Risk of mad cow disease to humans is very small, WUSTL experts say

WUSTL professors David N. Harris and John C. Morris were among experts commenting in a recent St. Louis Post-Dispatch story on public health implications of the discovery of mad cow disease at a Washington state dairy farm. Harris, who conducts research on prion brain proteins associated with the disease, said that no one yet knows what the protein’s regular function is. Morris, a neurology specialist, said that Great Britain’s experience with the disease suggests that most people who were exposed to tainted meat did not get sick. “It’s undeniable that there is this link (with BSE), and it’s such a scary illness that it overshadows the fact that the individual risk is quite small,” Morris said.

(Republished with permission from the St. Louis Post-Dispatch. This article originally ran in the Science and Medicine section on Sunday, Dec. 28, 2003.)

“Downer” is tested

Risk of mad cow disease to humans is very small, experts say

Diagnosis in dairy cow is “cause for concern . . . but not real alarm,” one says

By Tina Hesman
Of the Post-Dispatch

Doubts about the safety of the U.S. beef supply abound despite government assurances that a case of mad cow disease discovered in Washington is being handled with “an abundance of caution.”

Some consumer groups and foreign trading partners say the United States doesn’t look hard enough for mad cow disease and fails to take precautions to prevent the spread of the disease.

But risk analysis experts say that even in the worst-case scenario, the risk to humans is vanishingly small.

The sick cow, a 4-year-old Holstein dairy cow, is the first animal ever diagnosed with bovine spongiform encephalopathy – popularly known as mad cow disease – in the United States. The animal was one of 20,566 animals tested this year for the brain-wasting disease as part of a federal surveillance program.

That level of testing is enough to detect the disease in one in a million adult cattle with 95 percent confidence, said Ed Curlett, spokesman for the U.S. Department of Agriculture. That exceeds the international requirement, but skeptics say it’s not enough. The European Union tests every downed animal and all cattle over 30 months old. Japan tests every single cow sent to slaughter.

Even before the Holstein was found to have mad cow disease, the USDA planned to increase its surveillance to 38,000 animals, said Ron DeHaven, chief veterinary officer at the agriculture agency.

That number could increase. All of the policies concerning mad cow disease are now under review at the agency, Curlett said.

On Dec. 9, the sick cow was sent for slaughter. Like most of the other cattle sent for testing, the young cow was a “downer,” an animal that is unable to walk due to illness or injury. The cow had recently given birth to her third calf, a bull who was subsequently sold to a feed operation. Damage to a nerve located near the birth canal seemed responsible for the cow’s paralysis, and the U.S. Food Safety Inspection Service veterinarian at the slaughterhouse found no other cause for alarm when the animal was inspected after slaughter, federal agriculture officials said.

The cow’s brain and spinal cord were removed and sent to a federal diagnostic laboratory in Ames, Iowa. The rest of the carcass was sent to processing plants. Under federal regulations, brain and spinal cords from downed animals must be removed before slaughter, but spinal material from animals that appear healthy is fair game for the food chain. One survey estimates that up to 35 percent of processed meat could contain brain and spinal cord material.

Even when downed animals are selected for slaughter and human consumption, the riskiest material – the central nervous system, including the brain and spinal cord and part of the intestine – is removed from the carcass before meat is harvested. That practice leaves little risk that infectious particles, called prions, are left in the meat, agriculture officials stress.

Officials from the U.S. Department of Agriculture instituted a recall of meat from 20 cattle that were slaughtered on the same day as the infected dairy cow out of “an abundance of caution,” not because they believe there is any danger to beef consumers, said DeHaven.

But the use of downed animals in human food is a controversial practice.

Critics say it’s risky business that could expose consumers to infectious material.

“I can’t even offer an explanation for why a downed cow was even considered for slaughter,” said Leon Thacker, head of the Indiana Animal Disease Diagnostic Lab at Purdue University. “Our rule is that a cow has to walk onto the slaughter floor to be considered for inclusion in the human food supply.”

“The system works”

It’s no surprise that the cow was one selected for mad cow testing, Thacker said. Only a few healthy cattle are tested for mad cow disease, because experience in other countries has taught inspectors that sick and wobbly animals are the most likely to be at risk of developing the disease, he said.

But cattle can be downed for a variety of reasons, including broken legs, that have nothing to do with mad cow disease.

