Risk of BSE in USA is low, say US investigators

December 15, 2001 Lancet Volume 358, Number 9298 by Laura Newman
The US government will improve measures to prevent the spread of bovine spongiform encephalopathy (BSE) even though a report released by US scientists on Nov 30 suggests the risk of a BSE epidemic in the USA is minimal.

"No evidence that BSE nor its human form (variant Creutzfeldt-Jakob disease) has ever been introduced in the US exists--and if it ever were to emerge--we could quickly contain it", said George Gray, a toxicologist and lead investigator of the report by the Harvard Center for Risk Analysis, Boston, MA, USA.

Although the risk seems to be low "preventing BSE is a top priority of the USDA, said US Department of Agriculture (USDA) Secretary Ann Veneman, who commissioned the Harvard Center for Risk Analysis to examine the risk of BSE in the USA. The report is "an important tool for gauging the effectiveness of USDA's current programme" and will "help us improve and strengthen our firewalls", said Veneman.

A ban on animal feed containing waste bovine tissue is the key government strategy to keep BSE at bay, according to the report. Gray estimated that even with a worst-case scenario, the number of infected animals would be limited to the single digits each year, with "little spread".

Bans on importing live ruminants and ruminant meat and bone meal from the UK went into effect in 1989 and were extended to Europe in 1997. Also, in 1997, the Food and Drug Administration banned recycling of potentially BSE-infected cattle tissue for cattle feed. However, the USA still allows recycling of this tissue for chicken and hog feed.

The report is based on a mathematical model that simulated the introduction of BSE into the USA through various means, accounting for its natural history and the probability of human exposure to infectious cattle. According to the model, spread would most likely result from failure to comply with the FDA feed ban, misfeeding on farms, and mislabelling of feed and feed products prohibited for consumption by cattle. The disposal of sick animals could also influence spread, the report states.

Gray acknowledged that the model could not be formally validated, but he explained that it was tested on the BSE outbreak in Switzerland, and they found that the model's predictions were "reasonably close to empirical observations". However, the model calculated that Switzerland had 170 BSE-infected cows, whereas the actual number of infected animals to date is 398, noted the report.

The USDA will take several measures in response to the report, said Veneman. They will double BSE testing on cattle every year and propose a ban on certain stunning devices used in slaughterhouses because they can cause splattering of infectious brain tissue into other organs.

The department will also announce an options paper in the federal register that will outline possible regulatory actions to limit BSE exposure and will invite public comment. These include prohibiting the use of brain and spinal cord from certain cattle in human food; prohibiting the use of central nervous system tissue in boneless beef, including mechanically recovered meat, and prohibiting the use of the vertebral column from mechanically recovered meat taken from cattle showing symptoms of BSE.

Some experts have questioned whether the report and USDA's proposed actions provide enough insurance against a major outbreak. "A study does not replace very aggressive testing", warned Pierluiggi Gambetti (National Prion Disease Pathology Surveillance Center, Case Western Reserve University, Cleveland, Ohio, USA). "It is important to know whether we have zero cases, a few, or just what order of magnitude."

At first glance, doubling the number of BSE tests sounds impressive, until you realise that the USA slaughters 37 million cattle per year and they plan to test about 12 000, Gambetti explained. "The number is minuscule compared to the total number of cattle in the US", he added. And rules can only go so far, Gambetti explained. "It's another thing entirely to enforce them." Even with rigorous enforcement, Gambetti said that he would expect the BSE risk in the USA to be closer to that found in France, Switzerland, and Germany--not the UK.

Michael Hansen (Consumer's Union, Yonkers, NY, USA) questioned the model's ability to accurately predict safety. "For one thing, it only focuses on British BSE and not other TSEs [transmissible spongiform encephalopathies], such as chronic wasting disease or potential undiagnosed TSE in cattle in the US". Hansen also raised concerns about a September, 2000, Government Accounting Office Report, that revealed weak compliance with feed rules in the USA.

Other advocacy groups also questioned whether the report went far enough, including the Center for Science in the Public Interest and Public Citizens Health Research Group, both in Washington, DC, USA. Although the USDA paid for the study, several newspapers noted that a sticking point for many consumer and environmental groups was the Harvard Center for Risk Analysis' longstanding reputation as an industry-supported think-tank.

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