Norovirus, Salmonella responsible for most foodborne illness outbreaks in 2007

**CDC. MMWR. 2010; 59:973-979.**

CDC researchers identified 1,097 foodborne disease outbreaks, which lead to 21,244 illnesses and 18 deaths, in the United States during 2007. Although Salmonella and norovirus were the most common culprits for both outbreaks and illnesses, these numbers were down 8% and 15% from the annual average reported from 2002 to 2006.

CDC researchers gathered data on outbreaks occurring in 2007 from 48 states, the District of Columbia and Puerto Rico. Data reported include number of foodborne illnesses, hospitalizations and deaths; the confirmed or suspected etiologic agent and food vehicle involved, according to the researchers.

Etiology of an outbreak

A single etiologic agent was identified in 64% of outbreaks, resulting in 73% of illnesses. An etiology was unidentified for 363 outbreaks.

“Knowing more about what types of foods and foodborne agents have caused outbreaks can help guide public health and the food industry in developing measures to effectively control and prevent infections and help people stay healthy,” Chris Braden, MD, acting director of the CDC’s Division of Foodborne, Waterborne and Environmental Diseases, said in a press release.

Delayed reporting of illness to the health department, overconsumption and unavailable human or food sample test results were the most common reasons for not identifying an etiology or food vehicle, the CDC researchers said.

Of 497 outbreaks with a single etiologic agent confirmed, 52% were caused by bacteria (with 6,441 illnesses), 40% by viruses (with 6,120 illnesses), 7% by chemical agents (with 141 illnesses) and 1% by parasites (with 65 illnesses).

Norovirus and Salmonella were the most common cause of illness, causing 39% and 27% of outbreaks involving a single etiologic agent. Norovirus accounted for 97% of illnesses caused by a virus and Salmonella accounted for 53% of those caused by bacteria. Enteritidis was the most common Salmonella serotype identified and accounted for 28 single-etiology outbreaks. Forty of the single-etiology outbreaks were caused by shiga toxin-producing Escherichia coli, of which 39 were caused by serogroup O157.

Among the 18 multi-state outbreaks, 10 were caused by Salmonella, six by E. coli O157:H7, one by C. botulinum and one by norovirus, according to the report.

Contaminated food

The CDC classifies foodborne disease outbreak-associated foods into 17 categories. In the current report, the largest number of outbreaks were caused by finfish (41 outbreaks), poultry (40 outbreaks) and beef (33 outbreaks), and the largest number of illnesses were caused by poultry (691 illnesses), beef (667 illnesses) and leafy vegetables (590 illnesses).

Salmonella was the causative agent in two of the three largest reported outbreaks. The food vehicles responsible were hummus (802 illnesses) and commercially-processed pot pies (401 illnesses and 3 deaths).

Norovirus caused the second largest outbreak, which occurred at a conference hotel where shared foods were responsible. According to the researchers, the largest outbreaks associated with a single food commodity were caused by a chicken dish contaminated with C. perfringens (132 illnesses), leafy vegetable salad contaminated with norovirus (128 illnesses), chili beans contaminated with C. perfringens (125 illnesses) and beef contaminated with E. coli O157:H7 (124 illnesses).

“Timely investigation and reporting of foodborne outbreaks can provide public health, regulatory and agricultural professionals with information to target control and preventions strategies as well as to promote good food-handling practices among food employees and the public,” the researchers wrote. — by Stacey L. Fisher