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Outbreaks Where Food Workers Have Been Implicated in the Spread of Foodborne Disease. Part 3. Factors Contributing to Outbreaks and Description of Outbreak Categories

As Originally published by Journal of Food Protection,, February 2007

By: Dr. Ewen Todd, et. al.
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In this article, the third in a series of several reviewing the role of food workers in 816 foodborne outbreaks, factors contributing to outbreaks and descriptions of different categories of worker involvement are discussed. All the outbreaks had worker involvement of some kind, and the majority of food workers were infected. The most frequently reported factor associated with the involvement of the infected worker was bare hand contact with the food followed by failure to properly wash hands, inadequate cleaning of processing or preparation equipment or utensils, cross-contamination of ready-to-eat foods by contaminated raw ingredients, and (for bacterial pathogens) temperature abuse. Many of the workers were asymptomatic shedders or had infected family members and/or used improper hygienic practices. Outbreaks were sorted into categories based on how many workers were implicated, the origin of the infective agent (outbreak setting or off site), the degree of certainty that the worker(s) were the cause or were victims, whether or not the workers denied illness, the ability of the agent to grow in the food, whether only the workers and not the patrons were ill, and whether patrons were more responsible for their illnesses than were the workers. The most frequent scenarios were (i) a single worker causing an outbreak by directly infecting patrons; (ii) an infected worker fecally contaminating foods that were then temperature abused, leading to an outbreak; and (iii) multiple workers linked to an outbreak but with no clear initiating source. Multi-ingredient foods with limited descriptions were most frequently implicated and usually were served in restaurants or hotels, at schools, and at catered events. Identified contaminated ready-to-eat foods included produce, baked goods, beverages, and meat and poultry items. In some situations, it was not clear whether some of the workers were the cause or the victims of the outbreak. However, in other situations there may have been an underestimation of the role of the worker. For instance, workers sometimes denied infection or illness for a variety of reasons, but subsequent investigation provided evidence of infection.

This article is the third in a series of several reviewing the role of food workers in foodborne outbreaks. Members of the Committee on Control of Foodborne Illnesses of the International Association for Food Protection analyzed 816 foodborne disease outbreaks with 80,682 cases where food workers were implicated as the source of the contamination. Most of the outbreaks were from the United States, Canada, Europe, Australia, and a few other countries. Outbreaks were caused by 14 agents, primarily norovirus or probable norovirus, Salmonella, hepatitis A virus (HAV), Staphylococcus aureus, Shigella, Streptococcus, and parasites. Multiple foods and multi-ingredient foods were identified most frequently with outbreaks perhaps because of more frequent hand contact during preparation and serving. Some of the outbreaks were very large, and 11 outbreaks included more than 1,000 cases. In five of the outbreaks, more than 100 affected individuals were hospitalized, but in general, hospitalization and death rates were low (3.4 and 0.12%, respectively). Outbreaks were associated most frequently with restaurants, followed by catered functions, meals prepared in the home for special events such as parties, schools, hotels, hospitals, church social events, camps, and the workplace. Meals served on cruise ships, airplanes, and trains have also caused outbreaks where workers were responsible for the food contamination and sometimes for the further spread of the contamination when the agent was highly infectious, e.g., norovirus. The reports often did not contain enough information to be certain that the workers were the sole cause of the outbreaks; there is always some degree of uncertainty when analyzing such data. However, the 816 outbreaks we studied revealed some similar trends: workers were asymptomatic and excreted the pathogen unknowingly while working or continued to prepare food when it was obvious to them and sometimes others that they were ill and could be contaminating food. In this study, we analyzed the outbreaks for trends that could be important for developing appropriate control measures to reduce the likelihood of worker errors and outbreaks.


The database of 816 outbreaks where food workers have been implicated, the criteria used for collecting and collating these data, and the preliminary analyses have been described previously (10, 31). The factors contributing to outbreaks are important for both determining the cause and developing effective controls. The factors in the outbreaks where workers are implicated are mainly associated with contamination, i.e., how the agent got onto or into the food vehicle. All of the 816 outbreaks chosen for the study involved infected food workers or occasionally infected patrons or guests who handled food. The various factors contributing to these outbreaks have been previously described (10) and were based on definitions listed in investigative reports published by the Centers for Disease Control and Prevention (2) or were assigned descriptions by the authors. Examination of these factors and the narratives of the investigation reports implicating workers revealed that food workers are implicated in outbreaks in several different ways. A decision was made to rank the various factors contributing to outbreaks and sort the outbreaks by categories.


. . .Continue to read rest of article (PDF).

Dr. Ewen Todd is a Food Safety Expert with over 45 years of knowledge and experience in general food safety and Food Microbiology Issues. Specifically, he has extensive experience the reporting and surveillance of Foodborne Disease, as well as development of analytical methods for identifying foodborne pathogens. As a result of his work with the industry and his knowledge of foodborne disease, Dr. Todd has influenced research programs and regulatory approaches taken by the Health Protection Branch of Health Canada. In the US, he held the position of Director of the National Food Safety and Toxicology Center and the Food Safety Policy Center at Michigan State University.

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