How Do Tomatoes Get Salmonella?
From poop to produce.
Federal health officials are still trying to pinpoint the source of the salmonella-tainted tomatoes that sickened at least 167 people in 17 states since April and claimed the life of a Texas cancer patient. How can salmonella, a bacterium that normally lives inside animal intestines, get on your tomatoes?
Manure, runoff, and wild animals. Livestock animals, especially when kept in large numbers in confined spaces, can contract salmonella and carry the bug without showing any symptoms at all. Infected cows, pigs, and chickens shed the bacteria in their waste, which is sometimes used to fertilize nearby fields. The heat generated when manure is composted kills off most, but not all, disease-causing bacteria.
Contaminated water supplies can also put salmonella on your tomatoes. Runoff from livestock pastures, or from leaky or overtopped waste lagoons at industrial farming sites, can dirty streams, groundwater, and other bodies of water farmers draw on for irrigation. According to an FDA investigation, that was the likely cause of a 2002 salmonella outbreak in imported Mexican cantaloupes.
Since salmonella can infect anything with an intestinal tract, wild animals can spread the bacteria onto crops through their own droppings or from fecal matter they track in from elsewhere. The 2006 outbreak of E. coli in spinach, for example, was traced to a pack of wandering wild boars. The swine had picked up tainted cow manure on their hooves before breaking through the fence of a nearby spinach field to graze.
Producers do rinse their harvest with chlorinated water to remove most of the harmful bacteria, but enough can be left to make you sick. If the skin of a tomato is punctured when the fruit is picked from the vine or when presliced for sale in a supermarket or restaurant, then bacteria get inside, and no amount of washing will make it safe to eat. This is partly why on-the-vine tomatoes have been exempt from this most recent salmonella scare.
Salmonella and E. coli poisoning used to be primarily associated with the consumption of undercooked meat. But that's changing, as produce-related outbreaks become more common and more widely publicized. In 1999, produce was responsible for 40 separate food poisoning incidents in the United States. In 2004, that number climbed to 86. There have been 13 major outbreaks involving tomatoes alone since 1990.
Why the shift? One factor is a lack of inspections of farms and packing plants by the Food and Drug Administration, which means that more contaminated produce slips into the market undetected. The U.S. Department of Agriculture inspects every meatpacking plant in the country each day, keeping close tabs on safety conditions. By contrast, the Food and Drug Administration, which is charged with regulating produce, might inspect a vegetable packing facility once a year, and the number of inspections is shrinking. In 1972, the FDA inspected 50,000 farms and plants. By 2006, that number had dwindled to 10,000. Meanwhile, having increasingly centralized packing plants means that crops from a single contaminated field can mingle with clean produce and be shipped across a wider swath of the country than ever before.
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Explainer thanks Jeff Cronin of the Center for Science in the Public Interest, Jaydee Hansen of the Center for Food Safety, Robert Martin of the Pew Commission on Industrial Farm Animal Production, and Marion Nestle of New York University.
Ryan Hagen is a writer and catastrophe-response policy researcher living in Brooklyn, N.Y.
Photograph of tomatoes above and on Slate home page by Stephen Morton/Getty Images.