Attributing U.S. Foodborne Illness to Food Consumption

Over the past three years, USA Today has run a major story on a food safety problem almost every month. U.S. consumers may be a bit shell-shocked by the barrage of headlines warning of foodborne pathogens (disease-causing organisms) or harmful chemicals. American consumers—as well as those in the agriculture and food-processing industries—are undoubtedly asking, what next?

Unfortunately, with foodborne illness it is even difficult to say which foods have been the biggest problems in the past. The reasons are actually as simple as these: the evidence gets eaten or thrown out; illness may follow food consumption by days or even years; and, human memory, particularly when trying to remember what one had for dinner even three days ago, is frail. Just as unfortunately, it is important to know which foods caused the most illnesses in the past in order to reduce illness in the future.

Compelling, substantive reasons exist—for all parties involved—to want to invest time and effort in developing information on the sources of foodborne illness. Consumers need to know how to handle foods safely and be able to recognize the relative riskiness of particular foods to guide their purchase decisions. Producers would like to know whether the types of foods they produce are likely to be the next story on the front page of the New York Times so they can develop strategies to avoid potential financial risk. Supply-chain managers want to know about the relative riskiness of the different sources of a product so they can appropriately weigh the costs and benefits of each source. Governments want to know about the relative riskiness of foods to effectively design laws and target efforts to protect the public from health risks.

There are also important procedural reasons for wanting quantitative data on the sources of foodborne illness—reasons related to ensuring that regulations are actually needed and do not unfairly burden trade. Both industry and consumers are often concerned about special interests having undue influence on government agencies or about government agencies writing rules that favor one firm over another.

Government agencies in the United States and abroad rely on formal risk assessment as the primary means of understanding how health risks arise in the food supply. Risk assessment is a process of quantifying and modeling the pathway from contamination through exposure to health outcomes. It typically relies on dose-response relationships to predict illnesses or deaths. Estimating a pathogen dose-response relationship is difficult, however, because pathogens tend to be species specific and human testing is considered to be unethical. An alternative is to estimate disease incidence from epidemiological data and then attribute it back to the source of infection—in other words, a food attribution estimate.

Uncertainty abounds in estimates of the number of cases of foodborne illness each year. Health statistics depend heavily on reporting by physicians and medical laboratories, but most cases of foodborne illness are probably mild and never show up at a doctor's office. When someone with foodborne illness does seek medical attention, the physician or medical laboratory may not report the illness to public health authorities and, if it is reported, it may be identified only as a case of infectious disease. In fact, scientists from the Centers for Disease Control and Prevention (CDC) estimate that for many pathogens, only 1 in 38 cases of foodborne illness are reported.

Even greater uncertainty exists about the food sources of foodborne illness. Food safety managers and public health officials need to know which pathogens either in or on which foods are making people sick. Physicians can determine which pathogen made a patient sick by ordering a laboratory test, but that typically does not occur because such tests are more useful for public health surveillance than for patient care. Even if a physician suspects that an illness is foodborne, it will typically be difficult to pinpoint the cause. Individuals' ability to recall what they ate is notoriously poor. Often, a few days may pass between infection and illness, and then it is a guess as to which food was actually associated with the illness. Again, there is usually no clinical reason to investigate the matter further.

In response to these reporting problems, the CDC and state public health surveillance authorities have developed three major foodborne illness surveillance programs: OutbreakNet, PulseNet, and Food-Borne Diseases Active Surveillance Network (FoodNet). Although these systems provide helpful information about the sources of foodborne illness, further work is needed to make them truly useful for food attribution in policy analysis.

A number of efforts are under way within federal agencies to adapt this data or to create new data to meet the need for attribution estimates. Most of these efforts are targeted at specific regulatory needs. For example, the Food Safety Inspection Service is working on attribution of Salmonellosis to food products under its jurisdiction, using a sampling and genetic subtyping protocol developed in Denmark. The CDC is working on two food-system-wide approaches, one based on outbreak case data that could be updated in real time, and another that relies on a blend of outbreak and case-control study data. Microbiologists also continue to work on the problem of developing predictive dose-response models for human foodborne pathogens.

In the absence of hard data, judgment-based estimates are also used. Usually, this is done informally. Current estimates attributing the incidence of foodborne illness to specific pathogens rely heavily on the expert judgments of a group of researchers at the CDC to fill gaps in the literature. More formal methods are being developed; for example, evidence-based medicine has developed a set of criteria for evaluating studies through systematic literature reviews that are used to identify best clinical practices. Risk analysis in environmental and safety policy has long relied on structured analysis or elicitations of expert judgment for subjective estimates of missing parameter values. [callout text] It doesn't take much to have a big public health impact: four food groups—produce, seafood, poultry, and ready-to-eat meat—and just three pathogens account for a sizable majority of all foodborne illnesses.

