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

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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.

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