A raw deal

Tracking the way leafy greens spread disease

March 28, 2013

Wracking stomach cramps signaled the start of Mary Kozlowski’s illness in late October 2011. Pain begat bloody diarrhea. Kozlowski visited her doctor, who sent her home after taking a blood sample. The following day, she checked into a hospital near her home in St. Louis, Mo., as the number of platelets in her bloodstream fell. Kozlowski, it turned out, had been infected with a potentially lethal strain of E. coli bacteria that produces a chemical called shiga toxin. After attacking her intestines, the toxin ravaged her kidneys to the point of failure. Kozlowski, now 62, recovered from the illness — only to find out it began with the supposedly healthy act of eating salad. And the infectious ingredient? Bad romaine lettuce.

Kozlowski ate romaine lettuce from a Schnucks supermarket near St. Louis at least three times between October 12 and 20, 2011, according to the complaint she filed against the grocery store chain. And her case of food poisoning wasn’t an isolated incident. The Centers for Disease Control and Prevention (CDC) linked this infection to 57 other reports of E. coli that had occurred over the course of a month across ten states. The nationwide investigation led back to a single agricultural plot of lettuce. The CDC has not made public the identity or location of the farm where the lettuce grew, and whatever tainted the greens with E. coli remains unknown. A spokesperson for Schnuck Markets, Inc. did not return repeated phone calls.

When it comes to food poisoning, the biggest offender today is produce — not the usual suspects you might expect like seafood or undercooked meat. Plants, such as vegetables and nuts, are responsible for more than half of all foodborne illnesses in the United States, including viral and bacterial infections. Although the risk of death is low, about two million of the nine million annual cases of food poisoning came from leafy greens alone. The proportion of infections traced back to vegetables has risen steadily since the 1970s. And as the risk rises, surveillance may be diminishing: Congress’ current budget cutbacks may lead to a loss of as many as 2,100 Food and Drug Administration safety inspections.

The CDC and FDA presently have safeguards in place to minimize the harm from an outbreak. “Every E. coli infection must be reported,” said Laura Gieraltowski, a foodborne illness expert at the CDC and an author of the study detailing the 2011 outbreak, which was published recently in the journal PLOS One. Although death from an E. coli infection is rare — the bacteria claim about 60 lives in the U.S. annually — the CDC remains on the lookout for signs of new infections, as the shiga toxin causes almost 100,000 cases of diarrhea and 3,200 hospital visits a year.

The key to uncovering a potential illness outbreak is identifying a specific strain of bacteria. If a doctor discovers that a patient’s stool harbors harmful E. coli, the physician must send samples of the bacteria to a state laboratory.

Health care workers transport what is called an isolate, a yellowish nutrient broth or gel in which the offending bacteria grow, to their state’s health department. There, state microbiologists look for what Gieraltowski calls a “DNA fingerprint.” By comparing the DNA from one E. coli isolate to another, researchers can link related infections via a type of bacteriologic genealogy test.

And if similar fingerprints pop up in several states, the CDC and FDA step in. The organizations discovered that the E. coli which caused Kozlowski’s hospitalization in Missouri matched bacteria from patients as far away as Arizona and Georgia.

Knowing where a foodborne illness outbreak occurs geographically isn’t enough to determine the guilty comestibles. To find the culprit, disease trackers question the afflicted patients about what they ate, though it’s not always easy to get answers. “I don’t remember what I ate yesterday,” said Gieraltowski, “let alone three weeks ago.”

In the investigation into the 2011 shiga toxin outbreak, researchers compared the eating habits of 22 patients to those of 82 healthy individuals living in the same areas. The CDC found that 85 percent of infected patients — but only 41 percent of their healthy neighbors — recalled eating romaine lettuce in the week prior to the outbreak’s start. And 77 percent of interviewed patients ate at the salad bar of a Schnucks grocery store in St. Louis, compared to only five percent of the people who weren’t sick.

By checking the records from the Schnucks chain, the disease detectives figured out which farm grew the tainted lettuce. But the investigators needed organic material to dust for more DNA fingerprints. And by the time the FDA had found it, the farm was plowed under. Bereft of further leads, the researchers could reach no conclusions regarding the noxious E. coli‘s origins.

