On Science: What if mad deer disease came to Missouri
This article first appeared in the St. Louis Beacon, Sept. 23, 2009 - This month saw the solution to a puzzle that has chewed at the edges of my thinking for eight years. One of the very best things about being a scientist is that the puzzles our work creates do get solved. Not always right away, when they first present themselves in shadowy outline as we try to understand new data, but eventually.
Like woodpeckers, scientists keep pecking away at a problem, day after day, removing this bit of what we don’t know, and then that fragment, until finally the solution to the puzzle is revealed, an answer often so clear and simple you wonder “Why didn’t I see that?” This puzzle’s solution is one of those, a clear answer to a cloudy problem.
My story starts back in the fall of 2000, when my brother Paul sent me a large package of frozen elk steaks. Paul like my father is a big game hunter, and travels around shooting animals. This elk was living in Wyoming near the Colorado border when he had the misfortune to encounter Paul.
In November, only weeks after receiving the elk meat, I disposed of it (I would like to say I threw it out, but actually I fed it to my dog). I had just read alarming news, news that drove me to act quickly. In the very area where Paul had bagged his elk, elk and deer herds were reported to be seriously infected with a wasting disease very like mad cow disease. You will remember mad cow disease as a major news story a decade ago. I wrote several columns about it, a story as much in the news then as swine flu is now.
Mad cow disease seemed particularly scary, as it is not an infectious disease in the sense we are accustomed to dealing with. Unlike AIDS or flu or chicken pox, it is not introduced into your body by viruses, bacteria or other microbes. Unlike cancer or muscular dystrophy or cystic fibrosis, it is not the result of a DNA defect. Individuals affected with mad cow disease have no microbial infection, and their DNA is normal.
What Causes Mad Cow Disease
Mad cow disease results instead from misfolded proteins called prions that pass to an affected person's brain. At first, only a few proteins may be misfolded, and little harm to the brain results. But each misfolded protein induces other brain proteins to misfold in the same way, so that, spreading like a rumor, the mistake eventually becomes widespread. Sponge-like holes develop in the brain, which gradually loses the ability to function.
Mad cow disease is one of a family of prion-caused brain diseases called transmissible spongiform encephalopathies (that is, infectious hole-causing brain disorders), or T.S.E.'s. Each kind of animal had its own kind of T.S.E. Sheep have a form called scrapie, for example, and humans have Creutzfeldt-Jacob disease (CJD). Even hamsters have their particular form of T.S.E.
The first major outbreak of mad cow disease was in England in the mid-1980s. It took British officials quite a long time to come to grips with the problem, and a lot of British beef-eaters became infected; 77 died. The British eventually had to slaughter almost 4 million cattle to eliminate the disease-causing prion from their foodchain. Later, the same prion was detected in 90 cows in France in the year 2000, leading to further mass cattle slaughtering throughout Europe. Today it is generally assumed that the mad cow prion has been banished from European human diets.
It still makes the news, though.
From One Species to Another
The key lesson we have learned from the British outbreak of mad cow disease is that T.S.E.s can be transmitted from one species to another. Tests of British patients afflicted with mad cow disease point not to human CJD, but rather to the form of the prion found in sick British cows. Humans in Britain had become infected by eating hamburger containing brain or spinal cord tissue from infected cows.
Once they realized this, biologists began looking more closely at cross-species transmission of other forms of T.S.E.s. The results are not encouraging. The British experience has taught us that sheep prions can infect cows.
Earlier this year researchers demonstrated that prions from hamsters infect mice just fine, too. And deer? In laboratory experiments carried out in test tubes, elk and deer prions from chronic wasting disease animals were able to convert healthy human prions into infectious prions.
There was no clear demonstration that these test-tube-converted prions would actually produce a brain disease in humans, but the presumption seems strong. No traces of human disease have yet been traced to elk or deer meat, but the available scientific evidence suggests that transmission is probably possible.
