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Animals In Print
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From 9 February 2004 Issue
 

Mad cow's untold story
Studies quietly raise questions about threat to humans
By Todd Hartman, Rocky Mountain News
January 12, 2004

Below the drumbeat of reassurances from government and the cattle industry that mad cow disease poses no threat to public health, a small universe of scientists working on a family of related illnesses are finding disturbing evidence to the contrary.

Several little-publicized studies, as well as ongoing research at a government laboratory in Montana, continue to spark questions about human susceptibility not only to mad cow, but to sister diseases such as chronic wasting disease that mainly affects deer and elk, and to scrapie, which infects sheep.

Mice research and clusters of cases where humans contracted a disease similar to mad cow also has some scientists wondering if consuming infected meat might have killed far more people than medical experts have long assumed, not only in Great Britain, but in the United States as well.

For that matter, some say, it's possible that CWD, endemic in Colorado's wild deer and elk, has also sickened and proven fatal to humans, but doctor's haven't - or can't - yet recognize signs of the disease during an autopsy.

Little of this research, nor the wary comments of researchers toiling in relative obscurity, reaches a broad public audience, leaving most people to hear the oft-repeated promises of well-funded interest groups or high-profile public officials.

Just last week, for example, the National Cattlemen's Beef Association described mad cow disease solely as an animal and economic problem - not a human health problem. The U.S. Department of Agriculture and Colorado's own commissioner of agriculture have made similar pronouncements.

Such statements, offered frequently since the December discovery of a Holstein with mad cow in Washington state, spark criticism from scientists and consumer advocates who say that the government and industry are injecting certainty into a field where uncertainty remains the dominant theme.

Much remains unknown about so-called transmissible spongiform encephalopathies, or TSEs, the family of diseases to which mad cow, scrapie, CWD and two human versions - known as Creutzfeldt-Jakob, or CJD, and "new variant" CJD - belong.

This rogues' gallery of deadly illnesses features an array of frightening symptoms. In humans they can include depression, inability to conduct simple tasks like operating a telephone and loss of balance and speech. In every case, the victim's brain becomes riddled with holes; thoughts, emotions and memories disappear on the way to death, often in just a matter of months.

But much about the diseases remains a mystery.

Clusters of a rare disease

It's long been believed, for example, that only one of the two versions of human CJD can be linked to eating meat contaminated with mad cow disease. The other version, known as sporadic or classical CJD, has long been thought to occur randomly in about one in every million people, a distinctive illness with no link to meat from an infected animal.

But studies in the last three years challenge that view, suggesting that classical CJD, might, in some cases, also result from eating contaminated meat.

Scientists in Great Britain injected mad cow disease, also known as bovine spongiform encephalapothy, or BSE, into mice whose brains were genetically engineered with human genes. One set of mice fell sick with the human form of mad cow, or new variant CJD.

But, in a finding that stunned researchers worldwide, another set of the genetically altered mice developed what looked exactly like the sporadic form of CJD, the one scientists have long believed has no relationship to mad cow disease or meat-eating.

"This finding has important potential implications as it raises the possibility that some humans infected with (mad cow disease) may develop a clinical disease indistinguishable from classical CJD," the researchers wrote in November of 2002.

Another study by French scientists a year earlier, found a strain of scrapie, the version of the disease that infects sheep, also caused brain damage in mice similar to classical CJD.

Together, the two studies suggest that at least some of the hundreds of Americans who contract classical CJD each year could have been infected by BSE-contaminated meat, and not simply by biological bad luck.

If true, it would put an end to the belief that no American has contracted a TSE through meat consumption in this country.

Then there are the mysterious clusters of human cases. Because classical CJD is so rare, scientists don't usually expect it to occur in clusters. But such clusters have occurred, including among hunters eating venison.

The most recent example involves seven people who apparently died of classical CJD in New Jersey. All of them had a link with a racetrack in Cherry Hill where they ate, according to reporting this month in The Philadelphia Inquirer.

Most surprising: The deaths included a 29-year-old. It's extraordinarily rare for someone so young to die of classical CJD. Victims are typically over age 55. In contrast, victims of new variant CJD during the 1990s mad cow panic in England were often young people.

The U.S. Centers for Disease Control and Prevention wasn't initially interested in the cluster, but since the discovery of the case of mad cow in Washington state the agency is taking another look at the case, according to The Inquirer.

CJD may be underreported

Even so, experts caution that clusters can also be coincidences. Rare diseases don't always end up geographically spread out.

"The cluster is curious, but these sorts of things can happen by random chance," said Patrick Bosque, a neurologist at Denver Health and an expert on TSE. Bosque is open-minded about the possibility of humans contracting such diseases at a higher rate than is currently believed, but is also cautious about jumping to startling conclusions.

"With almost 300 cases of CJD expected in the U.S. each year, it isn't too surprising that over a few years one might occasionally find a cluster associated with some ill-defined event or place," Bosque said.

