it relates to the
    presence of livestock, poultry, and the interface between farmland, and other agricultural areas.
  • Consider additional tools such as pre-harvest and/or post-harvest sampling and testing of
    products to help inform the need for specific prevention measures.
  • Poultry manure, while valued for its fertilizer value, is a known reservoir for Salmonella
    spp. Proper application of a manure that has been treated with a validated and verified process
    to reduce pathogens (e.g., composting with time and temperature measurements) can
    significantly reduce the potential for the integration of Salmonella or other human pathogens into
    soils (as compared to the use of raw manures).
  • Inspect, maintain, and clean and, when necessary and appropriate, sanitize all food contact
    surfaces of equipment and tools used as frequently as necessary to protect against contamination.
  • When appropriate, use EPA-approved products according to the label for cleaning and sanitizing.
  • Inconsistent adherence to or deviation from existing SOPs for cleaning and sanitizing by farms can
    affect produce safety. Effective communication on farms about SOPs and any changes to those
    SOPs can help ensure that food safety practices are being followed.
  • Root cause analyses may be useful in identifying for growers how human pathogen sources in the
    broader agricultural environment may contribute to contamination.
  • Improve traceability through increased digitization, interoperability, and standardization of
    traceability records which would expedite traceback and help remove contaminated product from
    the marketplace more quickly, thereby preventing further illnesses. This is not only important for
    growers, but also critical for shippers, manufactures, and retailers as well, to improve overall
    traceability throughout the supply chain.

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