Since it first appeared in Great Britain in 1986, mad cow disease has spread to almost two dozen other countries, including Canada and now the United States.

On Tuesday, the U.S. Department of Agriculture (USDA) announced the first American case of mad cow -- or bovine spongiform encephalopathy (BSE), a fatal brain disease in cattle -- had been found in an adult Holstein cow in Washington state.

'Despite the very best efforts of the USDA, the spread of BSE to the U.S. was and remains inevitable,' said Dr. David Katz, of the Yale-Griffin Prevention Research Center at Yale University.

'For both animals and people, a global economy is linked to a global public health. West Nile virus and SARS are examples of recent outbreaks that did not respect international borders. Neither will BSE,' Katz added.

In Britain, the cases of BSE peaked between 1992 and 1993. At that time, three of every 1,000 cows were infected, British officials said. In 1995, the first human death occurred -- in England.

In response to the incidence of BSE, millions of head of cattle in Britain were destroyed, and the European Union banned the import of British beef.

In May 2003, Canada reported its first case of BSE. The Canadian government quarantined herds, but it is possible some of the meat was eaten by humans, according to a Canadian government official.

It is believed cows get the disease from eating commercial feed that includes bone and brain from animals.

'The spread of BSE is the result of animal feeding practices that are harmful in a variety of ways,' Katz said.

Feed animals that should be herbivores are provided meal that contains the ground-up parts of other animals. This, of course, carries with it the risk of spreading disease, but it also changes the quality and composition of meat intended for humans, Katz explained.

'Valuable nutrients have been 'domesticated' out of beef, and harmful qualities introduced, because of the way modern animal husbandry is conducted. Such changes are the result of the mass marketing of feed animals by large farms devoted to efficiency and profit rather than the care of individual animals,' he said.

The first human victim of BSE was Stephen Churchill, 19, who died in May 1995 from an illness that looked like Creutzfeldt-Jakob Disease (CJD). CJD usually strikes those over 55.

There were two other similar deaths in Britain that year. When scientists examined the brains of these patients, they found unusual spongiform symptoms and named this new condition variant CJD (vCJD).

It was later found that people could get vCJD from eating infected beef or nerve tissue, and possibly through blood transfusions. To date, 143 people in England have died from vCJD, as have 10 in other countries. So far, there have been no deaths from vCJD in the United States.

People with vCJD experience increasing mental impairment. As the illness progresses, the impairment becomes severe. Patients often develop involuntary muscle jerks, and they may go blind. Patients eventually lose the ability to move and speak and go into a coma. Patients often die from complications such as pneumonia and other infections.

The only way to confirm a diagnosis of vCJD is by brain biopsy or autopsy.

'The absolute risk of BSE for people who eat beef is extremely low,' Katz said. 'This one U.S. case is not a reason why people should feel obligated to change their diets. But for many reasons other than BSE, most Americans would benefit by reducing their intake of beef.'