Today (June 10) the Robert Koch Institute published an Information update on EHEC outbreak. It's a long document, with one passage of special interest: How the experts narrowed the culprit down to the sprouts. Excerpt:
Recipe-based restaurant cohort study
With a high probability, the results of the "recipe-based restaurant cohort study” finally permit narrowing down the source of the infection to the consumption of sprouts. It was possible to apply this methodological approach only after a sufficient number of restaurant customers could be identified to ensure adequate statistical power of this analysis.
To ascertain the consumption of raw fruit and vegetables by patients and controls more objectively and less dependently on memory, RKI used the following approach in the “recipe-based restaurant cohort study”:
Five groups (travel groups, clubs, etc) that comprised a total of 112 participants and included 19 individuals who acquired EHEC infection were questioned regarding the foods they consumed after eating in a common restaurant. Additionally, the menus ordered by the participants were identified by means of order lists and meal receipts.
The restaurant kitchen was questioned in detail regarding the preparation and the type and quantity of ingredients in each menu ordered by any of the study participants. Furthermore, available photographs taken by travel group members were analysed to confirm which food items, including toppings, were seen on the plates.
The data thus gathered was analysed in a cohort approach that permits the retrospective estimation of the relative risk of infection for the restaurant customers.
Results of this analysis showed that customers who ate sprouts had an 8.6-fold increased risk (95% confidence interval (CI) 1.5-∞) of EHEC/HUS illness compared to those who did not. This study also revealed that 100% of those who contracted the illness had eaten sprouts.
Insights from case-control-studies regarding the consumption of sprouts
Patients were questioned on the consumption of a large number of animal and plant origin food products, including sprouts, in the initial intensive exploratory interviews in Hamburg on May 20 and 21. In these, only 3 of 12 patients reported having eaten sprouts. Moreover, a relevant under-ascertainment of sprout consumption seemed unlikely since the participants of this survey demonstrated an exceptionally high dietary awareness.
Methodological standards in outbreak investigations call for inclusion of those exposures in the standardized questionnaires that can potentially explain a high proportion of outbreak cases. The inclusion of too many potential exposures may lead to false positive associations.
For these reasons, sprouts were not included in the questionnaires of the initial case control studies. Later, more extensive interviews subsequently conducted by RKI again included the consumption of sprouts. Overall, 16 (30%) of 54 patients who could answer questions on the consumption of sprouts in the context of these extensive interviews stated having eaten sprouts within the time frame consistent with the period of acquisition of the infection.
An additional case-control-study with the goal of further differentiating among plant food products was initiated on May 29, 2011 in Lübeck, Bremerhaven and Bremen using three healthy controls matched according to age, sex and place of residence to each of 26 HUS patients.
In this study, 6 (25%) of 24 cases stated having eaten sprouts in the incriminated period of infection, compared to 7 (9%) of 80 healthy controls. This association was statistically significant in univariate (odds ratio: 4.35, 95% CI 1.05-18), but not in multivariate analysis, thus limiting the explanatory power of this result.
Compared to controls, cases included in this study reported eating vegetables such as tomatoes, cucumbers and lettuce more frequently, a finding in accordance with the earlier case-control-studies. However, these associations were not statistically significant.
Overall, these findings are thus consistent with the results of the two previously published case-control-studies and show these vegetables to be frequently eaten together.
