How do you trace an outbreak? With difficulty!

The outbreak of E. coli O104:H4 food poisoning in Germany is possibly one of the worst outbreaks on record and certainly one of the most deadly.

Public health officials still don't know the source.  Spanish cucumbers were initially blamed, but it appears that might have been wrong, though it does appear that salad vegetables may have been involved.

The investigation of outbreaks like this is a science in itself, but must be frustrating for epidemiologists.  Our modern supply chains are extremely complex - foods are sourced from all over the world and suppliers may make up orders from many different primary producers.

André Picard, — Public Health Reporter of Canadian newspaper "The Globe and Mail" wrote a neat article with associated links and videos on the investigative process and its difficulties.  I have reproduced part of it below:

"...real life does not have tidy TV endings. Despite determined efforts and great expense, the precise source of poisoning is found in fewer than one-third of outbreaks. But here is a quick look at how public officials try to resolve those puzzles. 

First, there is a trigger event – usually people showing up in emergency rooms with violent cases of diarrhea and vomiting that are the hallmarks of food poisoning. Samples – stool, saliva, sometimes blood – are taken. Serious illnesses like E. coli get special attention: 
Reporting them to public health authorities is mandatory.

Data from lab tests are routinely sent to local public health authorities and to PulseNet, an electronic database. While most cases of food poisoning are sporadic, patterns can be spotted quickly – there can be a cluster in a single city, or a sudden spike over a wide geographic area. Labs do DNA fingerprinting of pathogens like E. coli that can show if the illnesses have a common source. 

In this instance, three deaths of patients with E. coli infection in Germany set off alarm bells. They all had the same DNA fingerprint, strongly suggesting a common food source.
When an outbreak is suspected, epidemiologists (disease detectives) start questioning those who are sick. In these “hypothesis-generating” interviews, patients are asked about specific high-risk foods (such as sprouts or unpasteurized milk) and for detailed recollections of what they have eaten in recent weeks. 

This can produce, fairly quickly, a list of suspected culprits. E. coli is a fecal bacterium, for example, so raw foods are a focus. In Germany, a day after the deaths, consumers were warned to not eat tomatoes, cucumbers or lettuce. Three of four cucumbers tested came from Spain, so the country was quickly identified as a culprit in media reports. 

Public health officials share information gathered from patients with food safety regulators (such as the Canadian Food Inspection Agency), who try to trace the commercial supplier of a common food source. 

In their interviews, epidemiologist collect information on where food is purchased; if many of the patients shopped at the same food chain, for example, the CFIA would identify their vegetable suppliers and try and trace the movement of food through distributors and back to the farm. 

But there can be hundreds, even thousands, of suppliers and contamination of food can occur at any point in the process of getting food from the farm to the table – during production, processing, storage, distribution or preparation.  Finding the “locus of contamination” becomes an almost insurmountable process of elimination. Currently, farms in Europe that produce vegetables that are consumed raw are being set upon by all manner of investigators who will test products, water sources and so on. 

The paradox is that the more people get sick, the easier it is to ultimately find the culprit because the number of food sources they have in common becomes smaller. Often, despite mountains of data, the leads run cold. But the demand for answers does not wane.
The challenge for public health officials and regulators is finding the right balance. They issue speculative warnings like “Don’t eat Spanish cucumbers,” but more often than not they are wrong, and that can cause grievous damage to food producers. 

At the same time, acting slowly and waiting for a definitive answer on the source can result in countless cases of illness and death, and still no definitive answer. 

To read the full article and access the video clips, click here.

Not all reporting is of this quality; even the BBC has broadcast a news item labeling E. coli as "a virus".  Didn't the reporter talk to an expert, or even a Level 6 food science student?