The common dogma, said David Rasko, a professor of microbiology at the University of Maryland, is that the introduction of bacteria occurs at some point in the food processing chain. Because E. coli grow best in human and animal intestines, not on plants, one possible source is fecal matter from farm animals such as cattle or goats in adjoining fields. And livestock aren’t the only potential cause of contamination. Deer and rabbits, for example, can sneak into strawberry fields, said Gieraltowski, so biosecurity is also an issue.

To make matters worse, it’s difficult to identify an infected animal. “In cows, pigs or sheep,” said Rasko, “E. coli has absolutely no effect.” Many strains of E. coli live harmlessly in humans, too. But the E. coli O157:H7 strains that produce shiga toxin are an exception.

Exposure to as few as ten to 100 shiga-producing bacteria can cause sickness. Once ingested, the E. coli churn out toxic molecules in response to the unfamiliar environment within the human body. These molecules spread throughout the intestinal cells, first entering into the linings of blood vessels. Once the toxin hooks to the outside of a human cell, a tiny piece of the membrane sucks in toward the cell’s center, forming a bubble around the chemical. The bubble floats away from the membrane and makes its way into the ribosome, the protein factory of the cell. There, the shiga toxin springs free from its enclosure and, like a butcher’s cleaver cutting through a rope of sausage, hacks away at the cell’s RNA.

Without RNA directing protein assembly in the ribosome, the cell dies. Widespread cellular destruction leads to bloody diarrhea and, in rare cases such as Kozlowski’s, kidney damage with a mortality rate of about five to fifteen percent.

Kozlowski gained 30 pounds of fluids after her kidneys stopped working, according to her lawsuit. And her case is emblematic of the change in the way E. coli. bacteria infect consumers. “We’ve seen a fundamental shift in the vehicle of delivery,” said Rasko, referring to the change from meat to leafy greens as the source of E. coli infections. Between 1996 and 2005, consumers increased the amount of greens they ate by nine percent — and the number of related viral and bacterial outbreaks rose by almost 39 percent.

The earliest known shiga toxin outbreak, a result of undercooked McDonald’s hamburgers, occurred in 1982. “I’ve represented hundreds and hundreds of people from 1993 to 2002,” said William Marler, the Seattle-based attorney representing Kozlowski in the lawsuit against Schnucks, “and most of the E. coli cases were hamburger cases.” In fact, a symptom of shiga toxin infection, hemolytic uremic syndrome — the type of acute kidney failure that afflicted Kozlowski — was nicknamed “hamburger disease,” in the 1980s and 1990s. “It’s why most microbiologists like their burgers well done,” said Rasko.

But since 2003, the majority of the people Marler represents have filed foodborne illness lawsuits with leafy greens as the cause. And the scientific consensus mirrors Marler’s observation. In 2004, CDC scientists published a study linking produce to an increased number of outbreaks. In the 1970s, produce was the culprit behind 0.7 percent of all food poisoning cases; by the 1990s, that share was six percent.

“It has to do with the way food is being processed,” said Rasko. Methods of bacteria detection in the meat processing world, he said, are much better than those used in the produce industry.

Experts suggest several ways to prevent food poisoning from produce. When possible, avoid bruised or damaged fruits and vegetables. Though prewashed and packaged greens are generally safe, Marler prefers to buy a head of unpackaged lettuce and wash it at home.

And, added the CDC’s Gieraltowski, always cook your sprouts. “There are a lot of different steps,” she said, “in the chain from farm to fork.”

About the Author

Ben Guarino

Benjamin Guarino holds a B.S. in bioengineering from the University of Pennsylvania. After graduation he joined Penn’s Spine Pain Research Lab, where he studied the motion of artificial intervertebral discs and the painful effects of whole-body vibration. Upon discovering that engineering journals discourage metaphor, Ben decided to shuck his lab coat and don a press badge at SHERP. He’s fond of long runs and bad science fiction, and his Erdos–Bacon number is seven.


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