The Why Files The Why Files --

Spinach epidemic: Controlling E. coli


Eaten any bagged, fresh spinach lately? Neither have we. On Sept. 14, the FDA announced a national "recall" of bagged spinach, after the leafy green vegetable was linked to a deadly outbreak of Escherichia coli O157:H7. Many E. coli strains live in healthy human intestines, but this bug makes toxins that Leafy greens packaged in bags at the grocery stordamage human organs and blood cells. The characteristic symptoms of "O157" infection are diarrhea, often bloody, and severe abdominal cramps, but the toxin can also cause hemolytic uremic syndrome (HUS), which can lead to kidney failure and death.

Bagged greens are fast and convenient, but they can carry dangerous pathogens. Photo: USDA

Paradoxically, treating O157 infections with antibiotics can stimulate the release of extra toxin, making the symptoms worse. With supportive care, most people recover from E. coli O157:H7, but those who get HUS can need prolonged dialysis or a kidney transplant. So far, 16 percent of the spinach cases have gotten HUS, but only two have died. The current epidemic had infected 192 people in 26 states by Oct. Here's an (update on the current outbreak.)

E. coli O157:H7 causes about 73,000 illnesses and 61 deaths per year in the United States, according to a 1999 estimate.

Cut-away of human torso, organs of the digestive system exposedThe high acidity of the stomach quickly kills most pathogens, but not E. coli O157:H7. Graphic: NIH

The first known cases of human E. coli O157:H7 disease occurred in 1982; the bug was carried in hamburger meat that apparently was contaminated by cow feces at the slaughterhouse. After a sporadic pattern of outbreaks and a series of scary headlines, the meat industry has gone to great lengths to change slaughterhouse processes to reduce the danger.

Michelle Rossman, director of beef safety research at the National Cattlemen's Beef Association wrote to say, "...beef producers have invested $22 million in beef safety research since 1993. One of the many beneficial outcomes of this investment has been development and implementation of best practices which serve as a roadmap to reducing E. coli O157:H7 and other pathogens from farm to fork. ... The beef industry has been successful in the fight against E. coli -- government data shows E. coli incidence in ground beef has declined 80 percent since 2000 -- but we are committed to continuing the decline in the future."

But against the backdrop of a reduction in meat-borne disease we have seen a rise in a second source of E. coli O157:H7 disease: vegetables like spinach that get contaminated by manure that is used as fertilizer, or by irrigation or surface water that contains O157 from cattle manure. The cattle industry is also concerned to reduce this source, wrote Rossman. "Beef Big spray of brown liquid in the distanceproducers across the country lead conservation efforts and carefully follow science-based, best management practices to protect our water and soil resources. Our livelihood depends on the land and we are dedicated to environmental stewardship. For us, environmental stewardship and good business go hand-in-hand."

When manure is sprayed onto fields, it can carry pathogens like E. coli O157:H7. Photo: USDA

An outbreak breaks out
The present outbreak was first recognized in Wisconsin, says the state's chief medical officer for communicable diseases, Jeffrey Davis. On Sept. 5, his office got a report of E. coli O157:H7 cases, largely among people who had visited a barn at a county fair. On Sept. 7, his office received a reports of four cases in Dane County, then a report from the Blood Center of Southeast Wisconsin that five adults hospitalized in two counties had gotten plasma exchanges to manage hemolytic uremic syndrome linked to O157. On Sept. 8, Davis and Division of Public Health colleagues notified the Centers for Disease Control and Prevention of these striking events, and the Wisconsin State Laboratory of Hygiene posted genetic details of the E. coli O157:H7 strains from Wisconsin on Pulsenet, a network for comparing pathogen genetics that is used by public-health laboratories.

Rod shaped bacteria seen up-close
They may look innocent, and most of them are. But some strains of E. coli make horrific toxins, and you can't tell which is which just by looking. Photo: Rocky Mountain Laboratories, NIAID, NIH

As tests showed that most of the Wisconsin residents carried identical pathogens, "We saw clearly that there was a relationship between these clusters, so we expanded the information collection instrument," says Davis, with a focus on learning what people had been eating. Within a few days, Pulsenet was showing identical pathogens from New Mexico, Oregon and Utah, and by Sept. 14, Wisconsin and Oregon had independently pinned the blame on those handy bags of pre-washed spinach. That evening, the Food and Drug Administration made its recommendation against eating bagged spinach.

If you discount the cases that apparently were caught by direct contact with animals in barns, the national campaign against spinach was mounted just one week after the first cases of O157 were reported in Wisconsin. Davis credits some luck -- the blood bank report of five patients at five hospitals all being treated for HUS -- for that rapid reaction, but mainly better preparation and communication. "Pulsenet is working extremely well, other states were picking up on the isolates, and our Wisconsin State Laboratory of Hygiene consistently does great, timely work. They have a mechanism for receiving samples rapidly, and doing [genetic analysis] on a daily basis, that's very important."

The limitation on eating spinach has now been reduced. The Food and Drug Administration is now advising consumers not to eat fresh spinach or salad blends containing spinach grown in the Monterey, San Benito, and Santa Clara County, California, which were implicated in the current outbreak. Spinach grown in these counties is often packaged elsewhere. If you cannot tell where fresh spinach was grown, do not buy or eat it. Spinach grown anywhere else in the United States can be eaten, and frozen and canned spinach are safe. E. coli O157:H7 in spinach can be killed by cooking at 160ยก Fahrenheit for 15 seconds. (Water boils at 212 Fahrenheit.) If spinach is cooked in a frying pan, all parts must reach 160 Fahrenheit.

Beefy black cow stands among rolling green hills
It's a beefy looking bull, but does this black angus carry pathogens in its intestinal tract? Photo: USDA

In the dungheap
The story of E. coli O157:H7 begins in the intestinal tract of a ruminant -- an animal that houses bacteria that ferment cellulose into sugar. E. coli O157:H7 is "part of the normal flora of cattle, it lives very easily in cattle without harming them," says Carolyn Hovde Bohach, a microbiologist at the University of Idaho who specializes in O157.

"The pattern is that the O157 comes and goes," says Hovde Bohach. "In individual animals there is tremendous variation. Some animals take in O157, have it for a few days, it passes through the GI tract and contaminates manure for a time, while other animals can acquire a stable infection, carry the O157 for months, or a year or more, continually passing O157 in fecal material. You can think of it as being almost ubiquitous in farms and ranches, although not in huge numbers."

In a test, one of her students tracked the survival of E. coli O157:H7 in a manure heap. "I thought she'd be done in 30 days because the average duration of bacteria in cattle is about a month." But O157 survived for 21 months, in sunlight, during freeze-thaw cycles, at lots of different temperatures in the manure, outside the animal's body. "We quit after 21 months, and the O157 was still alive," she says.

O157 is unusually infectious, adds B. Brett Finlay, professor of microbiology at the University of British Columbia, who has studied the devious bug's genetics and tactics. "Ten organisms can make you sick, while salmonella takes 10 million. And E. coli O157:H7 is resistant to acid in the stomach that normally kills most things."

If E. coli O157:H7 lives in cow guts, could changing their feed control the pathogen?


Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

©2022, University of Wisconsin, Board of Regents.