Anatomy of an Outbreak

Foodborne diseases are estimated to account for 76 million illnesses, 325,000 hospitalizations, and 5,000 deaths per year in the U.S. alone, according to the Centers for Disease Control and Prevention (CDC). Over the past several years, high-profile outbreaks involving produce and other foods have raised questions about whether our food safety protection system is adequate to detect and respond to outbreaks of foodborne illness. In 2006, contamination of fresh spinach with the bacteria E coli O157:H7 led to one of the largest outbreaks of foodborne illness in recent years. In 2008, public health officials investigated a multi-state outbreak of Salmonella Saintpaul, which led to more than 1,300 confirmed cases in 43 states, the District of Columbia, and Canada. Following a two-month investigation, jalapeno and Serrano peppers from Mexico were implicated, and it is possible that raw tomatoes were responsible for some of the earlier illnesses.

While produce is only one of many food categories linked to outbreaks of foodborne illness, it is the major one and poses special challenges. Fresh produce is especially vulnerable to contamination because it is grown in a natural environment (a field or orchard) and is often consumed without cooking or other treatments that could destroy bacteria. At the same time, consumption of produce has increased dramatically, a positive development from a nutrition perspective but a challenge from a food safety perspective because of the diversity of produce sources. Produce also is difficult to trace back to its source because it is perishable and usually no longer available for testing by the time consumers become ill. Fresh fruits and vegetables often are sold loose without any packaging that could identify the source, and repacking produce from multiple sources is common.

Although foodborne disease is a serious concern, in general, the U.S. has a very safe food supply. Public health agencies have made great strides in detecting and responding to outbreaks over the past 10 years, but challenges still remain in finding the source and removing product from the marketplace.

Detecting Outbreaks

Detecting outbreaks begins with identifying individual cases of foodborne illness. The U.S. has an active surveillance system called FoodNet—a cooperative venture among federal, state, and local agencies in 10 states. Staff routinely contacts all clinical labs to collect information on every confirmed case of foodborne illness. For a case to be confirmed, the ill person must seek medical care and submit a specimen in which the pathogen is identified. FoodNet provides a better picture of how many illnesses are occurring, where they are occurring, and what pathogens are responsible.

FoodNet is complemented by another system called PulseNet, which tracks disease-causing bacteria by “DNA fingerprinting.” Public health officials at the state, local and federal level perform DNA “fingerprinting” on disease-causing bacteria isolated from people who have become ill, and these “fingerprints” are uploaded to the national database located at CDC, where they are available instantly to public health officials nationwide. Database managers at CDC perform regular searches, looking for clusters of patterns that are the same. By finding similar patterns through PulseNet, scientists can determine whether an outbreak is occurring, even if the affected people live far apart.

Identifying the Source

Unfortunately, PulseNet cannot identify the source of an outbreak by itself. It identifies clusters of cases that likely have a common source, such as a food distributed to numerous states. Once the recent cluster has been identified, an epidemiological investigation is needed to determine if the cluster truly represents an outbreak and to identify the cause or causes. State and local public health agencies are primarily responsible for conducting outbreak investigations. At the state’s request, CDC will help. This plays a critical role in the national coordination of response efforts. Multi-state cooperation is key due to the wide distribution of food in the marketplace.

The investigation includes sample collection in facilities and a review of animal management practices (animals can contaminate produce in the fields), processing practices, and water use. In the case of the recent produce outbreak, epidemiological studies indicated that jalapeno and Serrano peppers grown, harvested, or packed in Mexico were the cause of some clusters of illness, but that raw tomatoes consumed early in the outbreak could have caused some illnesses. The case for identifying an outbreak source is strengthened when a genetic fingerprint of the pathogen in an affected person can be matched with the fingerprint in a suspected food. This occurred with one of the jalapeno pepper samples tested. The ability to make this match is a relatively new phenomenon.

Taking Action

Once CDC, working with state and local governments, identifies the possible foods associated with an outbreak, it notifies the appropriate regulatory agency.

USDA has regulatory authority over meat, poultry, and processed eggs, and FDA is responsible for all other foods.

At that point, the regulatory agency begins a trace-back investigation in an attempt to identify the specific source of contamination. This is done by tracing the food suspected of making people sick back through the supply chain from the retailer or restaurant and inspecting points in the supply chain to determine where the contamination most likely occurred. Bills of lading (receipts of goods accepted for transportation) and invoices will be examined, and information is obtained on the practices and conditions under which the product was stored and handled. If a source is identified, the government works with industry to recall the product if appropriate. In the case of Salmonella Saintpaul, a distribution center in McAllen, Texas worked with FDA to recall the contaminated product in the U.S. The agency also may issue a public health alert to advise consumers to avoid certain products.

Improvements Needed

The infrastructure that exists to detect and respond to outbreaks needs improvement. Not all states have robust systems for identifying and investigating illnesses or even standardized questionnaires with which to interview ill patients about what they have eaten.

Improvement in trace-backs also is needed to identify which foods are responsible, and which foods are not. The pathways that fresh produce travels from field to consumer have become increasingly complex, with items changing hands many times. FDA is working extensively with states and the fresh produce industry to encourage the use of traceability procedures and technology.

More research also is needed to better understand how pathogens are introduced into the environment and how they are affected by processing practices. At this year’s International Association for Food Protection conference in Columbus, Ohio, researchers described studies examining how pathogens might enter produce through the stem and cuts in the surface, and how the temperature of water used to wash or cool produce after picking might play a role in introducing pathogens into produce.

Prevention also is key—the best way to reduce outbreaks is to provide a safer product. FDA issues various guidance documents to provide industry with “best practices” to minimize contamination. For example, FDA has Good Agricultural Practices that address factors such as irrigation water, animal control, worker health and hygiene, and sanitation of facilities and equipment. The agency issued a guidance document for fresh-cut produce in 2008 (

As another step, FDA announced in August it would permit the use of irradiation to treat iceberg lettuce and fresh spinach. Irradiation already is approved for meat and poultry, molluscan shellfish, and spices. Irradiated products must be labeled to indicate that they have received this treatment.

In the years to come, we can expect improvements in the infrastructure that exists to identify outbreaks and their sources and more research to find ways to reduce contamination during and after production. Meanwhile, consumers play a key role in food safety. While they cannot prevent all illnesses, there are a number of steps they can take to reduce them. For produce, FDA advises consumers to take these general precautions:

  • Refrigerate or discard cut, peeled or cooked produce items within 2 hours.
  • Avoid purchasing bruised or damaged produce items, and discard any that appear spoiled.
  • Thoroughly wash all produce items under running water.
  • Keep produce items separate from raw meats, raw seafood, and other raw produce items (separation helps to avoid cross contamination).
  • Wash cutting boards, dishes, utensils, and counter tops with hot water and soap when switching between types of food products.