Salmonella sources in tomato-linked outbreaks still unclear

Apr 8, 2005 (CIDRAP News) – Follow-up investigation has failed to pinpoint how Roma tomatoes were contaminated with Salmonella last summer, sickening more than 500 people in the United States and Canada, according to federal health officials.

Three separate outbreaks involving a total of 561 cases were linked with eating Roma tomatoes in June and July of 2004, the Centers for Disease Control and Prevention (CDC) reports in today's issue of Morbidity and Mortality Weekly Report. The outbreaks made 2004 the worst year so far for tomato-associated salmonellosis cases.

Circumstances pointed to four tomato-packing sites as possible sources of contamination in one or more of the outbreaks, but investigators couldn't find clear sources of contamination at the facilities, the report says.

The largest outbreak involved 429 laboratory-confirmed salmonellosis cases that occurred in nine eastern and midwestern states in July, the CDC says. None of the patients died, but 30% were hospitalized. All had eaten at a chain of delicatessens, identified at the time as Sheetz Gas Station stores.

A case-control study indicated Roma tomatoes as the source of infection. The deli chain had bought sliced Roma tomatoes from a single processor for all of its stores in five states. The cases involved six different Salmonella serotypes.

In a second outbreak, investigators found 125 salmonellosis cases in 16 eastern and midwestern states, all featuring the same serotype, S Braenderup, and occurring between Jun 18 and July 21. A case-control study showed that Roma tomatoes were the only item significantly associated with illness, having been eaten by 41% of case patients but only 14% of controls.

Finally, seven people in Ontario fell ill in early July after eating at the same restaurant, the report says. No case-control study was done, but Roma tomatoes were the only food eaten by all seven people. The same serotype, S Javiana, was identified in all seven cases. S Javiana was also seen in the largest outbreak, but the pulsed-field gel electrophoresis patterns differed.

Investigators who traced Roma tomatoes from restaurants back through the supply chain identified one field-packing operation and three packing houses as possible sources of contamination in the largest outbreak, the report says. One of these, a packing house in Florida, was also seen as a possible source for the other two outbreaks.

However, environmental investigation of four packers and five associated farms in Florida and South Carolina in the late summer and fall "did not reveal a clear source of contamination, and the packing houses appeared to be following food-safety guidance," the article says. But only one packer and one farm were actually operating when they were inspected, and investigation will continue this year, it says.

Investigators also checked quality-control measures at the tomato-slicing facility linked with the largest outbreak and found no contamination.

Salmonella infections have been linked to tomatoes since 1990, the CDC reports. Nine outbreaks totaling 1,616 cases have been reported since then. Given how many foodborne illness cases typically go unreported, the agency estimates that 60,000 tomato-related cases have occurred since 1990.

The article says Salmonella can enter tomato plants through the roots or flowers and can enter the fruit through small cracks in the skin or through the plant itself. But it is not clear whether the pathogen can move from the roots to the fruit. Killing Salmonella inside a tomato "is difficult without cooking, even if treated with highly concentrated chlorine solution," the article says.

It concludes that research is needed to shed more light on how Salmonella contaminates tomatoes and how the problem can be controlled.

CDC. Outbreaks of Salmonella infections associated with eating Roma tomatoes—United States and Canada, 2004. MMWR 2005 Apr 8;54(13):325-8 [Full text]

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