It's been well over a week since something rotten turned up in Germany. But scientists still aren't sure where it came from.
They suspect that an unusual variant of E. coli could have contaminated cucumbers, lettuce and/or tomatoes and German officials are still warning against eating them.
But the ultimate source of the foodborne illness continues to stymie them.
This outbreak is a serious one with 18 deaths so far. Germany reported 1,733 cases of illness on Friday, with 520 of them involving a syndrome that can lead to kidney failure, according to the Robert Koch Institute.
Though some officials say the outbreak appears to be stabilizing, more than 200 new cases have been reported since June 1. Most of the cases are near the city of Hamburg, but they've also been reported in at least 11 other European countries.
"It's very important to note that up until now the cases that have been observed have all been linked to Northern Germany," WHO epidemiologist Andrea Ellis told journalists in a media briefing in Geneva Friday. "People have either been travelers to Northern Germany or they've been residents of Northern Germany, who have then been identified in other places. In fact, that's been a very unique feature" of the outbreak.
You can listen to the full WHO briefing by clicking on the audio player below:
American experts tell Shots they're a little surprised the Europeans haven't been able to identify the source yet. "As someone watching from sidelines, it's distressing to have so many sick people and no source," Doug Powell, a professor of diagnostic medicine and pathobiology at Kansas State University and director of the International Food Safety Network, tells Shots. "They should have figured it out by now."
But others say tracking the source of most foodborne disease outbreaks is some of the toughest epidemiological work around, and ultimately requires a lot of luck.
"It's tedious, gumshoe stuff," says Michael Dunne, a microbiologist at the Washington University School of Medicine in St. Louis. "It's the same kind of deal as why a murder can't be solved within a week if no one saw the perpetrator and there wasn't much evidence at the crime scene."
How do you trace a food borne illness to its source? Dunne tells Shots that first, scientists take detailed food and travel histories of sick individuals. They also ask people who didn't get sick and live in the same area what they ate to serve as a control group. Doing these surveys for E. coli can be especially tricky because the more serious symptoms might not show up for seven days, and patients might not remember what they ate that long ago.
"It can be hard to get an accurate history of what they consumed, and investigators might insert their own bias into it, and ask, for example, 'Did you eat Spanish cucumbers this week?' and the subjects might assume they did," says Dunne.
After the food detectives gather as much data as they can, they put it together and look for statistically significant differences in foods and locations. Then they might go and look in people's fridges to see if they still have the potentially offending items which can be tested for the bacteria, hoping to match it with the exact same strain found in the patients.
Then they head to the grocery store or farmers' market in search of the seller to find out more about where the foods came from. When they have a good handle on the point of sale, they'll work further back in the supply chain to the farm. They look for possible contamination from cattle waste, and whether fertilizer is being used. "Anything that comes into contact with fresh produce – soil, water, and human hands — has the potential to contaminate it," says Powell.
Ultimately, investigators hope to be able to isolate bacteria from the source and match with the patient using molecular techniques.
Powell says he's still mystified as to where it could be coming from, but "whatever contamination is going on is obviously on a massive scale to have this many people sick. It's not just a little stray contamination."
Dunne says that back when investigators were probing the 2006 E. coli outbreak in spinach in California, which killed 4 people, they got lucky because they knew the location and were able to trace the contamination within about 10 days to a nearby cow-calf operation. The Europeans don't seem to have as much information.
And it is possible they may never find the source of the current European outbreak, especially, "if things are being imported illegally and nobody fesses up," says Dunne. But he expect the Europeans to have an answer to their E. coli mystery very soon.