July 19, 1999 University of Idaho researchers say a change in cattle diets suggested earlier as a way to eliminate the threat of lethal E. coli in beef won't work.
The Idaho research refutes an earlier study that suggested switching to a hay diet would significantly reduce the E. coli threat to humans.
"That study claimed that if cattle were fed hay, the E. coli they shed would be sensitive to acid and would be killed in the acidic conditions in the stomach," said Carolyn Hovde Bohach, a UI associate professor of microbiology.
The research was reported in the July issue of Applied and Environmental Microbiology by a UI team of Carolyn J. Hovde; Paula R. Austin and Karen A. Cloud, both UI graduates working as technicians in Bohach's lab; Christopher J. Williams, associate professor of statistics; and Carl W. Hunt, professor of animal science.
Idaho research on a test group of eight Holstein steers experimentally dosed with E. coli O157:H7 harbored the bacteria longer while fed a hay diet than while fed a grain diet. The grain diet is similar to the traditional finishing diet fed before slaughter.
The Idaho study also showed no difference, as a result of the two diets, in the acid resistance of the bacteria passed by the cattle in feces.
The bacteria can contaminate meat during slaughter. Meat packing companies take precautions to prevent contamination and have adopted new methods, such as steam-sterilizing carcasses, to kill bacteria.
E. coli O157:H7 can be lethal to people, particularly children, the elderly and those with weak immune systems. Unlike people, cattle carrying the bacteria show no symptoms and only a small percentage of cattle test positive for it.
The diet switch suggested last fall attracted widespread attention because it promised a new way to further control or eliminate the problem. "Many people thought the problem of cattle carrying E. coli O157:H7 had been solved by switching animals to a hay diet," Bohach said.
Laboratory testing showed the suggested dietary cure was a false hope. "We think it would actually be detrimental to feed cattle hay just before slaughter because the cattle would likely shed more of the bacteria if they are carrying it in their intestinal tract," she said. More importantly, the acid resistance of E. coli O157:H7 from grain-fed cattle was the same as the acid resistance of E. coli O157:H7 from hay-fed cattle.
In the UI experimental trials, consistent high quality diets similar to those now fed cattle before slaughter to produce the marbled beef preferred by many consumers appears to minimize the presence of E. coli O157:H7, Bohach said.
Although their work increases understanding of how the bacteria operates, Bohach said, the eight steers tested in the Idaho study is a sample too small to draw far-reaching conclusions. Their research is the most comprehensive to date on the link between diet and the presence of E. coli O157:H7 in ruminants, however.
In the Idaho study, the eight steers were rotated between a hay diet and a grain diet. Cattle fed the grain diet harbored the bacteria for a shorter time than those on the hay diet. The E. coli O157:H7 from the feces of steers on either diet were equally able to survive in an acidic environment similar to the human stomach.
The research was funded by the U.S. Department of Agriculture, Idaho Agricultural Experiment Station, Idaho Beef Council and United Dairymen of Idaho.
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