Nov 1, 2006 (CIDRAP News) – Federal investigators have turned up few solid leads in a Salmonella outbreak that has sickened 171 people in 19 states, but some food safety experts are suggesting that contaminated tomatoes and infected food service workers might have played a role.
The Centers for Disease Control and Prevention (CDC), in a press release yesterday, said DNA fingerprinting revealed that Salmonella enterica serotype Typhimurium caused the outbreak. The organism typically causes fever and nonbloody diarrhea that resolves in a week. Of 73 patients for whom the CDC has clinical data, 14 (19%) were hospitalized; no deaths have been reported.
The CDC said the outbreak appears to be over: "At this time, few new cases are being detected, and there is little evidence of continuing risk to the public." The agency said the hunt for the source of the outbreak may take days to weeks.
The New York Times reported yesterday that the CDC detected the outbreak 2 weeks ago through a national database that identifies patterns in foodborne illness reports. The CDC said cases in the outbreak have been reported since Sep 1. Most of the states affected are in the eastern half of the nation.
Carlota Medus, PhD, an epidemiologist with the Minnesota Department of Health in St. Paul, said samples from 14 patients in Minnesota matched the outbreak strain on pulsed-field gel electrophoresis (PFGE). She said most of the Minnesota cases occurred between Sep 12 and Oct 13. Minnesota, like some of the other states involved in the outbreak, noticed the pattern and contacted the CDC.
Medus said a case-control study in Minnesota suggests the contamination source may be tomatoes, adding that five cases appear to be linked to the same fast-food restaurant. "Our study is pretty small, though. It would be nice to have more supporting information, so it's a little too soon to say," she said.
Salmonella bacteria are found in the intestines of animals and can contaminate raw fruits and vegetables that have been in contact with impure water, animal manure, or an infected food handler. Symptoms of infection usually begin from 12 to 36 hours after a person consumes contaminated food. Most cases infection resolve without medical treatment, but the pathogen can cause serious and sometimes fatal illness in children, elderly people, and those with weak immune systems.
Jack Guzewich, RS, MPH, director of emergency coordination and response in the Food and Drug Administration (FDA) Center for Food Safety and Applied Nutrition (CFSAN), said S. Typhimurium is the most common strain found in humans and that the CDC usually sees about 5 to 10 cases each month; the numbers usually peak in September and October. "It is found in many places in the food supply, but most often in poultry," he said.
Foodborne disease expert Craig W. Hedberg, PhD, an associate professor of environmental health sciences at the University of Minnesota School of Public Health in Minneapolis, called the outbreak fairly significant in its size and scope. "This outbreak is most likely due to tomatoes, and many cases were probably exposed through restaurants," he said, though health officials have not linked the outbreak to a specific product, restaurant, or store.
David Acheson, MD, chief medical officer for the FDA's CFSAN, told the Associated Press yesterday that if fresh tomatoes are to blame in the outbreak, it will be more difficult to trace the original source of the contamination than it was in the recent Escherichia coli O157:H7 outbreak linked to fresh spinach. "You can get a lot of information from looking at a bag. You don't get that information from looking at a tomato," he told the AP.
During the summer of 2004, three Salmonella outbreaks were traced to contaminated Roma tomatoes. The outbreak sickened 561 people in 18 states and one Canadian province, the CDC said in an April 2005 article in Morbidity and Mortality Weekly Report.
Hedberg said a major concern in restaurant-related outbreaks of salmonellosis is the role of infected food handlers in spreading the disease. He said food service workers can become ill from eating the contaminated product and then expose customers to the disease when they are still shedding the organism and don't use proper hygiene practices.
Medus was the lead author of a study in the August issue of the Journal of Food Protection that tracked the role of food service workers in restaurant-related Salmonella outbreaks in Minnesota between 1995 and 2004. Investigators found that 12% (129 of 1,033) food workers tested positive for Salmonella. About half of those who tested positive reported no recent gastrointestinal illness. Bacterial shedding lasted about 30 days in the workers who reported symptoms, but averaged only 3 days in those who didn't.
The authors concluded that the duration of Salmonella outbreaks in restaurants suggests an ongoing contamination reservoir, and that infected food workers are a likely source of disease transmission.
Hedberg, a coauthor of the report, said infected food workers, as secondary transmission sources, can increase the size and duration of the outbreaks. "Restaurants should be stepping up surveillance for illnesses in their food workers," he said, adding that restaurants should make sure that food workers who have gastrointestinal illnesses are evaluated and treated.
Guzewich said the FDA depends on the CDC and state and local health investigators to consider food workers as the possible source of contamination. "This investigation would be no exception," he said, adding that Massachusetts evaluated the food workers involved in its cases, and all tested negative for Salmonella infection.
Oct 31 CDC press release on Salmonella outbreak
April 8, 2005, MMWR article "Outbreaks of Salmonella Infections Associated with Eating Roma Tomatoes—United States and Canada, 2004" [Full text]
August 2006 Journal of Food Protection article "Salmonella outbreaks in restaurants in Minnesota, 1995 through 2003: evaluation of the role of infected foodworkers" [Abstract]
CIDRAP overview of salmonellosis