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Surveillance for Foodborne Disease Outbreaks – United States, 1998-2008

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Surveillance for Foodborne Disease Outbreaks – United States, 1998-2008. MMWR. Volume 62, Number SS2—June 2013

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Tracking foodborne disease outbreaks provides valuable insight into the germs and foods that sicken an estimated 48 million Americans each year.

CDC's Foodborne Disease Outbreak Surveillance System collects and periodically reports data associated with recognized foodborne disease outbreaks in the United States.

This report summarizes data associated with the 13,405 foodborne disease outbreaks reported to CDC from 1998-2008.

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Questions and Answers

What is this surveillance summary about?

The summary uses data associated with the 13,405 foodborne disease outbreaks reported to CDC from 1998-2008 collected through the Foodborne Disease Outbreak Surveillance System. This report, Surveillance for Foodborne Disease Outbreaks – United States, 1998-2008, describes 11 years of data about:

  • The causes of outbreaks and where they occur
  • The pathogens and foods that caused the most outbreaks, illnesses, hospitalizations, and deaths
  • Trends in the pathogens and foods associated with outbreaks over time

What are the key findings of the paper?

This report draws from 13,405 foodborne disease outbreaks reported to CDC from 1998-2008 accounting for 273,120 illnesses, 9,109 hospitalizations, and 200 deaths (Figure A) and includes six key categories:

  • Pathogens causing outbreaks
  • Foods causing outbreaks
  • Pathogens linked to foods causing outbreaks
  • Trends over time in foods causing outbreaks
  • Places where food is prepared
  • Changes in outbreak reporting rates

The following FAQs address the six key categories of findings from Surveillance for Foodborne Disease Outbreaks – United States, 1998-2008:

What are the pathogens causing outbreaks?

  • Norovirus (39% of outbreaks) and Salmonella (26%) were responsible for the largest number of confirmed outbreaks and outbreak-associated illnesses. Figure A
  • Salmonella caused the most outbreak-related hospitalizations (44%), followed by Shiga toxin-producing E. coli (such as E. coli O157) (14%), and norovirus (11%). Figure A
  • Salmonella caused the most deaths (60), followed by Listeria(48), and Shiga toxin-producing E. coli (22). Figure A

What are the foods causing outbreaks?

What are the notable pathogens linked to foods causing outbreaks?

  • The largest number of outbreaks was attributed to scombroid toxin acquired by consumption of fish, ciguatoxin from fish, Salmonella from poultry, and norovirus from leafy vegetables Figure A
  • The largest number of illnesses was due to norovirus acquired by consumption of leafy vegetables, Clostridium perfringens from poultry, Salmonella from vine-stalk vegetables, and Clostridium perfringens from beef.
  • The largest number of outbreak-associated hospitalizations was due to Salmonella acquired by eating items in the fruits and nuts category, Salmonella from vine-stalk vegetables, STEC from beef, and STEC from leafy vegetables.
  • The largest number of outbreak-associated deaths was due to Listeria from poultry, Salmonella from fruits and nuts, and STEC from leafy vegetables.

What are the key trends in food commodities causing outbreaks and what do they mean?

  • Eggs
    • The proportion of outbreaks linked to a single commodity associated with eggs decreased from 6% during 1998-1999 to 2% during 2006-2008. 
    • The percentage of Salmonella outbreaks attributed to eggs declined significantly, from 33% during 1998-1999 to 15% during 2006-2008. Figure D [PDF - 1 page]
    • The percentage of Salmonella outbreaks caused by serotype Enteritidis (SE) decreased from 44% during 1998–2000 to 24% during 2006–2008.  SE outbreaks were most commonly associated with eggs and poultry.
    • The continued occurrence of outbreaks linked to eggs highlights the importance of measures to improve their safety such as the Food and Drug Administration's Egg Safety Rule, which was finalized in 2010.
  • Leafy vegetables
    • The percentage of outbreaks associated with leafy vegetables increased, from 6% of outbreaks in 1998-1999 to 11% in 2006-2008.
    • The percentage of outbreaks associated with leafy vegetables might have increased because more outbreaks are being proven to be caused by norovirus as a result of improved diagnostic capacity in state health department laboratories.
    • Recently proposed regulations for the produce industry can help to prevent contamination of produce, including leafy vegetables. 
  • Dairy
    • The percentage of outbreaks associated with dairy products increased, from 4% of outbreaks in 1998-1999 to 7% during 2006-2008.
    • The increased number of outbreaks caused by dairy products is because of an increase in the number of outbreaks caused by unpasteurized (raw) milk. 
  • Other foods
    • There were no trends in the number of outbreaks caused by other foods, including beef or poultry, during this time period.

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Where are the foods prepared that are causing outbreaks?

  • The most common places where foods causing outbreaks were prepared were restaurants or delis (68%), private homes (9%), and catering or banquet facilities (7%). Figure E
  • Some outbreak settings are more likely to be recognized and investigated (e.g., restaurants and schools) so data on outbreak settings in this paper may not fully represent all settings where contaminated food is prepared and consumed. Figure E

What are the trends in outbreak reporting rates, and how should they be interpreted?

  • The number of outbreaks reported to CDC declined overall by 23% from 1998-2008.  
  • We believe the lower number of outbreaks during this period is mostly due to reporting declines in one populous state. There were no trends suggesting a national reduction in state outbreak investigation and reporting. 
  • Of the 42 states with enough information to measure changes in the number of outbreaks reported over time, many improved or maintained stable reporting (20 improved, 6 maintained stable reporting).
  • These numbers reflect a change in the number of outbreaks reported and are not a change in the overall rate, or number, of foodborne infections tracked in individuals.  That data can be found through CDC's FoodNet surveillance system 

What are some limitations of the study?

