Leafy Greens And Foodborne Illness

Atlanta, GA has been hosting the 2008 International Conference on Emerging Infectious Diseases this week. In a March 17th presentation at the conference, CDC researcher Michael Lynch reported that leafy green vegetables have been increasingly associated with food-borne disease over the last 35 years.

Lynch and his colleagues compared the increased incidence of food-borne diseases associated with leafy greens against the change in per capita consumption of these vegetables for the periods 1986-1995 and 1996-2005. They discovered that the magnitude of the increase is far greater than can be explained by the increase in consumption.

Lynch reported that food-borne disease tied to leafy greens rose by 60% in 1986-1995 compared to the previous decade, while consumption rose by only 17%. Similarly, the incidence of leafy green-associated food-borne disease outbreaks rose by 39% in 1996-2005 compared to the decade before, while consumption increased by 9%. According to Lynch, the widespread nature of some of the outbreaks suggested that contamination occurred early in the production/distribution chain – maybe even at harvest.

There are several possible explanations for this spectacular increase in contamination of leafy green vegetables with Salmonella and E. coli O157:H7. In my opinion, one important reason is the juxtaposition of irrigated lettuce and spinach cropland and cattle feedlot operations.

The December 2007 issue of Emerging Infectious Diseases contained a report on the source of the large 2006 spinach-associated E. coli O157:H7 outbreak. The contaminated spinach was traced to a single farm in Central California. According to the investigation report, the pathogen was most likely carried to the spinach field from a nearby cattle pasture by feral swine.

There are, of course, other contributing factors to the rise in lettuce and spinach-associated outbreaks. A recent congressional oversight investigation into FDA's fresh spinach inspection activities found several serious problems, some of which may be due to chronic underfunding of the agency's food regulatory activities:

• FDA only inspects spinach fields when conducting a food poisoning outbreak investigation;
• FDA inspects spinach processors, on average, once every 2.4 years, far less than the agency's stated program goal of annual inspections;
• FDA relies too heavily on voluntary compliance, even after observing and documenting repeated violations; and
• FDA lacks the authority to insist that processors make their records available for review during an inspection.

New packaging technologies also contribute to changes in the pattern of food-borne disease outbreaks. USDA researchers reported this month in the Journal of Food Science that modified atmosphere packaging can favor the development of acid-resistant E. coli O157:H7 in lettuce if the packaged produce is stored above 15ºC (59ºF). Acid-resistance increases the ability of the microbe to survive stomach acid and initiate an infection.

Many food industry representatives, as well as some scientists and consumer advocates, have urged the FDA to approve irradiation of fresh produce as a solution to the problem of contaminated lettuce and spinach. While irradiated fresh produce might some day be as commonplace as pasteurized milk, I hope that regulators think long and hard before moving in this direction.

We've already see the Law of Unintended Consequences in operation with the use of modified atmosphere packaging of produce. Let's not create tomorrow's food safety hazard in the effort to resolve today's problem.