A CDC Report on Salmonella Outbreaks Linked to Tomatoes
This week’s Morbidity and Mortality Weekly Report (MMWR) includes a CDC report on 4 large Salmonella outbreaks linked to tomatoes served at restaurants. The CDC report pointed out that there is a particular concern regarding fresh tomatoes served at restaurants “because restaurants often store and handle tomatoes in ways that allow for amplification of bacteria.”
In 2 of the Salmonella outbreaks, the tomatoes were contaminated at the farm. (Investigations of the other 2 outbreaks were inconclusive.) In a Salmonella Newport outbreak, the outbreak strain of Salmonella Newport was found in irrigation pond water near tomato fields associated with the outbreak. The environmental investigation of a Salmonella Braenderup outbreak revealed that “multiple potential animal reservoirs of Salmonella (e.g., cattle, wild pigs, wild birds, amphibians, and reptiles) were present in and adjacent to the drainage ditches.”
According to this CDC report:These recurrent multistate outbreaks indicate that the tomato-growing environment is an ongoing source of contamination of tomatoes.Possible sources for environmental Salmonella contamination of tomatoes include feces from domestic or wild animals (e.g., reptiles, amphibians, or birds) or contaminated habitats, such as ponds or drainage ditches. Although the mechanism by which tomatoes become contaminated is not known, certain possibilities are suggested by experimental evidence. Tomatoes can internalize Salmonella when they are immersed in water with a temperature less than the temperature of the tomato. Tomatoes also can become internally contaminated when tomato stems and flowers are inoculated with Salmonella, which can occur during growth if contaminated water is applied directly to plants. Contamination on the tomato surface also can be transferred to the interior of a tomato when it is cut. Once contaminated, cut tomatoes provide an efficient medium for bacterial amplification.
The report suggests the following to prevent future tomato-associated outbreaks of
Salmonella infections:
- Further environmental and laboratory research to determine the source and routes of contamination, mechanisms by which pathogens contact tomatoes and become internalized, the stages of development at which plants are most susceptible to contamination that persists, and procedures by which contamination can be reduced or eliminated.
- Traceback investigations in future outbreaks that consider all levels of tomato production, including the field and packinghouse.
The tomatoes involved in all of the outbreaks were grown in the United States, and the problem seems to be irrigation water. There will be a continuing problem with Salmonella contamination until federal regulations require frequent testing of irrigation water and prohibit the sale of tomatoes grown on fields irrigated with contaminated water.
