Jumble of Tests May Slow Mad Cow Solution
January 4, 2004
By SANDRA BLAKESLEE
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
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
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
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
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
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.