Over the past 35 years the proportion of foodborne outbreaks linked to the consumption of leafy green vegetables has substantially increased and that increase can not be completely attributed to Americans eating more salads according to research presented March 17 at the 2008 International Conference on Emerging Infectious Diseases in Atlanta, Georgia.
"Consumption of leafy greens has increased over the years, but it does not completely explain the increase in the proportion of foodborne outbreaks due to leafy green consumption," says Michael Lynch of the Centers for Disease Control and Prevention (CDC), a researcher on the study.
Prompted by the high profile E. coli outbreaks associated with spinach and lettuce in 2006, Lynch and his colleagues decided to investigate the incidence of foodborne disease outbreaks associated with leafy greens in the past. Using data from the CDC foodborne disease outbreak surveillance system they analyzed over 10,000 foodborne disease outbreaks reported between 1973 and 2006.
For the entire period, approximately 5% of all foodborne outbreaks were linked to leafy greens. Most of these (60%) were caused by norovirus, but some were caused by salmonella (10%) and E. coli (9%).
"Given recent experiences that was not a total surprise. What was interesting was when we compared the numbers to consumption data," says Lynch.
Using per capita availability of leafy greens in the United States as a proxy for leafy green consumption, the researchers compared per capita consumption of leafy greens with the proportion of foodborne outbreaks caused by leafy green consumption.
"During the 1986-1995 period U.S. leafy green consumption increased 17% from the previous decade. During the same period, the proportion of all foodborne disease outbreaks due to leafy greens increased 60%. Likewise during 1996-2005 leafy green consumption increased 9% and leafy green-associated outbreaks increased 39%," says Lynch.
Further investigation is necessary in order to determine other factors that may help explain the increase, says Lynch. While many foodborne outbreaks can be traced to a problem in food preparation, he notes that some outbreaks are fairly widespread, suggesting that contamination took place early in the production process, either on the farm or the processing plant.
"The proportion of outbreaks due to leafy greens has increased beyond what can be explained by increased consumption. Contamination can occur anywhere along the chain from the farm to the table. Efforts by local, state and federal agencies to control leafy green outbreaks should span from the point of harvest to the point of preparation," says Lynch.
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