Deadly bug in sprouts

SummarySeed sprouting provides an ideal environment with optimal temperature and humidity for bacterial growth. Once the bugs are there they are very difficult to remove from fresh sprouts. A German case study shows the serious impact of sprout contamination with a particularly nasty bug that can cause diarrhoea, kidney disease and death.

On 21 May 2011, Germany reported an outbreak of Shiga-toxin producing Escherichia coli (STEC), serotype O104:H4. At the conclusion of the outbreak at least 4,300 cases of diarrhoeal disease, 773 cases of haemolytic uraemic syndrome (HUS) and 50 deaths across Europe linked to the outbreak in Germany had been reported to the European Centre for Disease Prevention and Control (ECDC). In addition, outside the EU eight cases of STEC and five cases of HUS, including one death had been reported in the USA, Canada and Switzerland through the international health regulations (IHR), all with recent travel history to Germany.

The clinical onset of the last outbreak-related case in Germany was 4 July 2011. The Robert Koch Institute (RKI) in Germany announced the end of the E. coli outbreak on 26 July 2011 after more than two months of intensive investigative activities.

Sprouted seeds

Early case-control studies conducted by the RKI demonstrated that clinical disease was associated with the consumption of fresh salad vegetables. The high proportion of adult women among cases, was consistent with fresh salad vegetables as the source of infection. This led to a false warning about some Spanish grown vegetables causing havoc for some Spanish growers. Later, a detailed cohort study demonstrated an association with sprouted seeds. Epidemiological studies on an associated French outbreak also implicated sprouted seeds as the outbreak vehicle.

A tracing back and tracing forward study showed that most of the clusters could be attributed to consumption of sprouted seeds from one producer in Germany. Investigation of the production site showed no evidence of environmental contamination. This left the seeds used for the sprout production as the prime suspect vehicle of infection. Fenugreek seeds were found to be common to both outbreaks and that a specific consignment of fenugreek seeds imported from Egypt was the most likely link between the outbreaks.

Import ban

A ban on imports into the EU of Egyptian fenugreek seeds and certain other sprouting seeds was imposed in July 2011 after the European Food Safety Authority said this was the most likely cause of the E. coli outbreaks.

The European Commission’s Food and Veterinary Office (FVO) visited Egypt in August 2011 and found that Egypt did not differentiate between seeds for sprouting and seeds for planting. The trace-back exercise found that three implicated lots were produced in upper Egypt by the same farmer in separate farms grown under organic conditions.

The Egyptian investigation found no evidence of STEC O104:H4 presence, although there was plenty of potential for contamination from human populations and animals, and problems with analytical methods. The FVO said Egypt must ensure that seeds produced specifically for sprouting must comply with hygiene rules and microbiological criteria.

Given an exchange of information with the Egyptian authorities and new measures to prevent contamination, the ban on fenugreek imports from Egypt was lifted on 31 March 2012.

Rare strain

The published data for STEC O104:H4 are scarce as this is a very rare serogroup infecting humans in Europe and globally. According to the information reported to ECDC, there were 10 reported cases of STEC O104:H4 infection in the EU Member States and Norway during 2004-2010. Five of the 10 cases between 2004 and 2010 were related to travel to Afghanistan (2008), Egypt (2010), Tunisia (2009, 2010) and Turkey (2009).

In addition to those cases reported to ECDC, a review of the scientific literature revealed that STEC O104:H4 has been isolated twice in Germany in 2001 and once in Korea in 2005. The German isolates differed from the 2011 outbreak strain.

Inherent problems in sprout production

The preparation of fresh sprouted seeds rarely includes a step where bacterial contamination is eliminated. Hence, food preparation of fresh sprouted seeds is based on the understanding that they are sold as ready-to-eat, i.e. safe to eat as is, or following only minimal preparation. For fresh produce, this assumes and relies on a production process which prevents contamination and an ability to detect contamination when it occurs. These conditions have proven not to be satisfied in this case.