CWD: Mad Cow Disease in deer?

By Tom McCutchan | Jan 26, 2012
Photo by: Ken Bailey

If you’ve ever heard the word “prion,” it was probably in reports about Mad Cow Disease during the 1990s. Mad Cow Disease was caused by a type of prion. Prions are single proteins, far smaller than viruses or bacteria. They are passed from one creature to another and start a chain reaction of events that, in Mad Cow Disease, leads inevitably to death.

A different but related prion causes Chronic Wasting Disease (CWD), which was first detected in the western United States in elk, moose, and deer. CWD is rapidly spreading and has now been seen as far east as upstate New York.

Before you empty your freezers of venison, however, let me reassure you that: 1) there is no evidence that CWD can be transmitted to humans (I will be happy to sample your venison if you care to send me a tenderloin); 2) there is no evidence that CWD has reached Maine; 3) there are precautions that hunters can take to protect themselves (just in case) and help keep CWD out of Maine. A fourteen-minute, Internet video (cwd-info.org) gives pertinent advise on safely field dressing your animal.

Prions cause a number of horrible diseases that involve a slow destruction of the nervous system. In humans they affect our capacity to move, speak and think. At present, a majority of scientists, as well as the people who people who award Nobel Prizes, believe that the prion acts alone both in transmitting the disease and causing its symptoms. I am not alone in questioning the validity of this conclusion but, for the purpose of this column, will respectfully present the majority view (describing the minority view involves tales of illicit sex and the theory of evolution; hence, a column of its own).

Much of what we know about prions comes from studies of Mad Cow Disease in Great Britain, and of Kuru, a disease resulting from the practice of cannibalism, in isolated regions of Papua New Guinea.

In the 1970s, we learned that prion-related diseases could be transmitted from human to human. Cannibals in Papua New Guinea started dying off at an unusually high rate in the 1970s (and not for the reason you might at first imagine). As often happens, one scientist found this problem intriguing and headed out to investigate. His initial observations revealed that the deaths were mainly occurring among women and children, who ate only brains and internal organs at the communal meals, while men ate only muscle. This was an important clue. He found a second clue in the bizarre symptoms that individuals presented in the latter stages of the disease: uncontrolled shaking, acute depression along with sudden outbursts of laughter, and a lack of muscular coordination.

Recognizing that these clues pointed to a neurological disease, he had samples of victims’ brain tissue analyzed by microscopy. All the samples had sponge-like lesions similar to those found in the brain tissue of sheep that had died of Scarpie, a brain-wasting disease that is specific to sheep.

It turned out that all the damage caused by the disease was the result of structural changes in a single protein. Investigating scientists named these newly discovered proteins “prions.” Further research showed that prions are chemically identical to a human protein that is critical in neuron function: the ability to send messages from the brain along spinal column. The only differences between this human protein and the prion are that the human protein and the prion have a different shape, and that they serve entirely different purposes.

What makes prions really creepy is their ability to attack and transform chemically identical human proteins into prions themselves. Once they’ve been turned into prions, the formerly human proteins proceed to convert other proteins into prions, a theme similar to that of the horror movie “ The Invasion of the Body Snatchers.” The ever-increasing community of evil prions eventually destroys the human’s brain.

Mad Cow disease, first seen in cattle in 1987, was the result of feeding cattle by-products to cattle (in effect, turning them into cannibals). These animal by-products came almost exclusively from animals that were so sick they could no longer stand. There is no indication that healthy cattle can be infected simply through contact with sick animals; they have to eat them. People in turn became infected with the disease by eating meat from animals that were infected.

It was estimated that over 400,000 infected cows entered the human food chain before the British government stopped the practice of feeding cow by-products to cows in 1989. The length of time between becoming infected and showing signs of the disease is measured in years—possibly as many as 20 years. It was initially predicted that there would be a large increase in dementia and death among humans over the ensuing decades. It has now, however, been more than 20 years since the source of the disease was identified and stopped, and the numbers have not been as high as predicted.

Mad Cow Disease caused more than 3,000 cattle to die each month at its peak, so there were both medical and economic reasons to study it, as well as a large number of subjects to study. And yet, there are reasons why it has been very difficult to research prion-caused disease. For one thing, although we know that spongiform lesions in brain sections are an almost certain indicator of a prion-based disease, these are only identifiable after death. For this and other reasons, studies proceed slowly. It is fast-moving research that attracts funding institutions and science lives on highly competitive grants.

Despite this, in the past few years, we have observed a rekindled interest in the study of prions. There have been truly fascinating reports in respectable, peer-reviewed journals suggesting that prions may be associated with other human health issues, some bad and some good. There are reports that “good” prions may enhance long-term memory. We may be awash in a sea of prions that we don’t even recognize yet. Could they be connected to Alzheimer’s disease? To sleep disorders? Who knows what other effects — good and bad — they are having?

 

 

 

 

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