But scientists also believe that the true number of CJD cases is underreported in this country, and the disease is not as rare as once believed. Several studies and surveys have shown anywhere from 1 percent to 12 percent of Alzheimer's cases, for examples, are actually CJD, since some symptoms can be similar. Some other dementia-like diseases could also be misdiagnosed, experts say.

Misdiagnosis of Alzheimer's

If one considers that Alzheimer's disease kills some 50,000 Americans a year, then the true number of CJD cases is probably more than the conventional estimate of 200 to 300 cases a year, but more like 500 to 5,000 cases if one considers the range of misdiagnosed Alzheimer's cases.

"Given the new research showing that infected beef may be responsible for some (classical) CJD, thousands of Americans may already be dying because of mad cow disease every year," wrote Michael Greger, a medical doctor and crusader on the need for more steps to prevent the spread of mad cow disease in the United States.

Many scientists, however, caution that if mad cow were really fueling an increase of classical CJD in the United States, then we should also be seeing cases of new variant CJD, the form of mad cow that struck people in Great Britain.

"Could BSE have been transmitted to people in the U.S. but gone unrecognized because it looked like (classical) CJD?" Bosque said. "I suppose it is possible, but there would be no obvious explanation for the fact that (new variant) CJD has not been seen in the U.S."

But critics answer that the United States has no formal monitoring system in place to track CJD cases and autopsy the victims. That would enable scientists to more accurately detect trends in occurrences and study the pattern of destruction in the brain that reveals whether a victim died of classical or new variant CJD.

That's also why it's believed that some Alzheimer's patients, who aren't typically autopsied, may actually have contracted CJD but the diagnosis is never confirmed through study of the brain after death.

As it is, doctors at the National Prion Disease Pathology Surveillance Center at Case Western Reserve University in Ohio estimate they analyze tissues from less than half of the known CJD victims in the U.S. annually.

TSEs borne by silent carriers

At a National Institutes of Health laboratory in western Montana, researchers are focused on the question of what happens when TSEs move back and forth between different animals. What they've found is unsettling.

Their latest research is an extension of work they've been conducting for several years. It shows that TSEs can be passed to some animals without causing disease, but then passed onto another species where they cause illness.

The implications are significant.

Researchers began by taking brains of hamsters infected with scrapie and injecting that tissue into mice. Then, they killed the mice at various points and tested for presence of the infectious agents - called prions - in the brains of mice. They didn't find any.

But researchers took the next step, injecting tissues from these apparently prion-free mice into another set of healthy mice and hamsters. That's when the outcome turned frightening: the newly infected creatures developed TSEs and died.

What does it mean? The mice from the earlier part of the experiment were not free of the disease after all. They were silent carriers of a sort, not showing any signs of the illness but able to infect others.

This has dramatic implications for the real world. What if, for example, some humans carry mad cow disease or another TSE, but aren't affected by it. Perhaps they could still pass it on.

In the words of researchers: "Subclinical human carriers might pose a serious risk for contamination of surgical instruments, tissue transplants and blood products," according to a 2002 report on the mice-hamster experiments in Montana. Then there are the ramifications for animals.

Cross-breed infection

Michael Hansen, a scientist for Consumers Union, points out that cattle tissues that are mostly removed from the human food chain are still converted to feed for chickens and pigs, among others.

Remains of those animals, in turn, are still rendered into cattle feed - a practice derided by critics and banned in Europe but still legal and common in the United States.

"Those animals could become silent carriers and infect cattle," Hansen said.

The Montana researchers have continued their work, injecting tissues from the mice and hamsters that became sick into yet another set. The next generation of mice and hamsters got sick as well. The experiments have also revealed another disturbing fact.

The infectious prions appear to persist and adapt, evolving to infect a new species and even becoming more virulent as they move between species. In some cases, incubation periods change, growing longer or shorter depending on the experiment.

In short, behavior of the diseases has been unpredictable, making statements of certainty about them untenable.

The ability of the TSEs to adapt and change suggests a number of things: If mad cow, as believed, started as scrapie in sheep, did its adaptation during passage to cattle increase its ability to infect humans?

If so, the Montana researchers suggest a similar outcome could occur with CWD, where its transmission to other animals or livestock could change its characteristics and boost its potential to infect humans.

"In wildlife and agricultural settings, where TSE diseases might be transferred across species barriers, there could be other situations that lead to (silent carrier) infection and unexpected adaptation or spread to additional species," they wrote.

Bosque, the Denver neurologist, said the ability for TSEs to hide inside some animals and increase their infectivity is worrisome.

"It raises an additional level of concern, that these prions can persist in animals, and increase, even though (the animals) don't appear sick," Bosque said. "For one thing, it shows you that screening for just obviously sick animals may not be sufficient."

But currently, the United States focuses its testing almost solely on sick or "downer" animals. Industry officials have said testing every cow for mad cow disease as Japan does would be a waste of time and money.
    

http://www.rockymountainnews.com/drmn/state/article/0,1299,DRMN_21_2569718,00.html

Copyright, Rocky Mountain News
 

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