GUARDING AGAINST MAD COW DISEASE
One diseased cow in Canada sparks calls for stronger regulations in the U.S.
BETTE HILEMAN, C&EN WASHINGTON
Mad cow disease has been making headlines again. One animal with the disease--technically known as bovine spongiform encephalopathy (BSE)--was found in Canada on May 20. Although no other cases have been detected since, the one diseased cow has had a huge impact on Canada's billion-dollar beef industry.
The U.S., Japan, and more than 20 other countries have banned beef imports from Canada, causing that nation to lose $8 million a day in sales. Furthermore, Japan and South Korea are demanding that the U.S. ensure that none of its beef exports are commingled with beef from Canadian cattle. This is difficult to do because the Canadian and U.S. beef markets have been integrated for years, and before May 20, many cattle were shipped across the U.S.-Canadian border each day.
The Canadian BSE incident has also led to renewed calls in the U.S. for tighter government regulations to prevent the spread of mad cow disease. Several consumer groups want the number of U.S. cattle tested each year for BSE to be increased greatly. They advocate banning certain materials from cattle feed and are demanding changes in the feed given to poultry. They claim the U.S. is not following World Health Organization (WHO) guidelines for prevention of BSE.
At the same time, researchers have found suggestive evidence that BSE could be causing more disease in humans than has been estimated. According to European Union (EU) figures, 132 deaths from eating BSE-infected beef have occurred.
BSE is an always fatal neurodegenerative disease that has been found in animals in 35 countries around the world. Scientists believe it is caused by an aberrant misfolded prion protein that, by a chain reaction, induces normal prion proteins on nerve cells to misfold and form amyloid plaques in the brain. The disease eventually creates holes in the brain, hence the name spongiform encephalopathy. The infectious prion protein cannot be inactivated with ordinary disinfectants, nor is it completely degraded except by temperatures close to those used for incineration.
Cattle contract the disease through consuming meat and bone meal produced by rendering the remains of animals infected with BSE. Humans contract a similar disease, variant Creutzfeldt-Jakob disease (vCJD), through eating BSE-infected beef, scientists believe. BSE has an incubation period of two to eight years; for the human equivalent, vCJD, the period is as long as 30 years. In Europe, more than 5 million head of cattle have been destroyed since 1986 to stop the epidemic of BSE, but new BSE cases continue to crop up.
WHERE'S THE BEEF? More than 20 countries have banned Canadian beef following an isolated outbreak of mad cow disease.
CONSUMER GROUPS NOTE that the EU tests about one in four cattle for BSE each year, whereas the U.S. tests one in 5,000. They believe that if a larger fraction of the 35 million U.S. cattle slaughtered each year were tested with a fast, inexpensive prionics test, so-far unrecognized cases of BSE might well show up. They point to experiences in France and Ireland. Many more mad cow cases came to light in both countries when they initiated large-scale testing programs.
"Even though the U.S. has increased the number of cattle tested each year, we don't think it is enough, basically," says Karen L. Egbert, senior food safety attorney at the Center for Science in the Public Interest.
Lester M. Crawford, deputy commissioner at the Food & Drug Administration, disagrees. Such critics "have made a serious intellectual error," he says. "What we have done since 1989 is look for BSE in susceptible populations--that is, those animals that are old enough and are showing neurological signs. If you are ever going to find BSE, you will find it in animals that have symptoms," he says. "There is no use doing what some countries do--testing animals younger than 30 months--because you are never going to find BSE there when the average incubation period is about four-and-a-half years.
"If any country is set up to potentially find BSE, it is the U.S.," Crawford explains. "We are literally the only Western country that has never had a case of BSE because of the Draconian prevention program we put in place in 1989--five years before England did," he insists. In 1989, the U.S. prohibited all imports of cattle and other ruminants from countries with BSE.
Michael Hansen, senior research associate at Consumers Union, is not swayed by such arguments. He believes the U.S. should be screening all so-called downer cattle--those too sick or injured to stand up--with a quick prionics test, as is done in Europe. According to U.S. Department of Agriculture figures, at least 200,000 cattle are slaughtered each year in the U.S. because they can no longer stand or walk. BSE cases are most likely to occur among such cattle.
There is, however, still some controversy over which tests to use and just how effective each might be.
THE EUROPEAN COMMISSION, the administrative arm of the EU, has approved five rapid BSE tests based on the use of antibodies to the prion protein. "These tests are far cheaper and give results far more quickly than the testing methodology used by USDA's Animal & Plant Health Inspection Service (APHIS)," Hansen says. The newest test, and the one most recently approved by the EC, is based on a novel technology--conformation-dependent immunoassay (CDI). It appears to be more sensitive and accurate than the other rapid tests, he says.
The CDI test was developed in the lab of Stanley B. Prusiner of the University of California, San Francisco [Nat. Biotechnol., 20, 1147 (2002)]. Prusiner received a Nobel Prize in 1997 for the prion theory. This automated high-throughput procedure is manufactured by InPro Biotechnology and assays cattle brains for about $20 per sample.
Unlike the other four rapid tests, which are prone to false positive and negative results, Prusiner's does not use enzymatic digestion to degrade the normal prion protein and then test for any remaining (undigestible) infectious prions. Instead, it employs denaturation to expose a portion of the abnormal prion protein. Using high-affinity antibodies, it then is able to directly measure abnormal prion protein, detecting much lower levels of the infectious prion protein than can standard immunological procedures.
Recently, Japan and the U.K. have adopted the CDI method. Japan will employ it to test all of its cattle over 30 months that are slaughtered for meat, and the U.K. has similar plans.
