Mad cow disease

Last December, a cow from Sunny Dene Ranch in southern Washington state was sent to slaughter. She was a "downer" cow--one that couldn't walk because of illness or injury. That wasn't such an unusual event; sick and injured cows are into hamburger all the time.

What was unusual, however, was that the cow tested positive for mad cow disease--the first known case of the disease in the United States. Meat from the cow was recalled before it could reach consumers' plates. Still, some government regulators and meat eaters panicked, wondering how the disease wound up in the United States and whether it was here to stay.

A NEW CLASSIC Mad cow disease, officially called bovine spongiform encephalitis (BSE), is a disease of the brain. An infected cow has difficulty holding up its head and experiences uncontrollable twitching of its muscles. The animal stumbles and falls, eventually losing the ability to walk. Soon after, the infected cow dies.

BSE first made headlines in the mid-1990s when the British government announced an epidemic of the disease in the country's cattle. A few years later, people in the United Kingdom became iii with something that strongly resembled Creutzfeldt-Jakob disease (CJD). A rare brain disease, CJD causes progressive dementia, followed by paralysis and death. Dementia is a brain disorder characterized by loss of perception, memory, judgment, and the ability to control muscular movements.

The classic form of CJD occurs spontaneously in victims' brains, for reasons that aren't entirely understood. But many doctors linked the strange new form of CJD in the United Kingdom, known as variant CJD (vCJD), to eating BSE-infected beef. The variant disease affected mostly young adults, apparently striking five or more years after the victims had consumed infected meat. No cure exists for vCJD, and the disease is always fatal.

COW-NIVORES

Most foodborne diseases are caused by a virus or a bacterium, both of which are usually killed by thoroughly washing or cooking the food. Not so with mad cow. Though a handful of scientists disagree, most researchers believe that BSE is caused by an abnormal form of a protein called a priori (PREE-on). Prions persist even after meat has been cooked through.

Where do the mad cow prions come from in the first place? No one is sure. One possibility is that abnormal prion proteins arise spontaneously in a cow's brain, the way they do in people who have classic CJD. But many researchers believe that the outbreak of mad cow disease in the United Kingdom stemmed from scrapie, a prion disease in sheep. Cows are natural vegetarians, but cattle reared on modern farms are often fed protein supplements to fatten them up for the meat market. Protein supplements in the United Kingdom were sometimes made from ground-up sheep, some of them victims of scrapie. The disease might have jumped the species barrier when cows are sheep prions in the supplements.

Protein supplements in the United Kingdom also contained cow parts--possibly even parts from sick cows. Modern farming practices might have unwittingly encouraged the spread of abnormal prions.

FAILING THE TEST

Responding to the outbreak of mad cow disease and vCJD in Britain, in 1997 the U.S. Food and Drug Administration (FDA) banned the use of cow and sheep parts in cattle feed. But critics say too many loopholes exist in the law to make the ban effective. Parts from ground-up cows, including downer cows, are still used in feed for chickens and pigs. Those chickens and pigs can then be made into feed for cows. Furthermore, calves are sometimes given a milk replacement made from cow blood. Feed or supplements tainted with abnormal prions could trigger an outbreak of mad cow disease.

At present, no method of testing live cows for BSE exists, so any testing must be done after a cow is slaughtered. Japan currently tests a sample from every cow that enters the food chain, holding its meat in a refrigerator until the test comes back negative. The European Union tests about 70 percent of its cows. The U.S. Department of Agriculture (USDA) tests only 0.06 percent.

BETTER BEEF

That percentage sounds alarming, but don't panic. Human illnesses from BSE-infected beef are still very rare. Although nearly 200,000 British cows are thought to have suffered from mad cow disease, fewer than 150 people in the United Kingdom contracted vCJD.

After the U.S. case of mad cow disease was made public, the USDA announced changes to make beef consumption safer. Downer cows will be eliminated from the food chain, so meat from sick cows, whether infected with BSE or any other disease, won't wind up in your sloppy Joes.

The USDA has promised to introduce a system that tracks each cow from birth to death, so future disease outbreaks will be easier to trace. Politicians and consumer advocates have also called for improved testing for BSE. Ultimately, that one unlucky mad cow in Washington may actually end up being good for us.

HOW MAD COW WORKS

Prion proteins occur naturally in the cells of the central nervous system (the brain and spinal cord) An abnormally shaped prion can infect cells, causing mad cow disease, Creutzfeldt-Jakob disease, and several other diseases that belong to a class known as transmissible spongiform encephalopathies. When an abnormal prion makes its way into a neuron (nerve cell), it forces the normal proteins in the cell to change their shape as well. That change sets off a chain reaction, with more and more normal prions changing into the diseased variety. The abnormal prions build up in the brain, killing neurons and leaving the brain riddled with holes, like a sponge.

Objectives

Mari cow disease mode its first appearance in the United States last December. What threat does it pose to beef eaters?

Critical Thinking

* What other diseases are contracted from tainted food?

* Has the case of mad cow disease in Washington state mode you change your beef-eating habits? Discuss.

National Science Education Standards

* Personal and community health: disease prevention and control

Internet Links

* U.S. Food and Drug Administration: Bovine Spongiform Encephalopathy: http://www.fda.gov/oc/opacom/hottopics/bse.html

SKILLBUILDERS ANSWER KEY

1.C, 2. C, 3. A, 4. A, 5. B, 6. C, 7. C, 8. B, 9. B, 10. C

Multiple Choice

1. The disease scrapie occurs in (A) cows. (B) humans. (C) sheep.

2. The central nervous system includes the (A) brain. (B) spinal cord. (C) both A and B.

3. Nerve cells are also called (A) neurons. (B) neutrinos. (C) neutrons.

4. Mad cow disease is also known as (A) bovine spongiform encephalitis. (B) bovine supplement encephalitis. (C) brainstem encephalitis.

5. Most scientists believe that mad cow disease is caused by a (A) bacterium. (B) prion. (C) virus.

6. What percentage of cows does the U.S. government currently test for mad cow disease? (A) 100 percent, (B) 70 percent, (C) less than 1 percent

7. Eating beef infected with mad cow disease can cause which disease in humans? (A) BSE, (B) scrapie, (C) vCJD

8. Mad cow disease was first identified in the mid-1990s in (A) Japan. (B) the United Kingdom. (C) the United States.

9. Dementia includes (A) hearing loss. (B) memory loss. (C) both A and B.

10. Prions are a type of (A) cell. (B) neuron. (C) protein.

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