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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 shows.

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."

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