The Why Files The Why Files --

Spinach epidemic: Controlling E. coli


NYT's simple solution?
If E. coli O157:H7 is untreatable in humans, and if essentially all of it originates in cattle and their manure, then it's logical to try to reduce bovine O157 production. Could changing cattle feed make a difference? On Sept. 21, one week after the national spinach recall, an opinion piece New York Times (see "Leafy Green Sewage..." in the bibliography) blamed O157 on the grain-heavy diet fed to most livestock. According to the author, the deadly O157 is

small beef cow"not found in the intestinal tracts of cattle raised on their natural diet of grass, hay and other fibrous forage. No, O157 thrives in a new -- that is, recent in the history of animal diets -- biological niche: the unnaturally acidic stomachs of beef and dairy cattle fed on grain, the typical ration on most industrial farms. It's the infected manure from these grain-fed cattle that contaminates the groundwater and spreads the bacteria to produce, like spinach, growing on neighboring farms."

The news was alarming -- modern agriculture, a big, powerful industry, was to blame -- but also comforting: simply changing the feed would reduce or eliminate the hazard.

Snow-covered mountains rise in the distance as cows mosey by water
Herding Herefords to greener pastures. The food that animals eat may change the bacteria in their gastrointestinal tracts, but exactly how is hotly debated. The Why Files could not find scientists who think that changing feed -- by itself -- could control E. coli O157:H7. Photo: USDA

The Why Files tracked down the scientists whose work was apparently behind this indictment of grain, and they told us a completely different story. Todd Callaway, of Food and Feed Safety Research Unit of the U.S. Department of Agriculture in College Station, Texas, said the studies were conflicting. "Some studies show that switching to hay does reduce the shedding [of E. coli O157:H7 in manure], but there is just as much evidence that it increases it, or that it has no effect. There is a lot that is not understood. No two studies were done exactly the same way." Callaway, who summed up data on the relationship between feed and E. coli O157:H7 (see "Forage Feeding to Reduce ..." in the bibliography), says, "It was very frustrating to go through all the literature."

Pile of poop on grassScientists are just starting to explore the extent and meaning of bacteriological diversity in the intestine. In September, for example, scientists at University of Wisconsin-Madison discovered that the natural insecticide Bacillus thuringiensis (Bt) could not kill caterpillars unless they carried the normal complement of bacteria in the gut.

Cattle manure is the primary source of E. coli O157:H7, a deadly pathogen that can contaminate meat and fresh vegetables. Photo: USDA

False solution?
The Times's opinion piece, Callaway said, misinterprets a 1998 study which found that feeding hay reduced the total count of E. coli in cattle manure. "I was second author on that paper, and we were very specific. We didn't say anything about O157. But people jumped on this as the cure-all because it is so easy, and people were interpreting it for their own political ends, because there is so much argument about organic agriculture [which typically disdains the grain-heavy diet used in feedlots]."

However, it is also true that these scientists did suggest that feeding hay for five days before slaughter would reduce E. coli: "When cattle were abruptly switched from a 90 percent grain finishing ration to a 100 percent hay diet, fecal E. coli populations declined 1000-fold, and the population of E. coli resistant to an extreme acid shock declined more than 100,000-fold within 5 days (see "Forage Feeding to Reduce" and "Grain Feeding and the Dissemination ..." in the bibliography).

"Acid shock" refers to the intense acidity that microbes encounter in the human stomach, which kills most pathogens we eat. The claim was that changing to hay would make E. coli in general (but not necessarily O157) more likely to die in the human stomach, and therefore to make it harmless.

Subsequent studies of the effect of feeding hay on O157 have been confusing, Callaway adds. "The evidence breaks even, some is positive, some is negative, some is nil. Overall we can't say there is or is not an effect."

Washed-out solution
Francisco Diez-Gonzalez, associate professor of food science and nutrition at the University of Minnesota, who was primary author of the 1998 study (see "Grain Feeding and the Dissemination..." in the bibliography) confirmed Callaway. Man in glasses slices rings of cow intestine and places them in round dish"Based on the literature that exists now, there is no clear association between diet and O157. The effect on total E. coli, that may or may not be pathogenic, has been corroborated, it's very consistent. Hay feeding lowers the number of E. coli. But the problem is, when you try to observe the same effect with O157, there have been very conflicting results. It depends on what study you look at... We don't have a consensus on whether diet has much effect on [pathogenic] E. coli."

Sausage, anyone? At the National Animal Disease Center in Ames, Iowa, pathologist Joachim Pohlenz trims intestinal tissues to be examined for E. coli O157:H7 infection. Photo: USDA

One clue to the disparity, Diez-Gonzalez suggests, is that forage increased manure concentrations of O157 only when researchers deliberately fed O157 to the animals: "Any study that was done with naturally infected animals showed either no effect, or that pasture reduces prevalence." But many other factors could account for the different findings, he adds. "There are different [bacterial] strains that researchers use, different conditions, different feeds, and different animal genetics; there is a wide range of variations."

If changing the feed is no guarantee, what else might reduce E. coli O157:H7 in cattle manure?


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

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