FEATURE ARTICLE
Safer Salads
Contaminated fruits and vegetables are more common than ever. Why? And what can consumers do to protect themselves?
Jorge M. Fonseca, Sadhana Ravishankar
Of Pathogens and Produce
For packaged sprouts during the period from 1995 to 1999, and for leafy greens in more recent years, outbreaks of two bacteria, Salmonella and Escherichia coli O157:H7, have marked these items as particularly vulnerable to becoming vehicles for outbreaks. In the late 1990s, the sprout industry saw several undesirable records made and broken in outbreaks per year and cases per outbreak. According to the Centers for Disease Control and Prevention, during the 24-year period between 1973 and 1997, 32 states reported 190 produce-related outbreaks, which together involved 16,058 illnesses, 598 hospitalizations and eight deaths. More recent data from the Center for Science in the Public Interest show that in the 14 years between 1990 and 2004, produce was implicated in 639 outbreaks involving 28,315 cases—more than a threefold increase in almost half the time.
Among foodborne pathogens, Salmonella bacteria are the prime cause of outbreaks in fresh produce, causing one out of five such outbreaks between 1990 and 2003. The biggest culprits were tomatoes, melons and sprouts. Salmonella is acid-tolerant, which allows it to survive in fruits and vegetables with a low pH, such as tomatoes. Like many of the pathogens that can contaminate produce, Salmonella is an intestinal, or enteric, microbe. Animals shed the bacteria in their feces, and soil that contains fresh or incompletely composted manure from wild or domesticated animals can act as a reservoir for the bacteria. If produce that is grown in contaminated soil is not washed thoroughly, Salmonella on the surface can spread to the inside portion during slicing or cutting. In the case of sprouts, which are not usually sliced, bacteria probably enter the seeds before germination. The situation is complicated by the fact that consumers rarely cook and seldom wash sprouts.
Escherichia coli O157:H7 is a deadly strain of a bacterium that is normally found in all human intestines. As recently as fall 2006, a multistate outbreak linked to bagged spinach killed three people and sickened more than 200. This pathogen was once associated primarily with raw ground beef and undercooked hamburgers but now affects fresh produce, too. The rise in E. coli-tainted fruits and vegetables probably comes from cattle operations, which can contaminate fields through feces or feces-laced irrigation water. Cross-contamination between meat and fresh produce can also take place during processing or packaging, and contact with raw beef is suspected to have been at the root of outbreaks in cantaloupes, sprouts, lettuce and fruit salad in the 1990s. Nearly two-thirds of the outbreaks associated with E. coli-contaminated produce have occurred during late summer and fall, when warm temperatures and outdoor cooking can subvert good hygiene, and about half of the outbreaks have involved cross-contamination during food preparation.
Scientists aren't sure why this strain of E. coli has become so prevalent. Some investigators hypothesize that the type of food that cattle eat determines the quantity and acid resistance of the E. coli O157:H7 bacteria shed in their feces. However, the evidence to support this claim is inconclusive. One study reports that animals fed grain have larger populations of acid-resistant bacteria in their gut compared with animals fed hay. This finding makes sense, because cows lack the enzyme that breaks down starch, leaving it to ferment and acidify the rumen—conditions that create both food and ideal housing for acid-tolerant bacteria. Such bacteria, when shed in the environment, would be hardier than their nonresistant counterparts. If this dietary difference were true, then bacteria from grain-fed cattle would die more slowly in the environment or in the acidic conditions of a human stomach, presenting a greater risk to consumers. However, another study by different investigators found that E. coli O157:H7 shed from cows fed grain was no more acid-resistant than that shed from cows fed hay. The same report indicated that animals fed hay shed the microbe for longer periods than those fed grain. There is no clear epidemiological evidence correlating the presence of E. coli O157:H7 in animals with their diet.
Listeria monocytogenes is a foodborne pathogen commonly found in raw vegetation. The bacterium is ubiquitous and hardy in the environment, able to withstand refrigeration and even grow in such cold, dry conditions. Although fewer people get sick from Listeria than from many other microbes, this pathogen kills its victims more often than any other. In 1981, contaminated coleslaw killed 17 people and sickened 41; the source of the outbreak was believed to be manure from sheep that had listeriosis. In the same decade, this bacterium was a major problem in cheese and dairy products, leading industry regulators to adopt a "zero-tolerance policy" in ready-to-eat foods. Since then, stringent control measures have reduced the number of Listeria outbreaks significantly.
Although Salmonella, E. coli and Listeria get much of the attention, many other bacterial pathogens cause outbreaks too, including certain species of Bacillus, Campylobacter, Clostridium, Shigella, Staphylococcus, Vibrio and Yersinia. All are enteric pathogens, and all, with the exception of Shigella, taint fresh produce, most often because of cross-contamination with raw meat or eggs. In recent decades, these bacteria have caused outbreaks traced to many different fruits and vegetables, including cabbage, garlic (chopped, in oil), green onions, lettuce, mixed vegetables, parsley, sprouts, strawberries and watermelon.
Viruses also cause many outbreaks of produce-related foodborne illness, although the contamination is usually through an infected food handler. These viruses require a host and are usually too fragile to survive in the soil or on the produce itself for long periods of time.
Parasitic protozoans (single-celled animals) also account for some produce-related outbreaks. The most common culprit is Cyclospora cayetanensis, which hitched a ride to U.S. and Canadian grocery-store shelves aboard Guatemalan berries in the late 1990s. Other protozoan contaminants in recent years have included Cryptosporidium parvum and Giardia lamblia. These parasites infect fruits and vegetables through contaminated irrigation or wash water, causing profuse diarrhea in affected patients. In general, the quality of produce grown outside the United States is monitored by third-party agencies in the foreign country. Very little of the product is analyzed at the U.S. port of entry.
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