Editor¿s note: The U.S. Department of Agriculture announced on June 25 an "inconclusive" result on a rapid test used to detect mad cow disease (in the first three weeks of June, there were 5,668 negative results). Results of a confirmatory test with immunohistochemistry, announced on June 30, indicate that the suspect animal was not infected. Because the rapid tests produce a false positive rate of about 0.1 percent, and the U.S. is testing thousands of cattle a month, additional inconclusive results are expected.
Few ailments sound scarier than mad cow disease and its human counterparts. They incubate silently for years, slowly eating the brain away and leaving it full of holes. So it's not surprising that many people want the U.S. Department of Agriculture to test all cattle for the illness, formally called bovine spongiform encephalopathy (BSE). Certainly testing all 35 million cattle slaughtered annually would reopen trade with Japan, which has refused American beef since the discovery of a mad cow in Washington State last December. It might prevent BSE-free countries from dominating the export market. And consumers might simply feel better about their steaks, roasts and burgers. Too bad there's not much science to back up the proposal.
Commercial "rapid tests" are not designed to detect the disease reliably in most slaughtered bovines. They work best on those that have lived long enough to build up in their brains a detectable amount of prions, the proteins at the root of BSE. Typically those animals are older than 30 months or have symptoms, such as an inability to stand (called downer cattle).
Most U.S. bovines, however, reach slaughter weight before 24 months of age--before the tests can accurately detect incubating BSE. Most European countries recognize those limitations and target cattle 30 months and older. But using current kits on all slaughtered animals, at least 80 percent of which are younger than 30 months, may give misleading assurance about the safety of beef.
Do economic and emotional reasons justify that strategy? Testing costs about $25 to $35 per head, amounting to just a few extra pennies per pound. But in total, the "beef tax" would cost around $1 billion annually--for results that are equivocal.
When it comes to keeping consumers safe from prions, we can think of better uses for $1 billion. Like Europe, the U.S. should test cattle older than 30 months. Stricter and more complete enforcement of existing rules is even more critical. The USDA is supposed to check at least 200,000 cattle this year--what probably amounts to the bulk of U.S. downers, the category most likely to test positive. Yet reports of sloppiness have emerged. The most shocking occurred in Texas, where a downer somehow managed to avoid being tested after it was pulled by an inspector. The USDA's management, top-heavy with former beef officials, needs to take a more critical view of its relationship with the industry.
Also lost in the discussion is the surveillance of human prion diseases. Last year only about two thirds of all suspected human cases reached the national prion disease surveillance center at Case Western Reserve University, where brain postmortems are conducted. These examinations provide the evidence as to whether people are dying from prion infections--be they from mad cows or from deer and elk with chronic wasting disease. Additionally, they would help determine whether purported illness clusters, such as one tied to the now demolished Garden State Racetrack in New Jersey, have truly arisen from a common source.
Better assays are coming [see "Detecting Mad Cow Disease," by Stanley B. Prusiner]. They hold promise for detecting prions in young cattle and in cow parts not previously found to be infectious. They may also prove effective in uncovering new prion maladies and in testing live humans. Only when such assays become validated will it make sense to target all cattle. Right now other measures rank higher.