WASHINGTON — The percentage of foodborne illnesses linked to leafy vegetables and dairy increased substantially while the percentage of cases associated with eggs dropped, according to a study covering more than a decade of foodborne disease outbreaks.

The study, “Surveillance for foodborne disease outbreaks — United States, 1998-2008,” was published June 28 by the Centers for Disease Control and Prevention in its Morbidity and Mortality Weekly Report. The report examined outbreaks in which the first illness occurred during 1998-2008 that were reported by April 24, 2011.

During 1998-2008, the C.D.C. said it received reports of 13,405 foodborne disease outbreaks, which resulted in 273,120 reported cases of illness, 9,109 hospitalizations, and 200 deaths. Of the 7,998 outbreaks with a known etiology, 45% were caused by viruses, 45% were caused by bacteria, 5% were caused by chemical and toxic agents, and 1% were caused by parasites, the researchers said.

Encouragingly, the number of outbreaks declined 23% during the more than decade long study, and at no point after 2004 did the number of outbreaks exceed the median (1,267 outbreaks) of the 11-year period, the C.D.C. said.

Looking at outbreaks associated with food or ingredients, the researchers found 42% of the 7,724 outbreaks could be assigned to one of 17 predefined commodity categories: fish, crustaceans, mollusks, dairy, eggs, beef, game, pork, poultry, grains/beans, oils/sugars, fruits/nuts, fungi, leafy vegetables, root vegetables, sprouts, and vegetables from a vine or stalk. The most commonly implicated commodity was poultry, at 18.9%, followed by fish at 18.6% and beef at 11.9%.

Compared with the first two years of the study (1998-99), the percentage of outbreaks associated with leafy vegetables and dairy increased substantially during 2006-08, while the percentage of outbreaks associated with eggs decreased, the C.D.C. said. During 1998-99, the percentage of outbreaks linked to leafy vegetables was 6%, a figure that increased to 11% during 2006-08. For dairy, the percentage increased to 7% in 2006-08 from 3.6% in 1998-99. Meanwhile, the percentage of outbreaks linked to eggs fell to 2% in 2006-08 from 6% in 1998-99, the C.D.C. said.

The study also broke down where the outbreaks occurred. Of the 13,405 outbreaks, 86% were attributed to a single place of food preparation, the C.D.C. said. Of the total, 68% were associated with food prepared in a restaurant or deli, 9% with food prepared in a private home, 7% with food prepared by a catering or banquet facility, and the remainder with food prepared in another place. The largest outbreaks occurred in institutional settings, including prisons or jails, schools and camps.

“These findings underline the importance of targeted prevention measures for the specific foods that are associated with the most outbreaks and illnesses (i.e., beef, poultry, fish and produce) and provide insights into changes in causes of outbreaks over time (i.e., outbreaks associated with leafy vegetables and dairy increased during the surveillance period, while those associated with eggs decreased,” the C.D.C. said. “Most foodborne illnesses are preventable, and timely investigation and reporting of foodborne disease outbreaks provides information that might help to reduce foodborne illnesses. Public health agencies, regulatory agencies, and the food industry can use these data to help target efforts to prevent contamination of foods on the farm, in processing, and in restaurants and homes.”

For the full report, visit www.cdc.gov/mmwr/