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Understanding mad cow disease

There's a lot we still don't know about mad cow disease. But we do know how it spreads among animals and how the human form passes to people. This knowledge, combined with preventive action, is what the government is counting on to keep the disease out of our food supply. So far it seems to have worked.

Mad cow disease is a common name for bovine spongiform encephalopathy (BSE). There are many different spongiform encephalopathies, but each one affects a different species. Among the types are:

  • Creutzfeldt-Jakob disease and other diseases in humans.
  • Chronic wasting disease in deer and elk.
  • Scrapie in sheep.

Most scientists agree that these diseases are caused by prions (PREE-ons), abnormally shaped proteins in the brain.

When prions contact normal proteins, the normal proteins take on the prions' abnormal shape. This abnormal shape makes it impossible for the protein to play its normal role in brain functioning.

As more normal proteins take on this abnormal shape, brain function falters. This progressive damage to the brain causes the symptoms of the disease. In cows, symptoms may include aggressive behavior, difficulty standing or walking, and unexplained weight loss. Between two weeks and six months after the first symptoms show, most cattle with BSE die.

Where did it come from?

BSE was first identified in Great Britain in 1986. By 1993, the epidemic peaked at almost 1,000 new cases a week. Research suggests that it spreads through prion-contaminated feed.

Prions can get into feed when manufacturers use meat-and-bone meal (MBM) from infected animals. MBM is a powdery, high-protein dietary supplement made from the parts of sheep and cattle that humans don't eat.

To combat the spread of BSE, Great Britain now kills and incinerates suspect cattle and prohibits the use of mammal protein in cattle feed. When cattle are slaughtered, the organs most likely to harbor prions—for example, the brain and spinal cord—are removed and thrown away. Thanks to these measures, the number of new BSE infections dropped dramatically.

The cross-over

Unfortunately, before these measures were taken, BSE-contaminated meat entered the British food supply. Some people ate this meat and acquired the prion disease. Heat doesn't kill prions, so contaminated meat is dangerous no matter which way or how long it is cooked.

When these prions take hold in humans, they cause variant—also known as new variant—Creutzfeldt-Jakob disease (vCJD). The prions work the same way in a human's brain as they do in a cow's.

This disease should not be confused with classic or sporadic CJD, a prion disease that affects about 1 in a million people each year worldwide. Classic CJD has nothing to do with mad cow disease.

The earliest symptoms of vCJD include behavioral changes such as withdrawal, anxiety and depression. Within weeks or months, the person gradually loses control of movement and develops worsening dementia. The time between infection to the appearance of first symptoms is not known, but is thought to be a number of years. Once symptoms appear, most people die in about 13 months. Currently there's no way to treat or cure the disease.

What about us?

To keep BSE-contaminated animals and meat out of the United States, in 1989 the United States Department of Agriculture (USDA) banned the import of grazing animals and most of their products from all countries known to have BSE. The ban includes cattle, sheep and goats.

In 1990, the USDA's Animal Plant and Health Inspection Service started a BSE surveillance program. Cows are checked for BSE symptoms before they're slaughtered. Cows with symptoms are pulled out of the herd, killed and tested for BSE.

To prevent a BSE outbreak in American cattle, the U.S. Food and Drug Administration (FDA) outlawed the use of most mammal protein in feed for grazing animals. The FDA inspects feed factories to make sure they comply and sponsors educational workshops for state veterinarians and feed control officials nationwide.

The government has also put safeguards in place to make sure that cow tissues at high risk for carrying BSE do not enter the human food supply. And there is a response plan in place in case any animals in the U.S. do test positive for BSE.

Is it enough?

According to the Centers for Disease Control and Prevention (CDC) the risk to human health from BSE in the United States is extremely low.

Want to know more?

For more information about mad cow disease or vCJD, visit these sites:

Reviewed 8/1/2022

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