Outbreak investigators are looking for the source of Listeria monocytogenes that has sickened two dozen people across 13 states, killing two. The outbreak could be related to illnesses in Canada that have been linked to diced chicken. 

Whole genome sequencing, sometimes called DNA fingerprinting, shows the sick people in the United States are infected with bacteria that is closely related, meaning “people in this outbreak are more likely to share a common source of infection,” according to an alert from the U.S. Centers for Disease Control and Prevention.

“Whole genome sequencing showed that the type of Listeria making people sick in Canada is closely related genetically to the Listeria making people sick in the United States,” the CDC reports.

As of Friday, a total of 24 people infected with the outbreak strain of Listeria monocytogenes had been confirmed in 13 states. Specimens were collected from ill people from July 20, 2017, through Aug. 1 this year. All but three of the outbreak patients became sick from March through August 1 this year.

Sometimes it is difficult to identify Listeria patients and the foods that could have caused their illnesses because it can take up to 70 days after exposure for symptoms to develop. Also, special tests are required to diagnose Listeria infections, which can mimic other illnesses.

The sick people in the United States have ranged in age from 35 to 92 years, with a median age of 72. Sixty-three percent of the ill people are female. Of 23 ill people with information available, 22 hospitalizations have been reported in the U.S.

The Public Health Agency of Canada is also investigating an outbreak. The agency has found Rosemount brand cooked diced chicken as a likely source of the outbreak. Rosemount cooked diced chicken was supplied to institutions including cafeterias, hospitals and nursing homes wher many of the individuals who became sick in Canada resided, or visited, before becoming ill.

On Aug. 18 the Canadian Food Inspection Agency issued a recall for the implicated Rosemount brand cooked diced chicken. Whole genome sequencing showed that the type of Listeria making people sick in Canada is closely related genetically to the Listeria making people sick in the United States.

However, the CDC reports no definitive links to any specific food, grocery stores or restaurants have been identified by investigators in the United States.

“What is perplexing is why there is no confirmation by FSIS and CDC of the source of the outbreak,” said Bill Marler, managing partner at Marler Clark and publisher of Food Safety News.

“According to Health authorities in Canada, Rosemount brand cooked diced chicken has been identified as a likely source of the outbreak. And, according to FSIS, the recent recall of Tip Top Poultry in Georgia was prompted by The Food Inspection Agency (CFIA) notifying FSIS that a sample of product produced by Tip Top Poultry confirmed positive for the presence of Listeria,” added Marler.

“In addition Marler said, our own CDC has said that whole genome sequencing shows that bacteria isolated from ill people in both Canada and the U.S. are closely related genetically (presumably that applies to the Tip Top Poultry Listeria sample as well. This means that people in this outbreak are more likely to share a common source of infection.”