Foodborne Illness Source Estimates

This study does not address where contamination of food occurs along the food production chain.

A CDC study, "Epidemiology of Foodborne Norovirus Outbreaks, United States, 2001-2008," looked at this issue in relation to norovirus.

No, that is not the message from this paper. A healthy and safe diet is an important part of a healthy lifestyle. These findings do not mean that people should avoid certain categories of foods.

• Fruits and Vegetables: The 2010 Dietary Guidelines (also see the current guidelines) encourage Americans 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, such as chicken, often contain harmful bacteria such as Salmonella and Campylobacter. Washing poultry does not remove bacteria.
  • Kill these bacteria by cooking poultryto the proper temperature.
  • Use a food thermometer to ensure that cooked foods reach a safe internal temperature: 165°F for poultry, 145°F for whole meats (allowing the meat to rest for 3 minutes before carving or consuming), and 160°F for ground meats.

There is no single data source or analytic method that is best for estimating source attribution for all agents. CDC and its partners use several data sources and combinations of methods to estimate the number of illnesses associated with each type of food. Some are highly specific for one type of infection or one type of food, and others are more general. For example, by comparing the characteristics of Salmonella found in foods and animals with those found in people, we can attribute Salmonella infections back to specific animal sources. The best method may differ for different agents and foods.

Analysts use models to estimate the major food sources for illnesses. They use data from outbreaks, from sporadic illnesses, and from subtyping of pathogens:

Outbreaks

Outbreak data provide information about the role of various foods in causing illnesses. However, outbreaks contribute only a small proportion (less than 5 percent) of lab-confirmed foodborne illnesses. Only using outbreak data assumes that the food sources are the same for outbreak-related illnesses and the more common sporadic (non-outbreak) illnesses, which is more accurate for some food-pathogen combinations than others.

• The National Outbreak Reporting System (NORS) collects data electronically from state, local, and territorial public health agencies and includes reports of all outbreaks of enteric (intestinal) illness transmitted by food, water, animal contact, or by person-to-person contact that they investigated.
• CDC regularly publishes summaries of foodborne disease outbreaks reported through NORS.

Sporadic illnesses

Most health departments do not collect detailed data about exposures of individual people with illnesses, such as salmonellosis, that are often foodborne. Because most people do not know what made them sick, illnesses reported to a health department usually do not reveal the likely source. CDC and health departments do intensive studies that compare the exposures of people with sporadic cases of a particular infection, such as E. coli O157, with the exposures of other people who are not ill.1

• By comparing exposures reported by the two groups, we can determine which foods likely are responsible for some of the illnesses. This approach is called a case-control study.

Pathogen subtype

Pathogens of the same type are not exactly alike. In the laboratory, small differences between pathogens can be used to subtype them. The subtype is a marker, like a fingerprint, that sometimes can help distinguish strains from different sources.

• Investigators can compare the subtypes of pathogens isolated from animals and foods with the subtypes isolated from infected humans. They can combine this information with data on how often people consume certain foods, and then use mathematical models to estimate the number of illnesses associated with each source.
• CDC and the U.S. Department of Agriculture's Food Safety and Inspection Service (FSIS) recently collaborated to adapt a pathogen-subtype model for Salmonella infections to US data.2

Outbreak investigations definitively link illnesses to particular foods and pathogens.

• Outbreaks are caused by a wide range of agents, including bacteria, viruses, parasites, toxins, and other chemicals so they provide good coverage of foodborne agents.
• Outbreak investigations capture data on common and uncommon pathogens, and on common and uncommon foods.

• Estimates were not made for pathogens that did not cause outbreaks. Toxoplasma and Vibrio vulnificus did not cause outbreaks but have high fatality rates. Adding these pathogens might result in Meat and Mollusks (e.g., oysters, clams) being more important source of deaths than is indicated in the study.
• The study used data from an 11-year period and did not account for changes over this period. A shorter, more recent period is desirable when major implicated commodities have changed. For example, Listeria outbreaks caused by contamination of ready-to-eat meats markedly decreased after 2002.
• The estimates are presented as ranges of high, low, and most likely estimates for the number of illnesses, hospitalizations, and deaths attributed to each food category. The ranges reflect the uncertainty in the estimates.
• The estimates depend on the general assumption that the foods implicated in outbreaks are the same foods that cause individual illnesses not part of outbreaks.
• The risk for foodborne illness is just one part of the risk-benefit equation for foods; the health benefits of consuming a diet high in fruits and vegetables must also be considered.

The attribution of illness to foods can be done in several different ways, some highly specific for one type of infection or one type of food, and others more general.

• For example, by comparing the characteristics of Salmonella found in foods and animals with those found in people, one can attribute Salmonella infections back to specific animal sources.
• A working group of scientists from CDC, FDA, and USDA has been collaborating on several projects to improve these estimates. The Interagency Food Safety Analytics Collaboration (IFSAC) is bringing together more data from across the agencies, and evaluating methods for analyzing them, and will be providing more refined estimates in the future.