When a food poisoning outbreak occurs, the agencies responsible for regulating the food industry conduct an investigation into the specific illness and food product that caused the outbreak. Investigations are conducted by interdisciplinary teams consisting of epidemiologists, microbiologists, sanitarians, veterinarians, food scientists and various individuals from the food industry. When an outbreak is first discovered, the ultimate concern is to protect the public health. The investigation process is immediately implemented, and once an investigation uncovers a potential source, a recall is executed. After a recall has been successfully completed, further investigation is carried out to uncover new information about foodborne illnesses, such as identification of new illness-causing pathogens. This prevention part of the investigational process helps the food industry, public health officials, and the agencies responsible for food safety to identify problems in existing food monitoring and inspection systems, how to fix them, and ultimately, how to better prevent outbreaks in the future. Needless to say, there are many steps in the investigation process.

Detection of an Outbreak

                The first step in the investigation is, of course, detecting that an outbreak has actually occurred. Public health agencies usually discover an outbreak by monitoring the results of the foodborne illness surveillance systems in place.  If surveillance system reporting shows that an unusually large number of people in a given area fell ill due to the same pathogen, public health officials may deem this irregularity a “cluster.” If the individuals in the cluster all consumed a common food product linked to the illness, an outbreak is determined.

                A cluster, suggestive of an outbreak, can be identified in several ways.  Informal reporting can help public health officials determine a cluster that specifically identifies an outbreak and the food industry source responsible. Informal reporting occurs, for example, when individuals from a community inform their local health agency of several instances of foodborne illnesses occurring after exposure to a common food source, such as when a group of friends discovers each became sick after consuming food from the same food establishment.

                The formal reporting system helps public health officials determine a cluster, or several clusters, and the associated outbreak when the situation is more complex than that in an informal reporting scenario. Formal reporting may proceed in a number of ways. In some instances, outbreaks are discovered due to reporting requirements pertaining to a notifiable disease list. Physicians and laboratories must report each singular diagnosed infection that is included on the notifiable disease list, which is maintained by local, state, and federal agencies. Each singular report is organized in a surveillance system that reveals when the number of illnesses is higher than usual in a given area.  In the alternative, physicians may also suspect an outbreak when they are seen by a larger than normal number of people exhibiting the same symptoms, especially when laboratory tests confirm that those people have been infected by the same pathogen. Physicians will report such occurrences directly to the proper public health authorities, and thus make them aware of a possible outbreak.

                Formal reporting also occurs when laboratories themselves report cases of foodborne illness. When a physicians suspects, due to a patient’s symptoms, that the patient may have a foodborne illness, the physician will send a stool sample off to a laboratory for testing.  If the laboratory confirms a specific pathogen, it will send the information to the appropriate public health agency laboratory.  Agency laboratories will conduct further tests to identify the specific strain of bacteria.  Such tests are called subtyping tests, and they include serotyping and DNA fingerprinting.

                Serotyping will reveal the specific bacterial strain by examining markers on the bacteria.  When several samples are identified to have the same serotype or markers simultaneously, it is an indication of a possible outbreak. Bacterial DNA fingerprinting identifies the bacteria’s genetic pattern and the pattern is entered into to the Centers for Disease Control and Prevention (CDC) PulseNet database. The compilation of such data is a particularly effective method of identifying outbreaks that are widespread. Because PulseNet is the result of a collaborative effort of federal, state and local health agencies, the system can connect foodborne illness cases occurring nationwide; PulseNet enables specific pathogen identification, which leads to cluster identification and, in turn, widespread outbreak identification.

Finding Cases

                Identifying clusters is only part of the process of identifying a widespread outbreak.  Our food industry is complex and food products are distributed on a very large scale. Identification of initial clusters is only the beginning of the outbreak investigation process.  Public health agencies seek to find more infected individuals to effectively determine the severity, size, timing, and possible sources of the outbreak in order to prevent more individuals from becoming ill in the future.

