Holy cow! What now? Mad cow disease has hit the U.S. How worried should you be?
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?
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.
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.
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."
Some people believe the U.S. should test every cow that's slaughtered. But Sara Goodwin, spokesperson for the National Cattlemen's Beef Association, insists, "We have the systems in place to detect this disease, prevent its spread, and to protect animal and public health. U.S. beef remains the safest in the world."
Most of the ninth-graders interviewed don't plan to change their eating habits. But Rachal adds: "I'll think more about my food, and where it came from."
HOW MAD COW DISEASES SPREADS
Mad cow disease starts after a cow eats feed contaminated with nervous-system tissue from an infected cow or sheep. Normal PrP molecules, made by nerve cells, become abnormally folded. These misfolded PrPs are known as prions. Misfolded PrP force normal PrP to refold to match the misfolded ones. New prions are formed. Over time prions accumulate in the cow's brain cells, crippling the animal.
1 Nerve cell produces normal PrP molecules.
2 Misfolded PrP, or prion, invades causing normal PrP to misfold.
3 Prions build up and eventually kill the cell.
4 Prions invade other cells, where the process repeats.
Did You Know?
* In 2002, 1.3 million cattle were slaughtered in Japan; each was tested for bovine spongiform encephalopathy (BSE). In the U.S., however, only 20,000 of the 35 million cattle slaughtered in 2002 were tested for BSE.
* U.S. consumers spend approximately $50 billion per year on beef.
Directions: Read the story. Then test your comprehension by circling the correct answer.
1. Cows are naturally
2. Which of the following about prions is not true?
a. They are infectious agents made up of an abnormal form of protein.
b. They are folded into abnormal shapes.
c. Prions cause abnormal nerve cells to multiply.
d. Prions form tangled protein masses.
3. People can become infected with the human version of mad cow if they eat an infected cow's
a. nervous system tissue.
b. spinal cord.
d. all of the above.
4. Normal PrP molecules are made by
b. nerve cells.
c. amino acids.
d. blood cells.
5. BSE may have originated when British cows ate
a. scrapie-infected sheep.
b. tainted plants.
c. other cows.
d. an unknown source.
Take It Further:
Conduct a class poll. Will you eat beef despite the discovery of a case of mad cow disease? Why or why not?
1. b 2. c 3. d 4. b 5. a
For terrific information on mad cow disease, visit the companion Web site to the NOVA program "The Brain Eater." It includes a teacher's guide and activity pages. Visit: www.pbs.org/wgbh/nova/madcow/
Check out the U.S. Food and Drug Administration's Web page on bovine spongiform encephalopathy. www.fda.gov/oc/opacom/hottopics/bse.html
Also read these news articles:
"How Now, Mad Cow?" by Cathy Booth Thomas, Time, January 12, 2004.
"Mad Cow: What's Safe Now?" by Jerry Adler, Newsweek, January 12, 2004.
RELATED ARTICLE: New USDA regulations.
After the first cow in the U.S. was diagnosed with BSE, the U.S. Department of Agriculture created additional regulations to ensure the safety of the country's meat supply.
SOME OF THE NEW RULES:
1 Downer cattle, or cows that are unable to stand, are banned for use as food.
2 Cattle tested for BSE will be held until test results come back negative.
3 Brain, spinal cord, and other central-nervous-system tissues from cattle 30 months of age and older, along with the small intestine of all cattle, are banned for use in the human food supply. (Since BSE symptoms usually take three and a half years to show, cattle younger than 30 months aren't considered risky.)
4 Advanced Meat Recovery (AMR) is a technology that uses high pressure to remove muscle tissue from the bone. Spinal cord was already banned from being included in AMR product. Now dorsal root ganglia (clusters of nerve cells connected to the spinal cord) are also prohibited.
5 Mechanically separated meat is banned from use in human food, since it could contain bits of nervous-system tissue.