"I would say that what we know is about 15 percent of what's going on," said Suzette Priola, who works as an investigator in the Laboratory of Persistent Viral Diseases at the NIH's Rocky Mountain Laboratories in Hamilton. She is chairwoman of the Food and Drug Administration's Transmissible Spongiform Encephalopathy (TSE) Advisory Committee.
That level of knowledge makes the family of diseases known as TSEs more difficult to manage but ripe with opportunity for new science. The diseases have been in the public eye only in the past decade because of their appearance in cattle and then in humans who ate infected cows in Great Britain. In December, the first case of mad cow disease occurred in the United States, in Washington state.
BSE in cattle first appeared as an epidemic in Great Britain after a change in feeding practices in the late 1970s, Priola said. Cattle raisers rendered and ground dead sheep and cattle to feed to living cattle. The ground brains and spinal cords infected the animals that ate them. The first recorded case was in 1985.
"Cattle ate it, developed BSE," Priola said, "but no one knew it."
In 1996, scientists identified variant CJD in humans and connected it with eating BSE-infected cattle. It has infected 150 people in the last seven years.
"For the people who have it, it's a dreadful disease," Priola said. "It strikes particularly hard in young people."
It will probably not be an epidemic, she said, because the incidence dropped off to virtually none once the practice of feeding ground and rendered animals was banned in Britain in 1988.
The TSEs are hard to study because they take months to years to appear as disease, Priola said; the longest is Kuru disease, which has shown up in a cannibalistic tribe in New Guinea 40 years after the people were infected.
TSEs are also unusual because of the way they appear to transmit themselves. Most diseases are caused by viruses, which have DNA in their drivers' seats, invading cells and replicating themselves. But TSEs appear to be caused by infectious proteins, called prions.
"There's no DNA," Priola said. "There's nothing telling it what to do."
The prion appears to connect with the body's own proteins, in effect convincing them to come over to the dark side, Priola said.
They accumulate, bundle together, form long strings and cause holes in the brain.
"We're not entirely sure how this happens," Priola said, "but these two bond."
How well a TSE transfers between two species depends on how well their proteins match up.
"If it's the wrong ones, it completely prevents the two from bonding, and you don't get disease," she said.
BSE can cause human disease but not very well, Priola said; humans have to eat the brain or spinal cord of infected cows, which happened in Britain in ground meat. Scrapie does not pass from sheep to humans, but it's persistent in fields and can infect new herds after the fields have sat empty for five years. Chronic wasting disease does not appear to cross from deer and elk to cattle, but it circulates very efficiently among a herd of deer or elk.
There are no treatments for TSEs.
"Once a person is clinically diagnosed, it's 100 percent fatal," Priola said.
Testing is inadequate; science has not found a way to test a cow as soon as it is infected but is not showing signs of sickness. Those two events might be two years apart.
Now, the U.S. Department of Agriculture is testing high-risk cattle (so-called "downer" cows) before they enter the food supply. The FDA has banned feeding livestock other livestock and has banned risky bovine materials - brains and spinal cords - from medical products. Prions have never been found in milk or muscle.
Soon, the practice of feeding these byproducts to pigs and chickens will be banned, as will their inclusion in pet food.
Meanwhile, Priola said, science will continue to look for answers.
Priola addressed approximately 50 people at the St. Patrick Hospital Broadway Building Conference Center.
Reporter Ginny Merriam can be reached at 523-5251 or by e-mail at gmerriam@missoulian.com
|
 |
 |
Add your comment now! Write your comment in the form below.
(Email address is for verification only. If you'd like to email a story, look for the link above)