However, an APHIS veterinarian, who wishes to remain anonymous, claims the test the U.S. uses is superior to CDI. All the rapid assays are screening tests, and any positive samples they detect must be confirmed with additional testing, he says. "This confirmatory testing is immunohistochemistry--the gold standard," he says. "We are using immunohistochemistry as our primary test in our surveillance program. It is no more expensive--and in some instances may be cheaper--than the rapid tests."
Jiri Safar, the lead researcher who developed the CDI method in Prusiner's lab, disagrees. "Immunohistochemistry was never systematically evaluated for diagnostic accuracy and specificity over the time course of the BSE infection," he says. In 11,000 tests, the CDI method correctly identified samples of diseased and normal tissue with 100% accuracy. Its sensitivity is comparable with the most sensitive test available--a bioassay using mice genetically engineered to express multiple copies of the bovine prion protein, he says. Therefore, CDI does not need confirmation with immunohistochemistry, Safar explains. Because "many animals may be infected with BSE but never show clinical signs of central nervous system dysfunction," it is important to test cattle with a highly sensitive test, he adds.
There is also disagreement over whether the relatively small number of cattle tested each year in the U.S. violates WHO standards as laid out in its animal health code. USDA claims on its website that by testing 20,000 animals per year, it is doing more than the minimum recommended by the code--433 animals. This, however, is the required number of animals showing clinical signs consistent with BSE that should be examined each year as stipulated in Article 188.8.131.52 of the code. In contrast, Article 184.108.40.206 says: "Cattle that have died or have been killed for reasons other than routine slaughter should be examined. This population will include cattle who have died on farms or in transit." According to USDA figures, more than 200,000 cattle die or are killed for reasons other than routine slaughter each year.
A FURTHER KEY recommendation in WHO's animal health code is that "all countries should ban the use of ruminant tissues in ruminant feed." The feeding of meat and bone meal from rendered cattle to poultry may violate this provision. According to the Organic Consumers Association, "the major concern in feeding rendered cattle remains to other animals is that the cattle remains may directly or indirectly find their way back into cattle feed, which could potentially spark a British-style outbreak of mad cow disease."
One of the most direct ways to get cattle-derived meat and bone meal into cattle is the practice of feeding them poultry litter. Meat and bone meal from rendered cattle is often added to poultry feed. Chickens spill a portion of their feed on the floors of the poultry sheds. As a result, poultry litter may consist of a mixture of excrement, spilled food (including meat and bone meal), dirt, and feathers. An estimated 1 million tons of this material is scraped from the floors of poultry sheds, pelletized at 140 °F (the temperature of a sauna), and fed to cattle every year. This temperature is not nearly high enough to degrade infectious prions.
"There is a possibility that chickens waste so much feed that the litter can contain up to 30% meat and bone meal," Crawford says. FDA is considering two ways of eliminating potential problems from the feeding of meat and bone meal to poultry, he says. One is to ban the feeding of chicken litter to cattle; the other, ban cattle-derived meat and bone meal from poultry feed.
Ruminant tissues also end up as ruminant feed when calves are weaned on milk replacers consisting of spray-dried blood plasma from cattle, water, and fat. (Some milk replacers instead use whey, a milk product, as a source of protein.) The lymphocytes in spray-dried plasma could potentially contain prions at some stage when BSE is being incubated, Crawford says.
A FINAL CONCERN of consumer groups, such as Consumers Union, is that FDA's 1997 rule banning the feeding of cattle-derived meat and bone meal to cattle is not being rigorously enforced. In 2001, the Harvard Center for Risk Analysis said in its risk assessment of mad cow disease that compliance with the feed rule is the most important factor in preventing a BSE outbreak. However, a General Accounting Office (GAO) report published in 2002 concludes that FDA has been less than strict in enforcing compliance: "FDA has not acted promptly to compel firms to keep prohibited proteins out of cattle feed and to label animal feed that cannot be fed to cattle ... FDA's data on inspections are seriously flawed, and, as a result, FDA does not know the full extent of industry compliance."
The GAO report discusses problems FDA had when it first started the feed ban and inspections, Crawford responds. "We now believe we have better than 99% compliance."
Some researchers involved in the mad cow issue have uncovered evidence that the human toll from eating BSE-infected cattle may be higher than official estimates. First, there is evidence from animal studies that the consumption of BSE-infected beef may sometimes cause what looks like the sporadic form of CJD, which usually arises spontaneously and has not previously been linked to BSE. Research published in December 2002 suggests that BSE may cause both vCJD and CJD. When mice genetically engineered with the human prion protein are injected with a sample of a BSE-infected brain, most develop vCJD, but a small fraction come down with what appears, from the brain pattern of amyloid plaques, to be sporadic.
In another line of research, Laura Manuelidis, section chief in surgery in the neuropathology department at Yale University, and some other pathologists have found from autopsies that 3 to 13% of patients diagnosed with Alzheimer's disease actually had what appears to be sporadic CJD. She believes that some of the cases diagnosed as sporadic CJD may be linked to BSE. France, Switzerland, and the U.K. have experienced an increase in what looks like sporadic CJD since a BSE case was first found in the U.K. in 1989. It is impossible to estimate the true incidence of CJD in the U.S. because autopsies are performed on only 15% of subjects.
It is clear that the mad cow issue is not going away anytime soon. On July 25, USDA asked the Harvard Center for Risk Analysis to reevaluate its risk assessment. This could result in new recommendations for tighter regulations for BSE prevention.