A Complex Mystery: Finding the Sources of Foodborne Disease Outbreaks

I work in a group at CDC that investigates foodborne illnesses in the United States — illnesses like salmonellosis and E. coli infection.  One challenge we face during an outbreak investigation is trying to figure out the source of the outbreak. 

When a group of people consume the same contaminated food, an outbreak of illness can occur.  This group may be people who ate a meal together or people who happened to buy and eat the same contaminated item from a grocery store or at a restaurant. 

Here’s why outbreaks can be such a mystery:

• When people get sick from food, they often assume the cause was the last thing they ate before they started feeling ill.  That’s often not the case.  For many foodborne illnesses, it can take anywhere from several hours to several days before people start to feel sick.  The cause could have been something they ate several days ago, something they might not even remember eating. 
• The contaminated food usually looked, smelled, and tasted perfectly fine, making it sometimes very difficult to determine exactly what made them sick.
• If safe food production and handling practices were not followed,  the food could have become contaminated at any point, from the time the food was harvested or produced until it was eaten.
• People who get sick with a foodborne illness don’t always see a health care provider. When they do, the providers don’t always test for bacteria that cause foodborne illness.  These test results are very important, because CDC and other groups need the results to detect outbreaks.
• Because people are often not interviewed until weeks after they became sick, they may have trouble remembering what foods they had eaten or what spices and condiments they may have added to their food.

So, how do we figure out which foods are making people sick?  Some of the things we do are:

• Use technologies, such as “DNA fingerprinting” of bacteria from ill people to help determine which ones might be linked to a common source of infection.
• Interview people who have gotten sick to find out what foods they recently ate.
• Interview people who haven’t gotten sick to compare what foods they recently ate to the sick people.
• Study information from previous outbreaks to see which foods have often been a source before.
• Compare the types of bacteria found in food or ingredients during the outbreak to the types found in people who are sick.

How does this affect you? One thing to remember is that only a tiny fraction of foodborne illnesses are reported as part of an outbreak.  While it’s important to keep track of food recalls to avoid getting sick, it’s equally important to follow the basic food handling rules: Clean, Cook, Separate, and Chill. And, if you suspect that you have a foodborne illness, report it to your local health department. Often calls from concerned citizens like you are how outbreaks are first detected.

Questions and Answers

Posted April 14, 2010

Q. How about expiration dates for food in schools and hospitals?

A. The information we provide is intended for use by consumers in the home. For information about storage times of food in a food service environment, like a school or hospital, please contact your local or state health department. For information about food safety and the elderly, see Food Safety for Older Adults.

For consumer information on expiration dates and product dating for foods in the home, see Food Product Dating. The two charts under Storage Times at the bottom of the page provide details on refrigerator home storage of fresh and uncooked food products as well as processed products.

Q. Are there easy ways to determine whether an illness is caused by food poisoning when other types of illnesses have similar symptoms?

A. Unfortunately, many different kinds of illnesses can cause diarrhea, fever, or abdominal cramps, and they can spread in different ways, making it difficult to determine if your illness was caused by a food or by something else. Some common viruses can spread from one person to the next without involving food. Many bacteria and parasites can spread through food, and also water, or from animals or from other infected people. 

Determining which bacteria or other germ is the cause of the illness depends on laboratory tests. Even if once you know the germ that caused it, determining that it came from food can take a public health investigation that is not often done. If your illness is severe enough, visit your health care provider. If you are concerned that it really is from food, call your local health department.