Mad cow disease, otherwise known as bovine spongiform encephalopathy or BSE, affects the brain and spine of an animal. Those body parts are kept out of grocery stores and restaurants and have no contact with the meat that does make its way to consumers. Eating contaminated meat can cause Cruetzfeld Jakob (vCJD) disease in humans.
While many diseases are caused by bacteria and viruses, mad cow disease is caused by a transmissible protein called a prion, according to the U.S Centers for Disease Control and Prevention. While the function of these proteins in nature is unknown, what is known is that they act on the human nervous system to cause rapid brain degeneration.
An outbreak of mad cow disease infected thousands of cows in Britain in the 1990s and led to the deaths of nearly 150 British people who ate tainted meat. The outbreak was the result of tainted cattle feed, which at the time routinely included protein supplements made of ground spinal column material and brain tissue. That practice is currently prohibited by the FDA.
On Tuesday, U.S. officials announced that the cow had atypical BSE, meaning that the disease did not come from tainted cattle feed. Instead, scientists think atypical BSE is the result of a rare random mutation.
This is the fourth case of mad cow disease in the U.S. cattle supply since December 2003. The first U.S. BSE case was in 2003, found in a Washington state dairy cow that had been slaughtered. The second case was found in June 2005 in a cow in Texas, and the third was found in a cow in Alabama in 2006.
Officials are uncertain if the cow died of the disease and if others in the herd were infected. Officials are working now to identify where the cow came from and to test other cows in the same feeding herd, the Associated Press reports.
Dr. William Schaffner, chair of preventive medicine at Vanderbilt University, said it's unlikely that more cows will be infected.
"Mad cow occurs in animals as it does in humans -- rarely and sporadically. At this point, I would not expect there to be another cow to be found," he said.
Humans who have the disease tend to experience a progressive neurological or psychiatric disorder that lasts longer than six months with symptoms including dementia, seizure, unusual sensory symptoms, dizziness, or progressive unusual mood changes. The illness usually lasts 14 months, and the disease is always fatal.
As of last year, 221 cases of probable vCJD had been reported. This includes 172 in the United Kingdom, 25 in France, five in Spain, four in Ireland and three in the United States, with a smattering of cases in other countries around the world. The patients that were diagnosed in the United States were thought to have been infected while they were residing in the UK or living abroad in Saudi Arabia.
ABC News' Dr. Christopher Tokin contributed to this report.