How Mad Cow Disease Works

Mad cow disease, or bovine spongiform encephalopathy (BSE), is a fatal brain disorder that occurs in cattle and is caused by some unknown agent. In BSE, the unknown agent causes the cow's brain cells to die, forming sponge-like holes in the brain. The cow behaves strangely and eventually dies. The connection between BSE and humans was uncovered in Great Britain in the 1990s when several young people died of a human brain disorder, a new variation of a rare brain disorder called Creutzfeldt-Jakob Disease (CJD), which typically strikes elderly people. The new variation was called new variant Creutzfeldt-Jakob Disease (nvCJD), was similar to BSE and its connection to BSE was based on the following findings:

The British government concluded that BSE was probably the cause of nvCJD, and that the victims contracted the disease probably by eating meat from BSE-infected cows.

BSE is spread by contact with brain or other nervous-system tissue from an infected individual. Contact can be from eating food or food by-products that have been contaminated with nervous tissue, or from instruments that have contacted diseased nervous tissue. Once the infectious agent enters the brain, it can lie dormant for several years (even as long as 10 to 15 years). When activated, the agent kills brain cells, leaving large areas of spongy holes. Also, large clumps of abnormal prion proteins (plaques) are found in brain cells. Once the agent is activated, the disease runs its course in less than one year, and ultimately results in death.

We don't know the agent that causes BSE, but we do know the following:

Research into BSE and similar diseases (nvCJD, CJD, scrapie) have provided three theories about the agent that causes BSE. These theories lead us to believe that it is:

Click "Play" to see how mad cow disease gets started.

Let's examine the prion hypothesis more closely.

Prions are proteins that are found in the nerve cells of all mammals. Numerous prions are in each nerve cell, but no one knows for sure what the prion protein does. As mentioned above, many abnormally-shaped prions are found in the brains of BSE-infected cows and humans afflicted with nvCJD or CJD. So, the prion hypothesis goes like this:

Further research in this area may eventually reveal the nature of the BSE-causing agent.

Animal and Human Consequences

Cows infected with BSE lose weight, show abnormal behavior (skittishness), may become paralyzed and die. Humans afflicted with nvCJD begin with psychiatric problems (paranoia) or perhaps problems with their senses. They later develop problems in muscle coordination (balance, speech), muscle spasms, problems with their senses (hearing, vision) and memory loss. They may finally lapse into coma and die.

In 1988, the UK government banned using any ruminant (cattle, sheep, goats) or ruminant by-products in animal feed. Later, they banned exports of cattle to other countries. They have destroyed BSE-infected cattle and monitored herds for signs of BSE. In addition, the medical community has monitored the general population and reported any cases of nvCJD. These steps have contributed to a steady decline in BSE since 1992.

The U.S. government has instituted the following policies regarding BSE:

Several U.S. government agencies (FDA, USDA, CDC) monitor the meat supply in the United States as well as imports from other countries. European countries have instituted similar guidelines.

For more information on BSE and related topics, see the links on the next page.