  • Many foodborne disease outbreak reports have missing or incomplete information on certain aspects of the outbreak (e.g., the etiology or the implicated food vehicle), so conclusions drawn in this paper about etiologies or food commodities might not be applicable to outbreaks with etiology or food source unknown.
  • Additionally, some outbreak settings are more likely to be recognized and investigated (e.g., restaurants and schools) so data on outbreak settings in this paper might not fully represent all settings where contaminated food is prepared and consumed. Certain pathogens or food vehicles might also be more likely to be recognized or investigated.
  • Many outbreaks are undetected or are not reported to CDC. Consequently, it is unknown whether reported outbreaks and implicated foods represent an unbiased sample of all outbreaks occurring in the population. Therefore, any interpretation of observed statistical differences should be made with caution.

Does this mean I shouldn’t eat foods responsible for foodborne disease outbreaks, such as meat, leafy vegetables, and poultry?

  • No!  The study findings do not mean that people should avoid certain categories of foods, but they do point to the importance of following food safety recommendations during food preparation.
  • Fruits and vegetables: The Dietary Guidelines for Americans 2010 encourage us to eat more fruits and vegetables as a part of a healthy diet. Eating fruits and vegetables is associated with reduced risk of many chronic diseases including heart attack, stroke, and certain types of cancer.
    • When properly cleaned, separated, cooked, and stored to limit contamination, fruits and vegetables safely provide some essential nutrients that would otherwise be lacking in most American diets.
  • Raw poultry: Food items like chicken often contain harmful bacteria such as Salmonella and Campylobacter. Washing poultry does not remove bacteria.
    • Kill these bacteria by cooking chicken to the proper temperature [165°F).
    • Use a food thermometer to ensure that cooked foods reach a safe internal temperature.
  • A healthy and safe diet is an important part of a healthy lifestyle.

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Questions about Other Studies and Time Periods

How do these findings compare with the paper by Painter et al. (2013) on foodborne illness attribution to specific foods?

  • Outbreak data are the foundation for estimates of source attribution because outbreak investigations provide direct links between foodborne illnesses and the foods causing them.  Data from foodborne disease outbreak surveillance are our best data source for attributing illnesses to foods.
  • The paper published by Painter et al (2013) used these same outbreak data from 1998-2008 for the analysis. The authors of that paper combined outbreak data with previously published estimates of the total number of U.S. foodborne illnesses (both outbreak-associated and sporadic) caused by each pathogen to estimate how many illnesses occur in the United States each year attributed to specific food commodities.
  • In contrast, this summary of outbreaks from 1998-2008 analyzes the raw outbreak data, i.e., the actual number of outbreaks and outbreak-associated illnesses reported to CDC from state health departments.

Why did this study not include the data from more recent years?

  • This surveillance summary focuses on the 11-year surveillance period ending in 2008. Data from more recent years was not included partly because changes were made in the data collection system in 2009. Because changes in the data collection system can affect the data reported, this summary focused on the recent surveillance period when all the data were collected in the same manner.
  • CDC will continue to publish regular summaries of outbreak data. 
  • Data are also publicly available in the Foodborne Outbreak Online Database

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Other Questions about Foodborne Disease Outbreaks

What is already known about foodborne disease and outbreaks?

Woman sitting on bed grabbing stomach.
  • An estimated 48 million people (or 1 in 6 US residents) become ill from foods consumed in the United States every year. 
  • Only a small proportion of foodborne illnesses occur as part of recognized and reported outbreaks. When illnesses occur outside an outbreak setting, it is usually impossible to know what food or other exposure caused them. Therefore, foodborne disease outbreak data provide some of the most detailed and accurate data about the sources of foodborne illnesses.
  • Examining outbreak data can offer insights into the pathogens and foods causing foodborne illnesses, and into the factors that contribute to their occurrence.
  • Public health officials use this information for foodborne illness prevention, education, and public policy.

Why is the food vehicle unknown for so many reported outbreaks? 

  • There are several reasons, for example, sometimes most ill persons ate many of the same food items, so a single food can’t be pinpointed (for example, at a buffet or a banquet). 
  • Well-conducted investigations sometimes do not identify a food vehicle. There are many reasons for this.
    • CDC encourages states and territories to report all foodborne disease outbreak investigations, even if the food vehicle is not determined.
  • Although it is ideal to determine the food that caused an outbreak, much can still be learned from outbreaks that are known to be transmitted by food but for which the vehicle is not determined. 
    • For example, information about what pathogen caused the outbreak, where the outbreak happened (e.g., restaurant, home, school), how many people got sick, and whether someone died, is usually available, and this data is useful in designing prevention measures. 

What factors influence outbreak investigation and reporting by state, local, and territorial public health agencies?

  • Outbreak detection, investigation, and reporting are influenced by many factors such as available resources (time, staff, and laboratory capacity), health department priorities, and outbreak characteristics (size, severity). 
  • CDC typically becomes involved in outbreak investigations only when an outbreak affects multiple states or when a state, local, or territorial public health agency asks for CDC assistance with a particularly large, severe, or unusual outbreak.

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Where can I go for more government information on foodborne disease outbreaks and foodborne illness source attribution?

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