“Just because an animal is down does not mean it has BSE,” said Jeff Squibb, spokesman for the Illinois Department of Agriculture. Broken legs and other ailments can hobble an animal, too. And few downer cattle enter the food supply, he said. He estimates that only 10 to 20 downer cattle are slaughtered each month, representing a tiny fraction of the 586.3 million pounds of beef produced in the state each year.

And the fact that the downed cow was ultimately diagnosed with mad cow disease is proof-positive that agriculture officials are on the right track, supporters say.

“From our vantage point, this shows that the monitoring system works,” Squibb said.

Theories, but no answers

Mad cow disease is part of a family of diseases known as transmissible spongiform encephalopathies. The diseases, which include Creutzfeldt-Jakob disease in people, chronic wasting disease in deer and elk, and scrapie in sheep, are all caused when a normal brain protein goes awry. The brain protein, called PrP, studs the membranes of brain cells and cells in the spinal cord.

No one knows what the protein’s regular function is, said David Harris, a prion researcher at Washington University. Scientists have floated a number of theories, “but none of it is absolutely definitive so that it makes you say, ‘That’s it. Absolutely,'” Harris said.

And the jury is still out on exactly why, sometimes, the protein folds into the prion, or scrapie, form that causes disease. In the twisted configuration, the prion sets off a chain reaction, converting other normal molecules of the brain protein into prions. As the reaction progresses, neurons die, leaving holes, and tangles of prion protein litter the brain.

In people, the sporadic form of the disease strikes one in a million. No one knows why. No one knows how often the disease might pop up in cattle, but most researchers now agree that a spontaneous case of mad cow disease probably led to the epidemic among British cattle. The disease is not spread by cow-to-cow contact as other infectious diseases are. The primary route of infection is through feed contaminated with brain and spinal material from an infected animal.

“Holes in the safety net”

In 1997, the United States instituted a ban on using meat and bone meal from cattle and other cud-chewing animals in cattle feed. But the ban has been leaky – federal estimates say that 90 percent of feed producers comply with the ban – and critics contend that violators could be spreading the disease.

The General Accounting Office issued a report in February 2002 that indicated leaks in the feed ban could be cause for concern if a case of mad cow disease were to appear in the United States. Many critics say that the Washington case is evidence that the feed ban measures have been inadequate.

“I could say it doesn’t surprise me that a case popped up, because our previous report indicated that there were holes in the safety net,” said Larry Dyckman, a director at the General Accounting Office. His office is preparing a follow-up review of the feed ban, which could be available in the spring.

Prion diseases may incubate for years, even decades, before an infected person or animal shows symptoms. Creutzfeldt-Jakob, the human prion disease, usually shows up in patients after age 60. The form of the disease associated with eating tainted beef is known as variant Creutzfeldt-Jakob disease (vCJD) and typically strikes much younger people. The two diseases are clinically distinct, despite their similar names.

Scientists previously thought that only cattle older than 30 months old could develop the disease. But the discovery of a young bull in Japan – one that was less than 2 years old – showed that some younger animals also can get the disease. But whether young animals harboring an undetected prion disease are infectious is a matter of debate.

Risk is called “close to zero”

Even if mad cow material gets into the human food system, risk analysis experts say humans have very little to fear.

At the peak of the British mad cow epidemic, 1,000 new cases were detected in cattle each week, said John Morris, a neurologist at Washington University. Despite the prevalence of the disease in cattle, only 143 people in Great Britain contracted the variant form of the disease. That indicates that most people who were exposed to tainted meat did not get sick, Morris said.

“It’s undeniable that there is this link (with BSE), and it’s such a scary illness that it overshadows the fact that the individual risk is quite small,” Morris said.

How small?

“As close to zero as science will ever get,” said David Ropeik, director of risk communication for the Harvard Center for Risk Analysis. That’s little comfort to people who are inclined to worry about unlikely events, he admits.

“Ultimately, the only acceptable risk to all us is zero. Even a one in a million chance is too high if you’re the one.”

The Harvard center issued a report that calculated how fast and far a mad cow epidemic could spread in this country. In the worst case scenario – 500 cows with the disease and only 75 percent compliance with the feed ban – it would take 20 years to choke the disease out of the beef supply, but the chance of infectious material entering the human food supply and sickening people was still virtually zero, Ropeik said. That analysis is supported by the course of the disease in Great Britain, he said.

“There’s definitely cause for concern in this,” he said, “but not real alarm.”

Reporter Tina Hesman
E-mail: thesman@post-dispatch.com
Phone: 314-340-8325