Salsa and guacamole dip ingredients like tomatoes have been implicated in restaurant outbreaks in the U.S.Salsa and guacamole dip ingredients like tomatoes have been implicated in restaurant outbreaks in the U.S. (Shari Vialpando/Las Cruces Sun-News/Associated Press)

Contaminated salsa or guacamole were responsible for nearly one out of 25 foodborne disease outbreaks at restaurants over 10 years, according to U.S. scientists.

The study by the U.S. Centers for Disease Control and Prevention looked at traceable outbreaks between 1998 and 2008. The dips accounted for 3.9 per cent of outbreaks traced to restaurants.

Salsa and guacamole dips made with diced onions, tomatoes, peppers, avocados, herbs and other ingredients were all implicated.

The dips are often prepared in large batches, so even a small amount of contamination may make people sick, said Magdalena Kendall of the U.S. Department of Energy's Oak Ridge Institute for Science and Education in Oak Ridge, Tenn.

Lack of refrigeration is another possible reason the dips are linked to foodborne illness.

"We want restaurants and anyone preparing fresh salsa and guacamole at home to be aware that these foods containing raw ingredients should be carefully prepared and refrigerated to help prevent illness," Kendall, who worked on the study, said in a release.

Ways to lower infection risk

The risk can be lowered by following guidelines for safe preparation and storage of fresh salsa and guacamole to reduce contamination or pathogen growth.

The study was released Monday by the CDC at the International Conference on Emerging Infectious Diseases in Atlanta.

The U.S. has experienced outbreaks of foodborne illness over the last several years. Sources of the bacteria have ranged from spinach, to peppers to peanut butter products.

In 2009, Ontario health authorities investigating an outbreak of E. coli O157:H7 linked to a Harvey's restaurant in North Bay suspected contaminated onions in the outbreak that sickened 235 people.

Public health experts estimate that there are 11 to 13 million cases of foodborne illness in Canada every year, according to the Canadian Food Inspection Agency.