March 17 (HealthDay News) -- There was a significant increase in the proportion of food-borne illness outbreaks in the United States linked to leafy green vegetables from 1973 to 2006, but the rise can't be completely explained by increased consumption of leafy greens, researchers say.

"During the 1986-1995 period, U.S. leafy green consumption increased 17 percent from the previous decade. During the same period, the proportion of food-borne disease outbreaks due to leafy greens increased 60 percent. Likewise, during 1996 to 2005, leafy green consumption increased 9 percent, and leafy green-associated outbreaks increased 39 percent," researcher Michael Lynch, of the U.S. Centers for Disease Control and Prevention, said in a prepared statement.

Lynch and his colleagues decided to conduct the study after high-profile spinach and lettuce-related E. coli outbreaks in 2006.

Their analysis of more than 10,000 food-borne illnesses from 1973 to 2006 revealed that leafy greens were involved in a total of 5 percent of all food-borne outbreaks. Most of the leafy green outbreaks were caused by norovirus (60 percent), while others were caused by salmonella (10 percent) and E. coli (9 percent).

"Given recent experiences, that was not a total surprise. What was interesting was when we compared the numbers to consumption data," Lynch said.

He said further investigation is needed to determine why the rate of leafy green-related food-borne illness has increased more than consumption. Many food-borne disease outbreaks can be traced to a problem in food preparation, he said. However, some outbreaks were fairly widespread, which suggests that contamination occurred either on the farm or in 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," Lynch said.

The study findings were presented Monday at the International Conference on Emerging Infectious Diseases, in Atlanta.

In a three-part series published in January, HealthDay detailed the problems with tainted foods plaguing the U.S. food system. In little less than a year and a half, nationwide recalls of tainted products formed their own peculiar food pyramid: meats, vegetables, salads, snacks, fast food, even dessert items. The various pathogens in those products killed at least three people, sickened more than 1,300 others and touched almost every state in the country as well as Canada.

SOURCE: American Society for Microbiology, news release, March 17, 2008