Iowa Egg Recalls: Anatomy of a Food-borne Illness

This past summer, hundreds of people across several states were sickened by eating eggs contaminated with the bacteria Salmonella serotype Enteritidis. The illnesses led to one of the largest food recalls in recent times and officials continue to investigate the lapses that led to the widespread infections. While the incident emphasizes the need for better food safety to prevent food-borne illnesses, it also illustrates how the public, their doctors, and public health laboratories play an important role in helping to limit the number of new cases when an outbreak occurs.

Outbreaks of food-borne illnesses, more commonly known as food poisoning, occur when two or more people become sick after eating the same food or drink that is contaminated with the same bacteria or toxin. Identification of food poisoning outbreaks has been increasing in recent years, in part due to better detecting and reporting of cases but also due to the increasing globalization of the food supply.

When cases of suspected food poisoning arise, doctors are encouraged to get complete medical histories and to obtain samples for laboratory testing, most often samples of stool for culture and for other types of specialized testing. People who are ill are asked to cooperate by supplying a sample for analysis so that clinical laboratories can test the samples for pathogens.

Suspected cases of food-borne illness are reported by doctors and clinical laboratories to local or state public health agencies and investigations are set in motion to pinpoint the likely source of the illnesses. Pathogens or stool samples from affected people are sent to public health laboratories so that special testing can be done. These labs have the capability to perform molecular tests that can determine a "DNA fingerprint," also known as a pulsed field gel electrophoresis (PFGE) pattern, of the pathogen present in stool samples. This information is entered into PulseNet, a database used by local and state public health agencies, federal food safety regulatory labs, and the Centers for Disease Control and Prevention (CDC) to track illnesses. It allows for rapid comparison of DNA fingerprints in order to determine if illnesses and clusters of illnesses are related.

In May of this year, the CDC noted an increase in the number of cases of gastrointestinal infections due to a specific identifiable strain of Salmonella serotype Enteritidis. These bacteria cause symptoms such as fever, abdominal pain, and diarrhea. The symptoms typically first appear about 12 to 72 hours after ingestion of a contaminated food source and can last 4 to 7 days. The illness may require supportive care but usually resolves without complications. However, the very young, the elderly, and those with weakened immune systems are at an increased risk of severe symptoms and may need to be hospitalized.

By July, the number of cases reported to PulseNet rose dramatically. The CDC, the US Food and Drug Administration (FDA), and US Department of Agriculture cooperated to investigate possible sources of the outbreak. Eventually, two farms in Iowa were identified as suppliers of the eggs that were contaminated with Salmonella. Environmental samples taken from the farms were tested and the bacteria that were detected had the same DNA fingerprint as the bacteria from stool samples from sickened people. As a result, the two Iowa farms issued recalls of eggs between August 13 and August 20. The public was notified of the problem and advised to discard eggs and egg products linked to the two farms. (For the latest information on the egg product recalls, visit the FDA web site.)

Meanwhile, the FDA and other officials continue to work to improve egg safety. Clearly more needs to be done to avoid widespread infections such as those that occurred this summer. Nevertheless, the actions of the public, health care providers, public health labs, and other agencies helped to limit spread of the infections. An even greater number of people could have been affected if doctors had not recognized the symptoms and if sickened people had not agreed to be tested. The ability of public health laboratories to use sophisticated techniques to differentiate unique strains of food-borne bacteria was an important part of the investigation. Communication and cooperation across states and regions are likewise vital to improved food safety, especially when we eat foods that may be grown and transported from anywhere in the world.

Ideally, contaminated food would never make it into the food supply, but realistically, there will be times when this occurs. There are steps the public can take to help avoid becoming sickened by contaminated food. Also, if you think you have an illness caused by food from a restaurant or other business, the local public health agency needs to know. See the links in the Elsewhere on the Web section below for food safety tips and what to do when food poisoning strikes.

Tests: Stool culture
Conditions: Diarrhea

FoodSafety.gov: Keep Food Safe, The Basics
FoodSafety.gov: Report a Problem with Food
CDC: Tips to Reduce Your Risk of Salmonella from Eggs
Familydoctor.org: Food poisoning

(Update September 20, 2010) Centers for Disease Control and Prevention. Investigation Update: Multistate Outbreak of Human Salmonella Enteritidis Infections Associated with Shell Eggs. Available online at http://www.cdc.gov/salmonella/enteritidis/ through http://www.cdc.gov. Accessed October 2010.

(August 27, 2010) US Food and Drug Administration. Frequently Asked Questions and Answers: FDA's Investigation into the Salmonella Enteritidis Outbreak Involving the Recall of Shell Eggs. Available online at http://www.fda.gov/Food/NewsEvents/WhatsNewinFood/ucm223723.htm through http://www.fda.gov. Accessed October 2010.

(November 2009) Power Point presentation by Michael Pentella, PhD, D(ABMM), Associate Director, University of Iowa Hygienic Lab, Clinical Associate Professor, University of Iowa, College of Public Health, Iowa City, Iowa.

(January 2009) US Food and Drug Administration. What you should know about government response to food borne illness outbreaks. Available online at http://www.fda.gov/Food/ResourcesForYou/Consumers/ucm180323.htm through http://www.fda.gov. Accessed October 2010.

(August 10, 2010) Obasanjo O. Foodborne Illness Primer for Physicians and Other Healthcare Professionals: A Review. Medscape article. Available online at http://www.medscape.com/viewarticle/726313 through http://www.medscape.com. Accessed October 2010.

FoodSafety.gov. How Government Responds to food illness outbreaks. Available online at http://www.foodsafety.gov/poisoning/responds/index.html through http://www.foodsafety.gov. Accessed October 2010.