Vol. 8, No. 3
March 2016

Network keeps pulse on outbreaks

Reprinting with permission of the Association of Public Health Laboratories

In 1993, a large multistate outbreak of E. coli linked to Jack in the Box restaurants sickened more than 730 people. Four people died and more than 170 had long-term health complications. It took one month to determine the cause of the outbreak, and even longer to find the culprit.

A DNA fingerprint of Listeria is prepared for upload to the PulseNet database.

After that outbreak, public health officials realized there was a need for a national network that would speed outbreak detection. Together, the Association of Public Health Laboratories and CDC worked to develop a system that would ensure member laboratories were using the same testing methods so strains could be compared in a single national database.

Three years after the outbreak, PulseNet was born.

PulseNet revolutionized foodborne outbreak detection and investigations by using DNA fingerprinting technology to detect clusters of foodborne pathogens. By using PulseNet, outbreaks can be investigated in a matter of hours or days rather than weeks or months. Rapid investigation of outbreaks means they can be addressed and resolved more quickly, which results in fewer illnesses. Without this network, many large, national outbreaks may never have been detected or investigated.

The Iowa story

Iowa was not part of the infamous national E. coli outbreak in 1993 that sickened hundreds and spurred the creation of PulseNet, the national molecular subtyping network created to rapidly detect foodborne outbreaks. Iowa’s State Hygienic Laboratory, however, is part of PulseNet and has been since 1998, two years after the program began.

“This DNA fingerprinting program has been instrumental in recognizing regional, national and international foodborne outbreaks that may have gone undetected for months or years,” said Nancy Hall, manager of Environmental Microbiology. “This program allows outbreaks and their causes to be identified much more quickly, preventing further foodborne illness.

“In my opinion – and that of many others – this has been one of the most important foodborne outbreak surveillance tools in preventing illness in the last 25 years.”

Through PulseNet, public health laboratories have identified the source of many foodborne outbreaks, including Salmonella Newport in cantaloupe, Listeria monocytogenes in cantaloupe, and Salmonella in peanut butter and Italian deli meat.

How does PulseNet work?

After a clinical laboratory tests a stool sample to determine exactly which bacteria has made a person ill, they send an isolate of that bacteria to the public health laboratory, which then performs DNA fingerprinting of the bacteria using pulsed-field gel electrophoresis (PFGE). That DNA fingerprint is uploaded into the PulseNet national database where microbiologists and epidemiologists from around the country can view the data and determine whether there are clusters of similar patterns that might indicate an outbreak.

What does this mean in real life terms?

When you get sick and go to the doctor, you might be asked to provide a stool sample. The doctor may send that sample to a clinical laboratory that determines – for example – that E.coli O157 has made you ill. The clinical lab sends an isolate to your state’s public health lab where they perform DNA fingerprinting using PFGE. The DNA fingerprint information is uploaded into the PulseNet database.

At the same time, people in three other states become ill. The DNA fingerprint of the bacteria that made them sick is uploaded into the PulseNet database, too. A microbiologist in your state lab notices that the DNA patterns are all the same, indicating a need for further investigation to determine if all became sick from the same source.

Epidemiologists from your state department of health may contact you for more information. The four of you each mention that you ate at the same national restaurant chain.

The public health system quickly jumps into high gear. They begin to look for additional cases and to identify where the restaurant’s many food items originated. CDC works closely with other federal partners such as FDA and USDA to try to determine the exact source of this outbreak.

What’s next for PulseNet?

While PulseNet has used PFGE testing for DNA fingerprinting, new technologies such as whole genome sequencing (WGS), will someday replace this older technology. Some public health laboratories and CDC laboratories are using WGS alongside PFGE to demonstrate the utility of this technology for real-time outbreak detection.

“PulseNet has been one of the most significant public health programs in the last 20 years,” said Nancy Hall, Environmental Microbiology manager. “This program prevents hundreds of thousands of illnesses and saves billions of dollars annually. The new technology, WGS, will only strengthen this valuable program and hopefully continue to shorten outbreak detection, prevent illness and strengthen product recalls.”