                By developing what public health agencies call “case definitions,” what initially seem to be isolated incidents of illness can be included in the web that will ultimately include the large majority of the illnesses connected to an individual source. Case definitions define the outbreak’s exact characteristics and parameters, and may include the following details: the pathogen or toxin causing the illness; the most common symptoms reported; the time frame in which the illnesses occurred; the geographic area where individuals fell ill; and other defining characteristics of the illness, such as the DNA fingerprint if the disease has been tracked in PulseNet. By using the case definition, investigators are enabled to find more cases of illness. Given the case definition, investigators will review surveillance reports, the laboratory reports of PulseNet, emergency room records, and ask physicians and health officials in and around the geographic area of illness to be aware of and immediately report illnesses that they suspect could be connected to the outbreak.

                This process allows investigators to perceive the development of the outbreak, by tracking who becomes ill, where the ill individuals live, and when exactly their illness surfaced. Investigators often mark geographical maps, indicating exactly where illnesses occurred and the progression of its geographical spread. Another tracking device, termed the “epidemic curve,” graphs the number of cases that fall within the specific case definition parameters – the “case count.”

Identifying Probable Sources

                Since contamination can occur in many ways, such as by contact with infected individuals, animals or contaminated water, it is essential that investigators be able to narrow the possible culprits down to those most likely to be the cause of infection. Investigators generate hypotheses about the source of the contamination that caused the illness.  They do so by using the information obtained when developing the case definition.  Details such as the specific pathogen responsible for the illness, the geographic areas where individuals fell ill, and the age of the infected individuals help investigators contemplate the more likely sources of contamination.

                Once it is determined that the cause of the illness can be attributed to food, investigators develop a list of all of the possible food sources of infection. Investigators will narrow the list by eliminating foods eaten that, by timing, could not have caused the illness, and by interviewing sick individuals regarding what and where they ate during the days or weeks prior to becoming ill. Through such interviews, investigators seek to discover what was eaten in the correct time period, given the incubation period of the specific pathogen. The incubation period is the time it takes to experience symptoms after ingesting a particular pathogen, and incubation periods vary among pathogens. Interview questions will focus on those facts that make up the case definition determined by the investigators.

                When no food establishment or obvious source of contamination can be determined by interviewing and use of the case definition, investigators resort to the “shotgun” questionnaire technique. This questionnaire asks each sick individual to select which food items they ate out of a long list, to describe each meal they can remember eating in the days or weeks before they fell ill, and to explain their normal routines pertaining to grocery shopping, dining-out, event attendance, and travel. At times, investigators may inspect a sick individual’s home and record a list of food items kept in their pantry and refrigerator.

                After compiling the results, interviews or questionnaires are searched for common elements, and a list of possible sources is created, given the common information provided. Attempting to identify possible sources is time-consuming and complex; it requires repetitive testing and refining of the hypotheses produced in the investigational process. There are many challenges involved in creating testable hypotheses. Sick individuals may not remember exactly what or where they ate in the days or weeks prior to their falling ill or, when the source of the illness is a single ingredient, identifying the specific ingredient may require extensive research and time.

Testing Hypotheses

                Once investigators have identified the more likely sources of contamination, they must determine if any of these hypotheses are, in fact, correct. The two principal methods investigators use to test the hypotheses are: 1) analytic epidemiologic studies; and 2) food testing.

Analytic Epidemiologic Studies

                In analytic epidemiologic studies, investigators use groups of healthy unaffected individuals — “control groups” — against which to compare the symptoms and characteristics of ill individuals. In making the comparison, investigators look for conditions that would have made the ill individuals more likely to be exposed than the healthy individuals in the control group. Conditions, such as eating habits and the geographical area in which ill individuals lived or traveled when they became ill, can be particularly revealing. For example, an ill individual’s address may be entered into a system which will produce contact information for others living in the area.  By making calls and interviewing others living in the same geographical area as the ill individual, investigators can begin to narrow a list of exactly where the ill individual might have become exposed. If investigators make contact with an individual in the same geographical area who also became sick, but did not seek medical attention and did not report the illness, investigators can compare the shopping and dining habits of both to determine products or food establishments the individuals have in common. When a particular food is reported as having been eaten more often by sick individuals than those who remained healthy, the particular food may be the possible source of contamination.

