Jumble of Tests May Slow Mad Cow Solution

January 4, 2004

The nation's first case of mad cow disease has led to urgent calls for more and better tests to screen animals at the slaughterhouse door.

But the universe of testing for this elusive disease is murky. The extent and nature of testing varies from country to country. The tests are not foolproof, and there are many to choose from in a heated international competition.

The leading test manufacturers are Bio-Rad based in France, Prionics AG in Switzerland and U.S. Abbott Laboratories, which recently acquired rights to a test developed in Ireland. But in addition, at least 54 other companies are vying for position in the lucrative world testing market.

Consumers may want a quick testing solution that will reassure them, but it is not likely to arrive soon.

The Agriculture Department announced on Wednesday it would no longer allow downed cattle - those that have trouble walking to the slaughterhouse under their own power - into the human food supply and will test some of them. How many has not been decided, although more than half a million animals fall into the downer category each year.

Those cows will still be used for pet food and other products, including tallow and cosmetic ingredients.

Mad cow poses a challenge to testers because it is so different from most diseases. Most infections are caused by bacteria and viruses. Tests for them can show the infectious agents themselves, or the immune system's reaction to them. But most scientists believe that mad cow disease is caused by agents that are far more elusive - misfolded proteins called prions.

The prions, which are not broken down by a cell's natural cleanup system, accumulate in cells. Once they start accumulating, the theory is that the prions form a runaway chain reaction that ultimately destroys brain tissue.

Unfortunately, prions cannot be seen under a microscope or cultivated in a laboratory. They do not contain DNA or RNA and cannot be isolated with techniques developed to find those substances.

The main strategy of current tests is to use enzymes - digestive proteins that chew up other proteins. A tissue sample from the brain stem of a suspect cow is mixed with enzymes. Normal prions are broken down, but abnormal prions are sturdy and resistant and some are left over after the sample is treated. The key is finding ways to detect residual prions.

Conventional testing methods are cumbersome, said Dr. Stephen Dealler, a medical microbiologist at Royal Lancaster Infirmy in Lancaster, England.

But a greater problem lies in the mysterious course of mad cow disease. Worldwide testing shows that it can take from 20 months to 15 years for an animal to exhibit infectious prions in its brain stem after eating infected feed. No one knows if prions accumulate in small amounts in other issues early on. Current tests are devised to only find them in the brain stem.

In the United States, even under the new rules, testing will be quite limited, restricted almost exclusively to downed cattle. In the last 13 years, the Agriculture Department has tested about 57,000 animals out of about 390 million slaughtered. Last year, it tested 20,526 cattle out of 35 million killed. The tests were done on brain samples at a laboratory in Ames, Iowa. The testing method they use, which takes a week, is immunohistochemistry - a staining technique often referred to as "the gold standard."

The Agriculture Department has approved two very different and fast testing methods to diagnose wasting disease in deer and elk, a malady similar to mad cow disease which is endemic in this country. Because deer and elk show signs of disease much earlier in lymph tissue, samples can be obtained in live animals from tonsil biopsies.

The tests are based on similar methods for finding mad cow disease but have not been adapted for use in cattle.

In contrast, European countries use five approved tests to screen a million cattle a month - one in every four brought to slaughter. No tests are used on muscle meat or other body parts.

The result is that France, for example, tests more cattle in a week than the United States has tested in a decade. Most countries test animals over 30 months of age but several, including Britain, test at or above 24 months. The rationale for testing older animals is that prions are thought to take three to five years to reach the brain, although that has never been proved.

In all, European nations tested 19 million cattle last year and found 4,200 cases of mad cow disease, mostly in older cows. But several younger animals, one only 20 months old, have been diagnosed with the disease in recent years in Japan, England and Slovakia.

Animals to be tested are killed and their carcasses held in refrigerated warehouses overnight. Brain stem tissue, where prions are usually first found, is sent to a lab for testing. The diagnosis is sent to the slaughterhouse the next day. Infected animals are culled and incinerated or buried in special landfills.

In Japan, every cow headed for the dinner table is tested for the disease. Since testing began three years ago, only nine out of 1.2 million cattle that have been tested per year were diagnosed with mad cow disease. The youngest was 21 months old.

There is also a question of what tissue to test. Agriculture Secretary Ann M. Veneman said in a recent news conference that "scientific evidence shows that only nervous tissue like brain and spinal cord can carry the infectious agent."

But Dr. Stanley Prusiner, a neurologist at the University of California in San Francisco who won the 1997 Nobel Prize in Medicine for first describing prions as infectious particles, disagreed. "We don't know where and how prions move through the body before they show up in the brain," he said.

But there is a way to find out.

The test would be to infect calves, kill them at different ages, remove at least a dozen types of organ tissues - including muscle, heart and kidney - and place those tissues into mice engineered to have the normal cow prion. If any mice get sick, it would be possible to pinpoint where the infection exists outside the brain, Dr. Prusiner said. That experiment has not been done yet although he has been urging the British Food Standards Agency to do so.