Holy cow! What now? Mad cow disease has hit the U.S. How worried should you be?

Science World, March 8, 2004 by Jacqueline Adams

Until last year, mad cow disease wasn't high on Americans' list of worries. The disease had struck only in other countries, such as the United Kingdom (UK), Canada, and Japan. That changed on December 23, when a cow in the state of Washington tested positive for the disease. Worse, before anyone knew, meat from this cow was sold across several Northwestern states, Hawaii, and Guam. When the test results came back, the U.S. Department of Agriculture (USDA) recalled the beef. But for many consumers, the announcement came too late--they had already eaten the recalled beef. Luckily, USDA practices in place at the time limited the likelihood that any infected tissue was in the meat. Within a week, the USDA announced new regulations to keep beef even safer (see "New USDA Regulations," page 9).

How are teens across the country responding to the news? A group of ninth-graders from Pennsylvania had mixed reactions. "I'm not worried about mad cow disease. It has yet to infect more than one cow in the U.S., and they're taking many precautions," Karlie says. Justin, on the other hand, isn't convinced. "I don't eat beef at all now," he says. Whatever your opinion, you might find yourself in the cafeteria wondering: Just what is mad cow disease? Should you be worried?

SUSPICIOUS START

The drama first unfolded on Pitsham farm in the UK in 1984. A cow started losing weight, staggering, arching its back, and drooling. It acted like, well, a mad cow. But at the time, no one had ever heard of mad cow disease. The cow died six weeks later. Soon cows in other British herds were dying.

In 1986, scientists discovered the disease was a type of spongiform encephalopathy (in-SEH-fuh-LAH-puh-thee), a category of diseases that destroys neurons and leaves small holes in the brain. This class of disease is fatal. Scientists coined the cows' malady bovine spongiform encephalopathy (BSE), or mad cow disease.

BRAIN-EATING PRIONS

Spongiform encephalopathies are also called prion (PREE-on) diseases. Dr. Shu Chen, associate director of the National Prion Disease Pathology Surveillance Center at Case Western Reserve University in Cleveland, Ohio, explains why: "The current understanding is the prion is an infectious agent made up of an abnormal form of protein."

Cells make protein, strings of chemicals called amino acids. Nerve cells make normal priori protein (PrP) molecules, which can flip between shapes. When the PrP folds abnormally, the misfolded PrP molecules form prions that tan cause disease. According to the prion theory, misfolded PrP force normak PrP molecules to fold to match them. In this way, prions multiply in the cell and eventually kill it (see diagram, below). As these prions kill cell after cell, they leave small vacuoles, or holes, in the brain. Over time, they form plaques, tangled protein masses.

CANNIBAL COWS

BSE might have begun with a surprising notion--feeding meat to cows, which are herbivores, or plant eaters. Farmers began adding bone meal from slaughtered sheep to cattle feed. Another prion disease, called scrapie (SCRAPE-ee), strikes sheep. Scientists believe that cows that ate scrapie-infected sheep became infected. Then cow carcasses were ground up and added to cattle feed. The disease spread through British herds as cows ate their infected relatives.

In the decade following the discovery of mad cow disease, several humans died after developing symptoms similar to those of cows with BSE. Doctors thought the victims had a rare human priori disease called Creutzfeldt-Jakob (KROITS-felt YAH-kop) disease (CJD). But the victims were younger than most CJD patients, and the plaques on their brains didn't look like CJD plaques. "They look like daisy flowers because the central core of the plaque is surrounded by little petals of vacuoles," says Dr. Paul Brown, senior investigator at the National Institutes of Health. In 1996, scientists concluded they'd discovered a human version of BSE, variant Creutzfeldt-Jakob disease (vCJD).

One way humans can contract vCJD is by eating BSE-contaminated cow parts. An infected cow's brain, spinal cord, and other nervous-system tissues can carry the disease.

CAUSE FOR CONCERN?

Twenty-four countries have reported at least one BSE case. More than 183,000 instances of BSE and 143 of the world's 153 vCJD cases have occurred in the UK. To stop the outbreak, the UK banned farm animals from cattle feed and killed thousands of cows that may have been exposed to BSE. Sync 1992, the number of BSE cases there has been dropping.

In 1989, the U.S. banned the import of cattle products from countries with BSE. In 1997, the U.S. extended the ban to all European countries and banned the use of farm animals in cattle feed. These safeguards seemed to keep BSE out of the country--until the cow in Washington tested positive. So how did that cow slip in? Findings suggest it came from Canada.

Should you worry about mad cow disease? Chen says, "If the problem is limited to a single herd or very specific animal populations and is not spread to other places, then I don't think people should be too worried. But of course this depends on a very good surveillance system for BSE."