U.S. food safety and health agencies are continuing to expand the use of whole genome sequencing (WGS) to detect outbreaks of life-threatening foodborne illness, especially clusters of infection from Listeria monocytogenes. A 2014 Listeria YouTube video presentation shared recently by the Centers for Disease Control and Prevention depicts how the use of WGS solved a mysterious Listeria outbreak that was sickening pregnant women and infants in Maryland. The same outbreak caused a Listeria death in California. More recently, WGS was credited for helping to solve a multi-state outbreak of Listeriosis associated with mung bean sprouts produced by Wholesome Soy Products Inc. Currently, WGS is at the center of the public health response to a 10-state outbreak of Listeria associated with consumption of tainted caramel apples. The outbreak has caused four Listeria deaths in Texas, California and Minnesota.
“The faster we can find causes, the faster we can stop outbreaks and save lives,” said Fred Pritzker, a food poisoning attorney who represents victims of Listeria outbreaks.
Listeria is a microscopic bug that hides in many foods, can grow even when refrigerated and sickens an estimated 1,600 Americans per year. Of those who contract the infections, one in five die. And at least 90 percent of the outbreak patients who contract listeriosis belong to one of four basic risk groups: Pregnant, infant, over 65 or immuno-compromised. The dangerous profile of this pathogen prompted the CDC, FDA, USDA and National Institutes of Health do emphasize the use of WGS to stop outbreaks as fast as possible and save lives.
Whole genome sequencing is an advanced detection method that examines the DNA of microbes to discern one strain of pathogen from another. It’s considered a more exacting science than the more prevalent method of detection in food poisoning outbreaks: pulsed-field gel electrophoresis (PFGE). Compared with PFGE, WGS provides a clearer distinction of genetic differences among Listeria isolates. Strains of Listeria and other pathogens that are highly related under the WGS technique are more likely to have a common source, giving investigators greater confidence in grouping clusters of illness detected by our national public reporting system of foodborne illnesses.
In this year’s Wholesome Soy Products outbreak, WGS found five Listeriosis patients in Illinois and Michigan infected with highly related strains of Listeria. Interviews with two of the case patients helped identify sprouts as a possible source of the infections and WGS soon isolated the same strain of Listeria from mung bean sprouts produced by Wholesome Soy. All five known victims of this outbreak were hospitalized and two of them died, but the investigation halted production of the contaminated sprouts that had sickened people over the summer. During FDA inspections of the Wholesome Soy Products in August and October 2014, a field team observed unsanitary conditions.
“The high degree of genetic similarity between isolates from ill people and from mung bean sprouts and environmental samples collected at Wholesome Soy Products, Inc. shows that the food was contaminated with a strain of Listeria monocytogenes that can cause serious illness,” the CDC reported.
Highlighted in the CDC informational video on the use of WGS was 2013 Listeria death and outbreak associated with cheese products produced by Roos Foods of Kenton, Delaware. The products were recalled early this year and the FDA suspended the food facility in March. “Whole-genome sequences of the Listeria strains isolated from Roos Foods cheese products were… found to be highly related to sequences of the Listeria strains isolated from the patients,” the CDC said in its final report.
The dramatic outbreak was first detected in pregnant women and infants in Maryland starting in August, but then the cases seemed to stop. In October more cases were detected and a person in California died. As more health officials were alerted to the outbreak, the state of Virginia reported a finding of Listeria in soft cheese made at Roos Foods. WGS matched it to the strain that was causing illness and the outbreak was solved.
