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Mad Cow's Human Toll
The unfolding mystery of prion disease and its ultimate casualties

May, 2001 Scientific American by Philip Yam 

A TOUGH LITTLE
NEURO-INVADER
Malformed prions are thought to cause TSEs. But not all the evidence supports this so-called protein-only theory. A few researchers believe some kind of mini virus might be involved, but there has been no evidence of nucleic acids in infectious prions. In any case, the malformed prions are necessary to produce TSE, and getting rid of them is difficult, because the prions
  • Withstand typical cooking temperatures
  • Are impervious to radiation (one argument against viral involvement)
  • Resist proteases, enzymes that break down protein
Sterilizing instruments against abnormal prions can be tricky. Autoclaving at 134 degrees Celsius inactivates them, but paradoxically, autoclaving at 138 degrees C does not. A prior soak in sodium hydroxide is recommended.





BREACHING
THE SPECIES BARRIER
Cows probably first got BSE by eating feed containing rendered, scrapie-infected sheep. In the U.K., several dozen cats came down with a feline version of BSE after eating infected pet food. (Fortunately, none of the families with the cats appear to have contracted infectious prions.)

In the U.S., thereís a slim chance that a TSE called chronic wasting disease (CWD), seen in wild elk and deer in the Midwest, could find its way to cattle or to humans. In some areas, the CWD infection rate runs about 18 percent-some five times higher than BSE at its worst in the U.K. "Some in the U.S. may be being a little naive" about CWD, warns Adriano Aguzzi of the University of Zurich, because no one knows how it spreads in the wild. Moreover, studies have shown that CWD could infect cattle, albeit only when the diseased tissue is injected into the brain. But Paul Brown of the National Institute of Neurological Disorders and Stroke notes that CWD has been around for decades and has not spread or led to a single case of vCJD, even among hunters who may have eaten infected animals. "I'm not particularly worried about a wildfire spread, given the history," Brown says.

.

First, there are feelings of anxiety and depression. A wobbly gait and an uncertain grip soon develop. Within a few months come memory loss, confusion, an inability to recognize familiar faces. Body and mind deteriorate until death occurs. From the symptoms, one might conclude Alzheimer's disease-except that the illness completes its job in about a year, and patients are on average 29 years old. Only an autopsy will reveal, from the spongy mess that was the brain, that the patient died of variant Creutzfeldt-Jakob disease (vCJD)-the human form of the dread mad cow disease.

Since the first deaths in 1995, about 100 people have succumbed to vCJD-the vast majority in the U.K., where 15 died in 1999 and 27 last year, according to the U.K. Department of Health. The illness arises primarily through eating beef tainted by the substance that causes mad cow disease, or bovine spongiform encephalopathy (BSE). Between 1980 and 1996 in the U.K., 750,000 cattle infected with BSE were slaughtered for human consumption, and each cow could have exposed up to 500,000 people. Most of Britain's 60 million residents and untold numbers of tourists may therefore have come into contact with the BSE agent.

But grounding the risk in solid numbers has been nearly impossible, because so little is known about the relentless neuro-invader. Researchers are struggling to determine how much of a threat vCJD truly poses and to devise tests that can detect people who may be silently harboring the brain-wasting pathogen. Unlike other diseases, BSE, vCJD and other transmissible spongiform encephalopathies (TSEs) such as scrapie apparently do not arise from bacteria or viruses-or anything having DNA or RNA. The culprit appears to be malformed versions of protein particles called prions, which normally are coiled into a helix and help to maintain the integrity of nerve cells. Infectious prions are more sheetlike and somehow coax normal prion proteins to fold into the infectious form.

The incubation time is the key to determining the vCJD toll. (The infectious prions hide out in lymph tissue before assaulting the brain.) One estimate is 10 to 15 years, based on the assumption that the initial cases of vCJD stemmed from the earliest BSE outbreak, which began in the early 1980s and peaked in 1992. Such an incubation length would yield only several hundred vCJD cases, according to a study by epidemiologist Neil M. Ferguson and his colleagues at the University of Oxford. But 136,000 deaths are possible. In that case, "the incubation period of vCJD would have to be large-on the order of 60 years," Ferguson says. "This would make it unusual, but it cannot be ruled out." Complicating the issue is the unknown lethal dose. Most researchers assume that the more infected beef eaten, the greater the risk. But the type of beef also matters. Processed meats such as sausage may be the riskiest, because they are more likely to contain bits of brain and spinal cord, where prions abound. (One theory of why vCJD strikes younger people is that they consume a lot of processed foods.)

Genetics also plays a role. All vCJD patients thus far have had a particular variation on their prion gene, one that occurs in 40 percent of the Caucasian population. In fact, the Oxford estimates consider only these people. Whether the other 60 percent are immune to infectious prions or can resist them longer is unknown-if the latter, the ultimate number of casualties could jump dramatically.

A huge pool of asymptomatic, or silent, carriers could contaminate the blood supply or surgical instruments, if the experience with the conventional form of CJD, called sporadic CJD, is any indication. This condition results from a rare genetic mutation and is not transmissible the way vCJD is. But it has spread inadvertently through, for instance, the use of growth hormone or corneas taken from infected cadavers. In the U.K., 6.6 percent of sporadic CJD cases have occurred since 1985 because of medical procedures. The only surefire diagnostic, says Bruce Chesebro, a viral epidemiologist at the Rocky Mountain Laboratories in Hamilton, Mont., is to examine brain sections. Hence, many investigators are working on simple diagnostics, such as blood tests. It won't be easy. "There may not be enough prion protein in the blood to detect," notes Paul Brown of the National Institute of Neurological Disorders and Stroke. But picking out the infectious prions and then amplifying them to more obvious levels may be feasible. Last fall neuropathologist Adriano Aguzzi of the University of Zurich and his colleagues discovered that plasminogen, a natural blood component, clings to infectious prions but not to normal ones. Other researchers claim to have made antibodies that do the same thing. Alternatively, indirect markers of infection may exist: TSEs lead to a drop in the expression of a protein factor in precursor red blood cells.

A convenient diagnostic might enable what Aguzzi calls "postexposure prophylaxis"-preventing infectious prions from reaching the brain. "There are many possibilities one can think of to interfere with prion spread," comments Aguzzi, whose group has found a molecule from spleen cells that keeps prions from moving out of the gut. Researchers can "design little pieces of protein similar but not identical to prions to get in the way" of infectious prions, Brown suggests. Such approaches are more pragmatic than a cure, Aguzzi says, because by the time vCJD symptoms show, "the brain is a mess. There's so much damage, it's not realistic that something can be done with the current medical technology."

Strict controls on rendering throughout Europe-most notably, banning mammalian protein in ruminant feed-have reduced BSE cases dramatically. Violations, however, still pose a hazard: earlier this year two German abattoirs lost their licenses for mixing spinal cord material with feed. Such lapses are the only way the U.S. would see BSE, Brown thinks. "I am convinced we do not have BSE in this country," he states. "If these regulations are followed strictly, we never will." But mistakes happen: the government reported in January that about 25 percent of U.S. renderers were being lax, such as not labeling feed properly. And considering the popularity of global travel, a case of vCJD in the U.S. may be only a matter of time.



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