Sesame seed paste containing a rare strain of Salmonella sickened 23 people in 7 states and the District of Columbia last year, reveals a new report from the U.S. Centers for Disease Control and Prevention.
The article, published in CDC’s Morbidity and Mortality Weekly Report last week, marked the first time the government has told the public about the Salmonella Bovismorbificans outbreak, which lasted from August through November of 2011.
Illnesses were largely concentrated in the Mid-Atlantic region, with eight in Washington, D.C., seven in Maryland, three in Virginia and one apiece in Delaware and New Jersey. Three cases were also reported outside this region – one in California, one in Michigan and one in New Hampshire.
The hunt for the outbreak source was a long one, complicated by the fact that the vehicle turned out to be an ingredient mixed into a finished product.
While health officials had located the outbreak strain of Salmonella in hummus made at a D.C.-area restaurant by mid-November, it would be another six months before imported tahini was named as the contaminated ingredient in the hummus.
The outbreak was identified September 27, 2011 when the District of Columbia Public Health Laboratory found that Salmonella isolated from three different patients had matching DNA fingerprints. They checked the DNA patterns they had found against PulseNet, the national pathogen subtyping database used for disease surveillance, and discovered that six other infections of this strain had been reported over the last 60 days. A total of 23 cases would eventually be linked to the outbreak.
Interviews with 22 of the victims revealed that 20 had eaten in a restaurant in the D.C. metropolitan area in the week before getting sick. The focus was further narrowed when 14 out of 15 patients asked about restaurant type said they had eaten at a Mediterranean-style restaurant.
Hummus was the most commonly reported food, eaten by 10 out of 15 patients interviewed about specific food types.
When asked about restaurant names, 13 out of 15 patients had eaten at one of three Mediterranean-style restaurants, which CDC deemed Restaurant A, Restaurant B and Restaurant C for its report. All three restaurants turned out to have the same owner. Health officials then learned that food for all three locations was prepared at Restaurant A.
Investigators from the D.C. Department of Health visited Restaurants A and B and collected 15 samples of finished product. The outbreak strain of S. Bovismorbificans was found in a sample of hummus from Restaurant A.
This led health officials to ban distribution of hummus and all hummus ingredients from Restaurant A.
“When the investigation team felt like we had sufficient evidence of a possible exposure associated with one of the three restaurants, we worked quickly to restrict the sale of the suspected food to customers by issuing an embargo,” said Tiana Garrett, officer at CDC’s Epidemic Intelligence Service and part of the investigation team in an interview with Food Safety News.
“We were really diligent in looking for illnesses with an onset date after that embargo, but we didn’t find any,” Garrett said.
Health officials believed this was a sign that, by embargoing the hummus, they had likely curtailed the outbreak.
Ingredients – A Tricky Outbreak Vehicle to Uncover
But the question remained – what ingredient in the hummus was contaminated?
“We always have to take into consideration if there are particular ingredients associated with preparation of foods that may need to be investigated as well,” explained Garrett. “In this particular outbreak, when it was determined that the hummus was associated with foodborne illness, we knew that hummus was prepared with different ingredients, so we wanted to make sure that we looked at those individually.”
But samples collected from all ingredients used to make the hummus tested negative for Salmonella Bovismorbificans, meaning there would be no proof in the pudding.
And it didn’t look like patient interviews would provide any more answers.
“Ingredients are considered a “stealthy” vehicle for foodborne illness, said Garrett. “Patients may not be aware they were exposed to a particular ingredient used to prepare a food item, and that makes it difficult to trace.”
Another thing to consider was that a contaminated ingredient that ended up in the hummus may have come into contact with other foods, sickening those patients who didn’t report eating hummus.
“It’s also possible that other foods they had eaten may have been prepared in the same environment with the tahini and then somehow some tangential exposure may have happened there,” said Garrett. “We wouldn’t know about those instances if the patient wasn’t able to report them.”
Though the outbreak was over, the investigation into its source remained unfinished as 2011 drew to a close.
The missing link was found in May of the following year, when a traceback by the U.S. Food and Drug Administration revealed that the tahini used to make Restaurant A’s hummus had recently been associated with multiple Salmonella outbreaks in Canada. All tahini linked to these outbreaks had been imported from the same company in Lebanon.
FDA has now mandated that all tahini products coming from this foreign company be tested for Salmonella before entering the U.S. and has recommended that U.S. and Canadian officials partner to inspect the tahini manufacturing plant.
Why Wasn’t the Public Notified?
While a foodborne illness outbreak in ongoing, it is common practice for CDC (in the case of a multistate outbreak) or state and local health departments to issue warnings to the public, naming the implicated food if a source is suspected and illnesses can still be prevented, or simply alerting people that there has been a spike in infections from a certain pathogen.
However, a search of public health advisories issued by the D.C. Department of Health, the Maryland Department of Health and Mental Hygiene and the Virginia Department of Public Health from the time the outbreak was detected through when it ended reveals that no alerts about the outbreak were issued by the three health departments who reported multiple illnesses in their states.
Garrett explained that the outbreak investigation team, which included state health officials and representatives from CDC, chose not to issue a public health advisory because no more illnesses occurred after hummus was identified and embargoed on November 18.
“We didn’t have any additional cases with onset dates that occured after the embargo,” said Garrett. “As a group we collectively decided that it wasn’t necessary to notify the public; however if we had found additional cases after the embargo we would’ve considered notifying the public.”
While the onset of the last illness recorded was actually on November 21, 3 days after the embargo was issued, it is possible this victim ate the contaminated hummus before the embargo was implemented.
During its investigation of Restaurants A, B and C, the D.C. Department of Health also discovered multiple food safety violations at the establishments, including inadequate food temperature control, insufficient hand washing and the presence of pests and insects, reports CDC. Since these restaurants have not been named, Food Safety News could not access their inspection reports in D.C. Department of Health’s inspections database.
Public Health Lessons
S. Bovidmorbificans is rarely seen in the United States, having been identified in only five other foodborne outbreaks in the country since 2001. This is the first time the strain has been implicated in a Tahini outbreak in the U.S.
However, Garrett says, this outbreak highlights an important public health lesson: tahini, or sesame seed paste, is a known vehicle for foodborne bacteria. The substance is high in fats, and therefore provides an ideal home for bacteria, much as peanut butter does, explains the CDC report.
“The main message that we wanted to convey in the article is how important it is for public health officials and consumers to be informed that products that are made with imported sesame pastes have been shown to be associated with Salmonella outbreaks and that they should be considered as possible sources for foodborne illness in the United States in the future.”
In fact, contaminated sesame seed paste was in the news less than 10 days before the CDC article was published after a supply of tahini was stolen from a California importer’s warehouse, where it was being stored because a sample had tested positive for Salmonella. The tahini, which had been imported from Lebanon, was awaiting destruction. FDA warned the public that the stolen, potentially contaminated tahini may be on the market.
Garrett says this tahini is not known to be linked to the product that caused last year’s outbreak.
“To the best of my knowledge, the tahini manufacturer mentioned in this article is not the same as the one that was implicated in our outbreak,” said Garrett in an e-mailed statement to Food Safety News.© Food Safety News