Seven sick people, most of them middle-aged men, scattered across three states. As epidemiology goes, that’s not much to go on.
To track outbreaks of foodborne illness, the “epi” sleuths usually need numerous sick people, preferably concentrated in one or two areas. Larger “clusters” of illnesses greatly improve the chance of figuring out what made everybody sick.
But you go with what you got, says Josh Rounds, a staff epidemiologist at the Minnesota Department of Health in St. Paul. And just three weeks ago, in mid-February, what the MDH had was three cases of E. coli O157:H7 in Minnesota, three more in neighboring Wisconsin, and one in Michigan. Three of the case patients had been hospitalized.
None of the sick knew each other, nor had they eaten at the same restaurant or church potluck. They live hundreds of miles apart. Yet lab tests showed they had contracted the same strain of E. coli, so they were all contaminated by a single source. Minnesota launched its investigation, and Rounds got the job.
Step One: “Team D,” short for Diarrhea, students in public health studies at nearby University of Minnesota, who work part time for the state health department. Most work in the evenings, working the phones, interviewing people who have been ill. Each interview follows the same format – a 12 page questionnaire, inquiring what victims ate before they got sick.
Rounds knows that routine. As a student at UM, he worked two years on Team D.
Meanwhile, he also was working with his Wisconsin counterpart — epidemiologist Rachel Kols, who was running a parallel investigation 250 miles away in Madison.
The initial interviews produced several possible leads. All seven victims had eaten ground beef, Rounds says. E. coli still originates in cows and was once known as the “hamburger disease.”
“But when you looked closely at the details, it didn’t look like ground beef,” Rounds says. “For example, one case ate only privately slaughtered meat — not from the grocery store.”
Pork was also a possibility, but that didn’t hold up under further investigation either. Investigations are largely about “ruling things out,” explains Kirk Smith, foodborne disease supervisor at MDH.
But there was another common denominator — nuts. Six of the seven victims reported they had eaten nuts in the days preceding their illness. And in each case, the nuts were purchased in bulk, scooped from bins at the supermarket. When epis called the seventh victim back, he recalled that, come to think of it, he had eaten nuts from the local supermarket.
More interviews narrowed the possibility even further. While some had eaten mixed nuts, each of the seven had consumed hazelnuts.
Bingo: They had a suspect. “Well, we had a hypothesis,” Rounds cautions.
But a powerful hypothesis. When you have seven sick people, and all of them ate hazelnuts from supermarket bins, you have an “overwhelming probability,” Smith says.
But you still need a specific source, and that job belongs to the state agriculture departments. Using the interview results, officials start at the supermarket and work back, looking for one source that shipped hazelnuts to all seven stores. They quickly focused on a California wholesaler, Los Angeles-based DeFranco and Sons. While there was no laboratory evidence of contaminated hazelnuts, the epidemiology was “powerful enough to implicate the hazelnuts even without labwork,” Smith says.
On Friday, DeFranco voluntarily recalled its hazelnuts, and health officials issued public warnings to consumers. And this week, officials identified the outbreak strain of E. coli O157:H7 on hazelnuts found in the home of one of the Minnesota patients.
Investigators previously have found Salmonella in nuts, but E. coli is unusual. How could nuts come into contact with cows (or other ruminants)? Are they contaminated throughout, or just on the surface? Rounds doesn’t know. Nuts are typically shaken from trees and then harvested from the ground, “which presents the opportunity for contact with cow feces,” he adds. “They may go through a rinse process, to clean the dirt off the shells. But it would not be hot water, so it’s not a ‘kill’ step.”
Ultimately, it took about two weeks to locate the source of the outbreak and get it off the market — not bad, Rounds says. Although health officials don’t expect any more illnesses, Canadian authorities report two or more illnesses north of the border whose DNA fingerprints match the hazelnut strain, but haven’t yet confirmed a link.
In figuring out the source of this outbreak, “I think the key was the close collaboration between two state health deparments, and with the agriculture departments,” Rounds says. “Smaller outbreaks with fewer cases present challenges. But if you put resources in, you really can solve it.”