Outbreak strain often not found in E. coli investigations

Investigations into E. coli outbreaks often fail to find the outbreak strain in food or environmental samples, according to a review.

Scientists used Shiga toxin-producing E. coli (STEC) as a model pathogen to investigate the success of outbreak strain isolation from food or environmental samples during investigations.

The main reasons why the outbreak strain was not isolated were lack of sample availability, complex properties of food products, and methodological issues around laboratory isolation. 

A total of 223 outbreaks from January 2000 to August 2019 were included in the study published in the journal Eurosurveillance. Food and/or environmental samples were available for testing in 137 investigations, and the outbreak strain was isolated in 94 investigations. For 89 outbreaks no samples were available for testing.

There was no significant effect of STEC serovar or size of outbreak on likelihood of successful outbreak strain isolation. Isolation success ranged across commodities from 86 percent for beef-related outbreaks to 50 percent for salads and leafy greens. In 27 outbreaks with samples available for testing, an additional STEC strain was isolated alongside the outbreak strain. Risk management action was taken on epidemiological evidence alone in 21 incidents.

Providing evidence to support hypothesis

When an outbreak occurs, epidemiological, microbiological and environmental intelligence are used as part of the investigation to identify the food vehicle(s) involved, the source of the pathogen and any other contributing factors. Isolating the outbreak strain (as identified from the clinical samples of cases) from food or environmental samples is considered strong evidence of a causal link.

Scientists said it is recognized that for many STEC investigations, the outbreak strain is not isolated from the food or environmental source implicated by epidemiological investigations. They added the scientific value given to epidemiological findings in the absence of microbiological evidence is sometimes called into question.

Of the 223 outbreaks identified, 184 were caused by E. coli O157. Outbreak case totals ranged from two to 3,842, and the review covered 13,003 illnesses and 2,664 hospitalizations.

Outbreaks from 17 countries were included. The highest number were from the United States, the United Kingdom, Canada, and France. Nine outbreaks affected multiple countries.

For 65 outbreaks, both food and environmental samples were available for testing, with the outbreak strain isolated in 42 of these. For 10 investigations the outbreak strain was isolated from both food and environmental samples, for 16 it was only detected from the environmental sample, and for the remaining 16 investigations the outbreak strain was only isolated from food.

Outbreak size doesn’t matter

Beef, salads, and leafy greens and unpasteurized dairy products accounted for 62 percent of all outbreaks. Raw dairy products were more commonly associated with smaller outbreaks and there were no outbreaks of fewer than five cases linked to salads and leafy greens.

In a fifth of outbreaks, a STEC strain genetically distinct from the outbreak strain was detected and in a third of these, only a genetically distinct STEC strain was found in the implicated food. Researchers said the presence of different STEC strains shows that the food safety management systems in place are not sufficient to prevent contamination of the final product.

Although larger outbreaks were more likely to have samples taken, there was no statistically significant association between the likelihood of outbreak strain isolation and size. Scientists said this was an unexpected finding as outbreaks with more cases will often have more patient information, more food for sampling and potentially more resources for epidemiological investigations. Reasons for this could include a lack of available food for testing due to a short shelf life or product turnover.

For seven outbreaks, an alternative batch was tested but was negative for the outbreak strain.

In 16 incidents, the risk management decision was to take no action. When the rationale was given, four outbreaks were over before action could be taken, in one case the investigation was inconclusive and in another a home-made product was implicated. In 21 incidents, some form of risk management action was taken based on epidemiological evidence and mostly involved a product recall from consumers.

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Source: https://www.foodsafetynews.com/2025/01/outbreak-strain-often-not-found-in-e-coli-investigations/