                By compiling these statistics and comparing their results, investigators may be able to determine an association between ill individuals and a food product that is particularly strong. After discovering a strong association, investigators will analyze the regularity of exposure (i.e., how often the ill individual consumed the food item), the level of exposure (i.e., how much of the food item was consumed in each sitting), and the food product’s production, preparation, service and distribution chain (i.e., the composition and method of dissemination of the food item). Any connection may highlight the potential source.

Food Testing

                Food testing is the most direct method of determining whether a hypothesis concerning a potential source is correct. If a food product consumed by an ill individual can be tested, and the results show that the DNA fingerprint of an unopened package of food is the same as that found in the stool sample of the infected individual, this constitutes strong evidence that that food product is the source of the illness. However, food testing is often difficult and may produce unreliable results.  Food products that have a short shelf-life, such as produce, may no longer be available for testing once an outbreak has been determined. To add, when samples are taken from a food product, they are generally taken from a portion of the product in the entire distribution chain, which may not be the portion that actually contained the illness-causing pathogen.

                Hypothesis testing does not always prove to be helpful in an investigation.  In nearly half of all investigations, no link between a particular food and illness is discovered, despite the fact that the illness is clearly established as foodborne.  A link may not be discovered for a number of reasons.  At times, public health agencies do not discover that an outbreak has occurred until long after those affected have recovered and the pertinent time for testing has expired. Even when an analytic epidemiologic study has been conducted, a link may not be discovered between the food product and the illness because the sample of ill individuals was too small, multiple food items were the source, or because a “stealth food” was involved.  Illness is deemed to be caused by a “stealth food” when the food source is overlooked because ill individuals do not remember eating the particular food that caused the illness. When an outbreak has come to an end, but no particular food source has been identified, the source is simply declared “unknown,” and public health agencies must move on.

Pinpointing the Source

                Investigators find the source of the contamination in the food product causing the illness by tracing the food’s history. Beginning with the food’s preparation and presentation, an investigation will be tracked through the food product’s chain of distribution, processing and finally, to its origin. In some instances, investigators need only trace the source of contamination to its first-analyzed step: preparation and presentation. For example, if the food was prepared and presented for consumption by a singular establishment, the food may be discovered to have become contaminated by the hands of an infected food handler. Investigators would identify such a source by interviewing the food handlers or testing the kitchen of the particular establishment suspected to be the source. Agencies can also check the inspection reports of the particular food establishment for a history of safety violations.

                Unfortunately, identifying the source of the infection is not usually as simple as the above scenario.  Where a particular establishment is not the source, the food may have become contaminated during its distribution to numerous food establishments or grocery stores, or the food may have become contaminated during processing or production. In such a scenario, investigators will conduct a “source traceback” to identify the contaminating culprit. Investigators will inquire as to the suppliers of the food establishment or grocery store suspected of carrying or preparing the contaminated food.  Any suppliers then are asked about their distribution techniques, as well as what foods they are responsible for distributing.  Any processers or producers of foods that could be associated with the illness are then investigated by the appropriate responsible agency, whether local, state or federal. The testing and inspection of food products and the facilities of processers and producers may reveal the source of contamination.

Controlling and Stopping the Outbreak

                When the source of the foodborne illness is finally determined, public health agencies tackle the extensive and complicated task of identifying each location the food product has been distributed to.  The food product can be stocked on grocery shelves, in the kitchens of food establishments, and in the pantries and refrigerators of the public. To prevent further illness, it is essential for agencies to identify all possible places where the product has come to be located. The most effective way to warn the public of danger is through the recall process.  Not only will agencies inform and warn grocers and food-service establishments, warnings to the public will be posted on any agency websites and disseminated through the media.  When a specific product is suspected, an agency may give appropriate warnings even without solid confirmation.

Determining an Outbreak’s End

                The end of an outbreak can be determined when the number of illnesses drops back down to normal levels. Surveillance system data, when analyzed, will identify when the number of reported illnesses are in decline. However, even when the number of illnesses does, in fact, seem to be in decline, public health agencies will continue to monitor data to ensure that another spike in illnesses does not ensue after the initial decline.  If another spike occurs, the investigatory process continues.