Thursday, May 31, 2012
Critics Question Benefits of Broader E. coli Screening
Every week, dozens of kilograms of raw beef from meat-processing plants across the United States are rushed to one of three federal laboratories to be picked apart, pulverized, incubated and analysed. Working for the US Department of Agriculture (USDA), technicians in these labs are searching for Escherichia coli O157:H7, a toxic variety of the bacterium that has been monitored in US beef since 1994. But beginning on 4 June, the search will expand to include six more E. colisubtypes or ‘serogroups’.
US agriculture secretary Tom Vilsack announced the change last September, saying it would “stop problems before they start”. The USDA portrays the measure as a way to reduce risk, but it has not quantified the number of people that would be spared illness by the change. With the new screening protocol now imminent, meat producers are questioning the rationale behind the move and at least one outspoken expert agrees.
“I think it’s a fundamentally flawed policy, and it’s not based on the best science,” says Michael Doyle, a microbiologist and director of the Center for Food Safety at the University of Georgia in Griffin. He says that the USDA risks wasting limited resources chasing bacteria whose effects on human health are still unclear.
Most types of E. coli are harmless or even beneficial, such as those that colonize the healthy human gut. But some, such as O157:H7, can produce Shiga toxin, which can be harmful. The USDA began screening raw beef for this microbe after it was linked to the deaths of four children who ate contaminated hamburgers in 1993. Readily identifiable through rapid screening, O157:H7 can sometimes trigger haemolytic uraemic syndrome, which can cause life-threatening kidney damage.
Over the past decade, improved diagnostic tests have linked a growing number of food-borne illnesses to other Shiga-toxin-producing E. coli (STEC) serogroups, including the latest additions to the USDA’s testing regimen, dubbed the Big Six. Last year, an STEC subtype traced to contaminated fenugreek sprouts made thousands of people across Europe ill, and killed at least 53.
The Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, estimates that 173,000 STEC infections occur annually across the United States, with O157:H7 responsible for roughly 36% and the Big Six accounting for most of the remainder. By targeting these, the USDA is bringing food regulation in line with public-health data, says Robert Tauxe, deputy director of the division of food-borne, waterborne and environmental diseases at the CDC. “We think this is an important step forward in preventing human disease,” he says.
Industry representatives counter that the expanded testing will be redundant, because existing O157:H7 testing is an indicator of overall cleanliness and food safety. “We have done extensive research over the past several years that shows that the preventative controls in our facilities today work equally well for the non-O157:H7 as they do for O157,” says James Hodges, executive vice-president of the American Meat Institute (AMI), a trade association in Washington DC.
The AMI estimates that the new tests will cost the industry between US$172 million and $324 million a year, because it contends the extra sample analysis will force processors to hold onto meat longer before distributing it. More tests will also mean more false positives that require investigation, says Doyle, delaying release of the meat and lowering its market value.
The USDA acknowledges that most infections linked to the Big Six do not stem from contaminated beef but from other sources, such as vegetables. The tests will not eliminate risk of E. coli infection, says David Goldman, assistant administrator for the Office of Public Health Science at the USDA’s Food Safety and Inspection Service. Nonetheless, he says, “we expect that this is a truly preventative measure”.
The pay-offs of such a precaution are far from certain. Although the Big Six are increasingly seen as sources of illness, their toxic effects are often less severe than those of O157:H7. The serogroups are also divided into strains, not all of which produce toxins. Shiga toxins themselves occur in types that vary in toxicity by up to a factor of 1,000. Scientists are still trying to determine which strains are likely to produce the most harmful Shiga toxins, and to identify auxiliary molecules that predict a given STEC’s virulence in humans.
“We’re learning more and more, but it’s very complex,” says Phillip Tarr, a microbiologist at Washington University in St Louis, Missouri. Tauxe concedes that more research is needed to better understand STEC pathology. “This is an area that we expect to evolve,” he says. But if the meat-packing industry challenges the rule in court, as it has done for some previous USDA measures, the science of E. coli toxicity could soon find itself again under the microscope — this time, a legal one.