Since the December 23 discovery of the first U.S. mad cow, from Mabton, Wash., federal regulators have been under pressure to strengthen beef-safety rules. The pressure seems to have worked: this past Tuesday, Secretary of Agriculture Ann M. Veneman announced new and expanded rules to cut the risks associated with mad cow disease, formally known as bovine spongiform encephalopathy (BSE).

Most scientists believe that cows contract BSE through feed contaminated with an infectious form of a protein called a prion. Normal prion proteins probably help maintain brain cells; misshapened, the prion becomes pathogenic and can force normal prion protein to adopt the misfolded shape. These rogue forms resist a cell¿s attempt to break them down, so the malformed prion protein builds up and eventually ruptures the cell.

In August 1997 the U.S. Food and Drug Administration banned the feeding of most mammalian protein to cows. In the U.K. that practice amplified the disease agent to create the BSE epidemic, which began in 1985 and peaked in 1992. In 1996 British researchers realized that BSE could jump to humans, who develop variant Creutzfeldt-Jakob disease (vCJD).

The 1997 feed ban, along with import restrictions and other measures, was supposed to prevent U.S. and Canadian cows from eating contaminated feed. The U.S. case was born (apparently in Canada) in April 1997, so it consumed risky feed before the material was banned later that year. The cow probably got infected when it was young, because of BSE¿s long incubation, which can run upwards of eight years (the average is about five). But the cow could have gotten sick during the ban period, because feed mill operators and rendering plants did not fully comply with the regulations at first: in a January 2000 report, the Food and Drug Administration revealed that about one quarter of the firms were out of compliance. The latest survey found only two out of 1,826 firms to be out of compliance, reported Steve Sundlof of the FDA at a December 24 press briefing.

Some of the newest protection measures announced by Veneman go into effect immediately. "Downer" cattle--those unable to walk on their own --will no longer be served up. Specifically, food inspectors will condemn any animal showing signs of neurological illness, making illegal its use in human food. Moreover, cows suspected of having BSE will be held until test results return. Both measures directly address the example of the U.S. mad cow, which was a downer that was turned into human food and animal feed before test results were in.

Other measures to be implemented include a better, national system of animal identification and tracking and the banning of particularly risky material from cattle over 30 months of age from use in human food. These high-risk parts include the brain, spinal cord, eyes, dorsal root ganglia and other nervous-system tissue. Also to be banned, from cattle of all ages, is the distal ileum, a part of the small intestine known to harbor infectious prions for a brief period in a cow¿s life. The USDA has additionally forbidden the stunning of cattle with air prior to slaughter. In this procedure, air is blasted into the cattle¿s head, scrambling its brain. The pulse of air is so powerful, however, that bits of brain are blown out of the skull, down the spinal column and into the bloodstream. (Brain matter, in fact, has been found in kidneys, organs not normally thought of as risky.) Air-injection stunning, however, is rare; the dominant method relies on a "captive bolt," in which a bolt, not unlike those used as door locks, is fired into the skull and then retracted back into the gun.

Still being considered is more widespread testing of cattle. Currently, the USDA relies on the examination of downers, in which BSE is more likely to be found. But, early on, the USDA did not look very hard: in the 1990s it examined only a few hundred downers at most, far too low to detect BSE even if mad cow disease were as rampant as it was in France (the third hardest hit country, after the U.K. and Ireland). In 2001, France¿s peak BSE year, the country uncovered 191 mad cows out of 134,358 tests on high-risk cows. This ratio suggests that about 704 cows must be tested to find one BSE case. The closest the U.S. came to this number in the 1990s was in 1999, when it tested 651 downers. In fact, the U.S. should have examined many more, based on the presumed prevalance of BSE. France saw roughly 30 BSE cases per million head of cattle in 2001; if BSE occurred in the U.S. at a low rate of one case per million cattle (as has been seen in some European nations), then the country should have been testing several thousand downers every year.

Only in the past two years has surveillance reached acceptable levels, hitting 19,900 in 2002 and 20,526 in 2003. (The detection of the U.S. cow could therefore be seen as a success of the testing strategy.) The USDA aims to test 38,000 downers in 2004, according to the USDA¿s chief veterinarian Ron DeHaven.

Consumer advocates, some scientists, and even Democratic presidential hopeful Howard Dean have proposed BSE testing for every cattle that has been slaughtered for food. (Existing BSE tests are postmortem, as they require a brain tissue sample.) The vast majority of cattle, however, are slaughtered when young: 88 percent are killed at under 18 months of age. Existing tests have not reliably and consistently shown an ability to detect incubating BSE cases in such young cattle. (The youngest detected so far were 21- and 23-month-old cows in Japan.) Testing all 35 million cattle that go to slaughter in the U.S. every year could therefore lead to artificially high negative results. The testing of all slaughtered animals needs to wait for newer, more sensitive tests, or even better, a test that works on live animals.

The U.S. might consider following Europe¿s lead of testing all cows over 30 months of age. These older animals are much more likely to pose a mad cow threat. Such a strategy would undoubtedly require the USDA to adopt the "rapid tests" used in Europe and Japan. The USDA currently relies on a traditional method called immunohistochemistry, which demands antibody staining of a brain sample and microscopy to analyze the stains. Several companies, such as the Swiss firm Prionics and Bio-Rad of Hercules, Calif., have invented "rapid" tests that can produce results in hours. These tests rely on enzymes known as proteases to break down normal prion protein in a sample. The rogue prions, which resist protease digestion, are left behind for detection by custom-designed antibodies.

The latest batch of control measures addresses many BSE concerns and closes some of the loopholes in existing regulations. There is still room for debate--brain and spinal cord from young cows can still enter the food chain. But the expanded rules should go a long way to easing fears at the freezers. ¿Philip Yam

 

Philip Yam is Scientific American¿s news editor and author of the book The Pathological Protein: Mad Cow, Chronic Wasting, and Other Deadly Prion Diseases, published in June 2003 by Copernicus Books.