Pass the Veggies; Hold the Viruses
Food-borne Disease Caused by Fresh Leafy Greens Is a Growing Problem in the U.S.
March 18, 2008 -- Contamination of fresh, leafy green vegetables with
disease-causing bacteria and viruses is definitely on the rise, a CDC study
Why? That's not clear, says study leader Michael Lynch, MD, a medical
epidemiologist at the CDC.
But the study, which looks back to 1973, shows that the increase in
contamination is not simply because people are eating more greens or because
health departments are keeping a closer eye on the problem.
"It is a bit of a surprise. We thought the increase was all due to
greater consumption and enhanced surveillance, but these things don't
completely explain it," Lynch tells WebMD. "We see a wide range of
pathogens causing food-borne illnesses in leafy greens. Outbreaks can be
restaurant associated or they can be spread across several states, so this
suggests the problem has multiple causes."
Lynch and colleagues looked at data on food-borne disease outbreaks linked
to leafy greens from 1973 to 2006. Each outbreak caused two or more illnesses
traced to lettuce, cabbage, mesclun mix, spinach, or salad items containing at
least one leafy green.
Over the 33-year period, there were 10,421 food outbreaks – some 500 of them
from leafy-green vegetables. Norovirus -- the easily spread, hard-to-kill bug
sometimes called winter vomiting virus -- was behind 58.3% of the outbreaks. Salmonella caused 10.4% of the
outbreaks; E. coli O157:H7 caused 8.9%.
"Some of the pathogens are shown to survive quite well on leafy
greens," Lynch said. "E. coli, for instance, can adhere quite
well and even be internalized into these foods."
CDC researcher Karen M. Herman, MSPH, reported the findings
at this week's 2008 International Conference on Emerging Infectious Diseases,
held March 16-19 in Atlanta.
More than 60% of the outbreaks involved contamination by germs spread by
humans, Herman said, and about 30% of the outbreaks involved germs spread by
animals. Eight percent of the outbreaks were of uncertain origin.
"Contamination can occur at the point of preparation, such as an ill
worker at a restaurant or preparation on a contaminated surface," Herman
said. "Contamination during harvest or processing can cause widespread
outbreaks, such as the recent 11- state outbreak."
In the decade 1986-1995, Americans ate about 17% more leafy greens than they
did the decade before. Yet the proportion of food-borne illnesses caused by
leafy greens went up nearly 60%. From 1996 through 2005, consumption went up
another 9% -- but the proportion of food-borne illnesses linked to greens went
A consumer group, the Community Alliance with Family Farmers (CAFF), says
its own analysis of FDA data suggests that since 1999, 80% of E. coli
O157:H7 outbreaks on leafy greens in California have been traced to processed,
bagged salad. The group calls for increased monitoring of fresh-cut, bagged
Lynch says that the pre-bagged greens are only part of the problem.
"We have seen outbreaks due to both prepackaged and head greens, so it
is hard to say that one of those is safer than the other," Lynch says.
"Even if they are listed as prewashed, washing leafy greens is prudent.
Meanwhile, a lot of effort is being undertaken by industry and food-safety
officials to figure out how these greens are becoming contaminated and to
prevent this contamination."