Published on 06/05/03

U.S. measures protect against mad cow disease

By Cat Holmes
University of Georgia

When a single cow tested positive May 20 for Bovine Spongiform Encephalopathy (BSE) or "mad cow disease" in northern Alberta, Canada, the U.S. Department of Agriculture went on high alert. It temporarily closed U.S. borders to Canadian beef and related products, including animal feed.

That's one of the reasons BSE, a disease that causes fatal brain degeneration in cattle, has never been found in the United States, said Ronnie Silcox, an animal scientist with the University of Georgia College of Agricultural and Environmental Sciences.

"Since 1989, the U.S. has banned imports of live animals or animal products from countries that are at risk for BSE. So closing our borders to Canada beef was par for the course," Silcox said. "We know how BSE is spread, and we've put a number of systems in place to keep it out of our country."

This wasn't the case when BSE first appeared in Great Britain in 1986.

"Then, it was a strange new disease, with an unknown cause or source," Silcox said. "It took several years to see the link between using animal byproducts in animal feed and the disease."

During that time, the numbers of BSE-infected cattle in the United Kingdom skyrocketed, though not in the way contagious diseases do.

Unlike other feared cattle diseases like foot-and-mouth, BSE isn't contagious. "It's not transmitted by contact with other infected animals," Silcox said. "It's not caused by a virus or bacteria. It's spread by feeding byproducts from infected cattle to other cattle."

It can take two to eight years after a cow eats infected feeds for signs of BSE to show up, he said.

Since 1997, the United States and Canada haven't allowed protein from cattle, sheep, goats, bison, elk or deer, animals also known as ruminants, to be fed to other ruminants.

"This 'animal feed rule' eliminates the only cause of BSE, as far as we know," Silcox said. "Banning byproducts like meat and bone meal in cattle feed ensures that we're not feeding products that could spread the disease even if it were discovered."

Indeed, investigators from the Canadian Food Inspection Agency (CFIA) have found that the BSE-infected cow didn't enter Canada's food supply. The cow's remains were rendered and may have been used for the manufacture of a dry dog food. Dogs aren't at risk for BSE or any similar disease.

BSE belongs to a family of diseases known as transmissible spongiform encephalopathies, which include chronic wasting disease, transmissible mink encephalopathy and variant Cruetzfeldt-Jakob disease (vCJD). All are fatal. And the latter, vCJD, effects humans.

The Centers for Disease Control and Prevention Web site notes that as of April 2, 2002, 125 cases of vCJD had been reported in the world, most in the United Kingdom. Evidence strongly suggests that vCJD is caused by eating meat from BSE-infected cattle.

"CJD is like lightning striking -- it doesn't happen often but that's not much comfort if you're hit," Silcox said. "It's fairly rare even in countries where there was a lot of BSE infection."

Of course, the question remains as to how Canada's BSE-infected cow got it in the first place. According to the CFIA Web site, the BSE-infected cow, which died in January, was born before the 1997 animal feed rule. It could have contracted the disease before then.

Several other theories are under investigation.

(Cat Holmes is a science writer with the University of Georgia College of Agricultural and Environmental Sciences.)

Cat Holmes was a science writer with the University of Georgia College of Agricultural and Environmental Sciences.