The largest recorded foodborne illness outbreak in German history was caused by frozen strawberries imported from China, according to a study published in the February issue of Eurosurveillance.
At least 11,000 cases of norovirus were reported by 390 institutions – mostly schools and childcare facilities – between Sept. 19 and Oct. 7, 2012.
Epidemiologists were able to quickly identify dishes containing strawberries prepared in regional kitchens of a single catering company as the source of most of the illnesses. They also further traced the strawberries to one 22-ton lot imported frozen from China.
Investigators estimate that, assuming a similar level of contamination, potentially 11,000 cases of norovirus were prevented by the quick determination that strawberries were the source of the outbreak and the prompt withdrawal of one-half of the lot of strawberries from the market. They noted that additional illnesses were likely prevented since only a fraction of the delivered strawberries had been prepared for consumption, and they praised local public health agencies from the five impacted German states for rapidly communicating local outbreak information to the state level, allowing for a prompt, coordinated outbreak response.
The German Task Force on Food and Feed Safety found that some of the regional kitchens that received the contaminated frozen strawberries had heated them during preparation, while others had not. The study’s authors point out that heating of the berries could in part explain why not all institutions supplied by the regional kitchens that received the berries reported cases of norovirus.
In response to this outbreak, Germany has updated its recommendations for institutions caring for vulnerable populations, including schools, child-care facilities and elder-care facilities, to specifically include advice to heat frozen berries.
The authors note that, in recent years, several large norovirus, Hepatitis A and E. coli outbreaks in the European Union and the United States have been traced to fresh or frozen produce. They point out that trace-back investigations have, at times, been complicated or prevented due to a variety of factors, including political and economic issues.
While the authors state that contaminated water used in the production of the strawberries was likely responsible for the outbreak, they also note that, “A better understanding of how the berries became contaminated is crucial for developing long-term prevention measures upstream from the retailer.”
A Jan. 2, 2013, EU regulation requires that 5 percent of consignments of frozen strawberries imported from China be tested for norovirus.