Recently, colleagues and I conducted an expert elicitation on foodborne illness source attribution as part of an effort to develop a foodborne illness risk ranking model for use in broad federal-level policy evaluation. Over 40 of the country's leading food safety experts participated in the survey. They were able to draw on a broad range of knowledge to inform their judgments—knowledge of microbial ecology, food science, consumption patterns, and food-handling practices as well as epidemiological data. For each of 11 major foodborne pathogens, experts were asked to provide their best judgments of the percentage of cases caused by the pathogen that is associated with consumption of different food categories in a typical year. The food categories spanned the food supply. We then applied these percentages to CDC estimates of the incidence of illness, hospitalization, and death caused by each pathogen to estimate the cases of foodborne illness caused by the pathogen on different foods. These estimates were examined individually and aggregated to provide estimates of foodborne illness by food categories.

The purpose of the study was threefold. First, we needed a consistent set of estimates—spanning all foods—of the association of foodborne illness with food consumption. Second, we aimed to capture information on sporadic illnesses as well as outbreaks. And third, we intended to assess the extent of agreement among experts and the degree of confidence that food safety experts have in their own understanding of the association between foodborne illness and the consumption of specific foods.

The most marked finding is the relatively high public health impact of a small number of pathogens and foods (see Table 1). Prior research indicates that the three highest-ranked pathogens account for 97 percent of all foodborne illnesses. Our results suggest that incidence is also highly concentrated by food. Four food groups (produce, seafood, poultry, and ready-to-eat meat) accounted for 60 percent of all illnesses, 59 percent of all hospitalizations, and 46 percent of all deaths.

The results also show the importance of focusing public and private intervention efforts on particular food-pathogen combinations. A small number of such pairs account for most of the public health burden from foodborne pathogens. Fifteen out of 121 food-pathogen pairs accounted for 90 percent of all illnesses, 25 pairs accounted for 90 percent of hospitalizations, and 21 pairs accounted for 90 percent of deaths. It is worth noting that these foods and pathogens do not rank highly if they were ranked by themselves.

Our study characterized the uncertainty around attribution by pathogens, foods, and food-pathogen pairs by evaluating the level of agreement among experts, the 90 percent confidence bounds they provided, and how their judgments matched up to outbreak data (see Table 2). This information on uncertainty about attribution provides part of the foundation for deciding where to invest in further research and data collection on disease surveillance.

For some food-pathogen pairs, such as Vibrio on seafood, experts' best judgments are highly correlated with each other and with the outbreak-based attribution estimate, and their mean confidence "intervals" (the distance between the 5 percent upper and lower confidence bounds) are narrow with little variation among experts. For others, such as Campylobacter on produce, the mean and variance of experts' confidence intervals are small, but the correlation between expert judgment and outbreak-based attribution estimate is low. This is a case where experts agree that outbreak data do not provide a good attribution estimate but do agree based on other information, such as strong microbial ecology data. And then there are cases, such as Toxoplasma on many foods, where expert's best estimates are not highly correlated with each other or the outbreak-based estimate, and the mean and variance of their confidence intervals are relatively high. This scenario indicates a clear lack of evidence and strongly suggests that more research is needed to understand Toxoplasmosis attribution to food.

U.S. agencies are proposing to or currently make use of food attribution estimates in a number of ways including risk-based inspections, health-based performance standards, and the rationalization of federal food safety policy. In an effort to prioritize the use of limited inspection resources, the Food and Drug Administration's Food Protection Plan includes risk-based targeting of inspection of both domestic plants and imports. The Department of Agriculture's Food Safety Inspection Service has also proposed risk-based inspections of domestic meat-processing and slaughter facilities. Both efforts have proven controversial: consumer groups have expressed concern that a move from random or uniform allocation of inspection resources to risk-based allocation may not ensure product safety and that existing data are not adequate to support the shift. Improved source attribution estimates could play a role here.

Every industrialized country should have good information on how foodborne illnesses are distributed across the food supply, at least in theory. But data on these relationships are more difficult to collect than one might imagine. Changes in international trade law have also made the collection of such data more crucial than it may have been in the past. Governments around the world, including that of the United States, have made a focused effort over the past 10 to 15 years to improve the quality of information on the distribution of foodborne illness across foods. Eventually, this information will help both government agencies and private firms do a more effective, more efficient job of protecting the public from foodborne illness. But for now, a great deal of work remains to be done. A longer version of this article appears in the summer 2009 issue of Choices, a publication of the Agricultural & Applied Economics Association.