And this suggestion is where my brother's elk meat comes in. A sickness called "chronic wasting disease" was first reported in Colorado in 1967 among the deer populations living along the border shared by Colorado and Wyoming. As my luck would have it, this is just where my brother bagged his elk.
But Deer Are Vegetarians
Wildlife biologists estimate that in these populations, chronic wasting disease affects 1 percent of elk and from 6 to 15 percent of deer. It leads to emaciation, staggering and death. Over the following 42 years, this wasting disease has spread rapidly through deer, elk and moose herds throughout the West and Midwest. It is now found throughout 14 states and two Canadian provinces.
And here is where the puzzle arises: How in the world is the chronic wasting disease prion spreading? In humans, Creutzfeldt-Jacob disease is only spread by canabalism – you have to actually eat the prion protein for it to infect you. In England, the mad-cow prion was spread to people from cows only when people ate cow meat infected with it.
So how is the chronic wasting disease prion protein passing from deer to deer? Surely the deer are not eating one another. That would be like a horse chowing down on a steak – wild deer just don’t eat meat. Ever.
Over four decades, until this month, the puzzle remained unsolved. “Mad deer” disease continued to spread rapidly, in some unknown way, without a single deer being eaten by another deer.
This month, the solution was revealed. In the prominent journal NATURE, Stanley Prusiner (the man who received the Nobel Prize for discovering prions) announced research findings demonstrating that the infectious prion that leads to chronic wasting disease in deer is spread by the feces of infected animals. The prion protein is retained in the soil, binding to clay particles. When other deer graze on plants growing through the infected dirt, they consume the clay particles – and the prions.
And that is how the disease is spread from deer to deer: by fecal-oral transmission, among animals a classic way to spread any infection. An infected animal, even a perfectly healthy one that will not express the disease for a decade, releases the prion protein in its feces. Each deer excretes about two pounds of fecal pellets a day. As wild herds move around, more and more land becomes prion-infected. The pellets soon dissolve on the ground, releasing the prion proteins into the soil to bind to clay particles. There the prion-studded clay particles lay, stable and infectious, like little land mines, until another deer grazes on that soil.
In captive herds, up to 90 percent of the animals develop the disease. In wild herds, a third of the animals can become infected. In Prusiner’s study, the research team deliberately infected five captive mule deer by feeding them food pellets containing brain tissue from an infected animal.
They took fecal samples before infection, and at three to six month intervals afterward. Four to nine months after infection, the deer feces all began to exhibit prion proteins, even though the deer themselves were still healthy. The deer all did come down with wasting disease 16 to 20 months later, after having released prion-laced fecal pellets daily for a year. It is difficult to imagine a more effective way to spread the disease among wild deer.
What About Local Deer?
Most deer hunters are not aware of the problem, although biologists are trying to get their attention. In the meantime, while the scientists explore the potential problem, it seems to me prudent to be cautious. Would you eat a hamburger from a cow in a herd where 30 percent of the herd had mad cow disease?
A lot of my friends in Missouri will go deer hunting this fall. Should they worry? I am not aware of any instance of chronic wasting disease among Missouri deer, although it seems to me quite possible that Missouri herds, like those of other Midwestern states, are infected.
People have been eating deer meat in our state for hundreds of years with no evidence of ill effects. While I would love to see a serious screening of Missouri deer herds for T.S.E.s, I see no reason to panic yet over the possibility of "mad deer disease" here in Missouri. On the other hand, don't bother to offer me any deer meat this year.
George B. Johnson's "On Science" column looks at scientific issues and explains them in an accessible manner.
Johnson, Ph.D., professor emeritus of Biology at Washington University, has taught biology and genetics to undergraduates for more than 30 years. Also professor of genetics at Washington University’s School of Medicine, Johnson is a student of population genetics and evolution, renowned for his pioneering studies of genetic variability. He has authored more than 50 scientific publications and seven texts.
As the founding director of The Living World, the education center at the St Louis Zoo, from 1987 to 1990, he was responsible for developing innovative high-tech exhibits and new educational programs.